Feature Request: Why don't pump controllers dynamically set RPM for heating with solar panels

[Dirk]

I'm pretty sure you have this wrong. Running less flow in the morning is going to save on electricity, but the heat exchange will also drop. And it is not a direct ratio. You'll be getting less heat exchange not because there is less available heat, but because you are getting more heat loss (as others have explained above). Even though the panels may be heating the water in them some amount, without enough flow you won't be effectively increasing the pool's temperature. You won't be saving electricity, you'll just be wasting it.

Think of it this way. It's a common misconception, when DIYer's are trying to fool pool-heating physics, they'll loop a bunch of black drip tubing around and around and hook up a little pump to it, and run pool water through it. They'll feel really hot water coming out of the tube and declare themselves genius because they've cracked the solar heating industry wide open, for just a few dollars!! (And they'll swear their pools are getting really warm from doing so.) Hooey. Yes, the slower you run water through any sort of heat exchange tubing the hotter the water will get. But that hot water, at that reduced flow rate, won't heat your pool any noticeable amount. And even though these miracle machines are using very little electricity, pumping a little water with a little pump, that electricity is virtually 100% wasted. That setup is not doing anything, at all. You thinking that reducing the flow rate in the morning to sneak out a little "free heat" is just another variation of this DIYer fantasy.

Contrast that with a solar installer that doesn't have a proper flow meter to optimize flow through a "real" set of panels, he'll get the system going and stick his hand in the pool in front of a return. He'll increase the flow rate until the water coming out of the return is just barely warmer than the pool, because he understands the principal, the physics, that is actually at work. Moving a lot of water that is just slightly warmer than the pool will heat it faster than moving a little water that is much hotter than the pool. It's counterintuitive, but the physics are what they are.

If you're trying to save money because of your usage rates, you're going about it wrong. You think pushing water through your panels at the flow required for your SWG will get you a little "free heat." There is no such thing. All you'll be doing is pushing water up on your roof and wasting the electricity to do so, without significantly adding heat to your pool, even though you'll think it's working because the water out of your returns feels warm. And it will be. It just won't heat your pool worth a darn. You would be much better off starting the pump and the solar heating an hour or two later, but at the panels' optimal flow rate, and heating the pool more efficiently. The only way to save electricity without running the optimal flow rate through the panels is to not run your pump at all. Four hours of very inefficient heat exchange will not work out to be cheaper than one hour at optimal heat exchange, in terms of actually heating your pool.

Pardon the tough love: there are many threads here that describe how to cheat the physics, with tricks or gizmos to heat pools for practically nothing. They don't. You won't.

 

[TaylorN]

That is crazy. Many people in Texas wouldn't be able to use their air conditioners at even your lowest rates. My lowest monhly usage is usually around 1,500 kwh in February. The Texas heat and my 10.5 tons of AC pushes the usage to around 5,000 kwh in July & August.

We don't have any time of use billing. On my last bill the all-in rate was $0.1233 per kwh. I have an Emporia Energy monitor on each of my pool circuits. The total cost to run all pool related-power in August was $46.69. I am not trying to rub it in - I do eny your weather! I understand why it makes sense to carefully control the level and time of usage of your pool equipment. If you don't have an energy monitor already, I reccomemd getting one on your whole house or at least your pool. The Emporia version works well and is very easy to install and use.

Wow, Emporia Energy is a really neat product. Thanks for sharing!

I am certainly jealous of your all-in rate of $0.1233 per kw.

 

[TaylorN]

I'm pretty sure you have this wrong. Running less flow in the morning is going to save on electricity, but the heat exchange will also drop. And it is not a direct ratio. You'll be getting less heat exchange not because there is less available heat, but because you are getting more heat loss (as others have explained above). Even though the panels may be heating the water in them some amount, without enough flow you won't be effectively increasing the pool's temperature. You won't be saving electricity, you'll just be wasting it.

Think of it this way. It's a common misconception, when DIYer's are trying to fool pool-heating physics, they'll loop a bunch of black drip tubing around and around and hook up a little pump to it, and run pool water through it. They'll feel really hot water coming out of the tube and declare themselves genius because they've cracked the solar heating industry wide open, for just a few dollars!! (And they'll swear their pools are getting really warm from doing so.) Hooey. Yes, the slower you run water through any sort of heat exchange tubing the hotter the water will get. But that hot water, at that reduced flow rate, won't heat your pool any noticeable amount. And even though these miracle machines are using very little electricity, pumping a little water with a little pump, that electricity is virtually 100% wasted. That setup is not doing anything, at all. You thinking that reducing the flow rate in the morning to sneak out a little "free heat" is just another variation of this DIYer fantasy.

Contrast that with a solar installer that doesn't have a proper flow meter to optimize flow through a "real" set of panels, he'll get the system going and stick his hand in the pool in front of a return. He'll increase the flow rate until the water coming out of the return is just barely warmer than the pool, because he understands the principal, the physics, that is actually at work. Moving a lot of water that is just slightly warmer than the pool will heat it faster than moving a little water that is much hotter than the pool. It's counterintuitive, but the physics are what they are.

If you're trying to save money because of your usage rates, you're going about it wrong. You think pushing water through your panels at the flow required for your SWG will get you a little "free heat." There is no such thing. All you'll be doing is pushing water up on your roof and wasting the electricity to do so, without significantly adding heat to your pool, even though you'll think it's working because the water out of your returns feels warm. And it will be. It just won't heat your pool worth a darn. You would be much better off starting the pump and the solar heating an hour or two later, but at the panels' optimal flow rate, and heating the pool more efficiently. The only way to save electricity without running the optimal flow rate through the panels is to not run your pump at all. Four hours of very inefficient heat exchange will not work out to be cheaper than one hour at optimal heat exchange, in terms of actually heating your pool.

Pardon the tough love: there are many threads here that describe how to cheat the physics, with tricks or gizmos to heat pools for practically nothing. They don't. You won't.

I very much appreciate your tough love!

It's a common misconception, when DIYer's are trying to fool pool-heating physics, they'll loop a bunch of black drip tubing around and around and hook up a little pump to it, and run pool water through it.

Though I am not looking for magical/instagram/DIYer/"I can feel hot water out of the return at slow speeds" scenarios. Your phrasing and tone got me totally cracking up. I usually have the same feeling about what I see/read. We are on the same page.

I am however looking for an optimal Delta T with all variables considered. I will need to find the paper I was reading in the past, but want to say the correct Delta T is somewhere between 5-8 degrees F (but I could be off). Supposedly should be barely perceptible to your hand when under water. Certainly not "oooh that feels really warm".

Contrast that with a solar installer that doesn't have a proper flow meter to optimize flow through a "real" set of panels, he'll get the system going and stick his hand in the pool in front of a return. He'll increase the flow rate until the water coming out of the return is just barely warmer than the pool,

Its also funny because you totally nailed it with the installers. Our solar installer did exactly what you described (hand in water and adjusting RPM) and the results were reasonable, but I have since installed a FlowViz Flow Meter so I can tune every run scenario of the system [Pool SWCG (low speed), SPA SWCG (low speed), Pool circulation (higher speed), spa circulation (higher speed), Solar, Pool Cleaner, Pool heat, Spa Heat], which of course includes the solar GPM.

On multiple occasions (early morning, midday, late afternoon, cloudy, windy, 75degrees, 90degrees, etc) I manually measured the input and output temperature to decide on an optimal flow rate. So I still disagree that there is a one size fits all GPM. But its reasonable to assume that the window of RPM isn't maybe as dramatic, that is, not reaching all the way down to SWCG low speed RPM. Lets say SWCG speed is 1700rpm (230watts), and Solar installer set solar heat at 2800rpm (1,000watts), then maybe the compromise on efficiency for off peak solar heating could be down to 2000-2200rpm.

...because you are getting more heat loss (as others have explained above).

also remember we have an auto-cover pool and it remains closed all the time until we go for a swim. So we aren't going to have nearly the same heat loss as an open air pool (ie putting 100 units of heat in but evaporating 100 units of heat, instead of dumping 10000 units of heat and only evaporating 100 units of heat). We have very little evaporative cooling loss during the daylight. So I think we can definitely benefit from a little bit of off-peak lower rpm heat added to the pool in the mid morning, cloudy day, etc, especially since the the pump is already running.

Ultimately I am trying to get the pool warmer earlier in the day, but more importantly not wasting electricity midday/afternoon when the outdoor temp is not optimal for pool heating at high RPM. So if the Solar thinks it should be on full blast.... maybe it should actually be ramped down because it won't be able to hit is optimal Delta T. On the cloudy day the goal wouldn't be to heat the pool to its ideal temperature. For those days if we are going to have a pool party or something, I'll just run the gas heater to maintain the temp.

you think pushing water through your panels at the flow required for your SWG will get you a little "free heat." There is no such thing. All you'll be doing is pushing water up on your roof and wasting the electricity to do so, without significantly adding heat to your pool,

again, I would have to work through the math on this and first programmatically collect hourly data (air temp, input water temp, output water temp, rpms, watts, etc) to prove it out. However, I think it still might be reasonable to attempt to prove it out considering the RPM/Watt difference between running at normal low speed mode for SWCG and then pushing water onto the roof for solar is not hugely different.

As an example, with NO solar, 1700rpm is needed to hit 25GPM for SWCG and uses 230watts.
With water running through solar (it goes up to the second story of our house), 1900rpm is needed to hit 25gpm and uses 300 watts.
That is only a difference of 70watts.

If the difference was 300watts, I would definitely concede, but running water up to the roof isn't nearly as inefficient as I once originally thought. You have to pump it up, but then it also pulls as the water flows back down from the top, so it offsets more than you would think.

And we aren't talking about 1-2 gallons per minute through some home depot black tube. With our Solar panels at a minimum we are running 25gpm (300watts) no matter what, and at maximum running close to 45gpm (1,000watts). So not even double the GPM. 25gpm is a respectable amount of water.

In the end... its on me to prove it out with real world numbers, I just have a hunch there is room for optimization and it sure would be nice to have programmatic control.

 

[matthewsunshineflorida]

Respectfully, I do understand the physics @Dirk, but you're entirely misunderstanding and thus misrepresenting my actual claim. I'm NOT talking about running the pump slower to get more heat. I'm simply not, so please reconsider what I am suggesting because we actually seem to somehow completely agree on the premise and yet not the conclusion:

He'll increase the flow rate until the water coming out of the return is just barely warmer than the pool, because he understands the principal, the physics, that is actually at work. Moving a lot of water that is just slightly warmer than the pool will heat it faster than moving a little water that is much hotter than the pool.

AND

the more water you pump through solar panels, the warmer your pool will get. But there is a point of diminishing return, and it's exponential. At some point, it'll cost too much in electricity to pump for very little increase in heating.

These are the EXACT principles I am leveraging. What you're telling me is that the water coming out of the return should only be slightly warmer than the pool to maximize heat transfer and minimize heat loss - but without using unnecessary energy beyond the point that it becomes a diminishing return, correct? Is that an accurate summary of where we agree? From there, all I'm saying is that very temperature differential that you're saying the solar installer correctly cares about changes based not ONLY on flow but on the available heat on the roof relative to the water and air. Do we agree on that too?

This means there is an equilibrium point at which the Temperature Differential (water temp difference at the returns) is at a point that comes close to maximizing heat transfer without going beyond the point of diminishing returns - JUST like in your example quoted above.

This very principle you have explained so well is why I operate mine with a variable speed at a fixed TD, to replicate a fixed heat transfer efficiency equilibrium: Heliocol recommends my panels to flow 50-70gpm for their peak BTU output, which is observed at the hottest part of the day. During that time, when the roof is 150 degrees, this flow creates a temperature differential of let's say 3-4 degrees. However, during the morning when the roof is 95 degrees, this same speed creates a temperature differential of say 0.25 degrees - past the point of diminishing returns and thus wasting electricity. If I run the pump at 30gpm, I can then get it back to around a 2 degree temperature differential so that I am lowering the cost of running the pump while only marginally reducing the efficiency of heat transfer - since I'm simply lowering flow to equilibrium right around the point of diminishing returns that you correctly referenced.

Varying flow relative to temperature differential means I'm always running my solar with approximately the same heat transfer efficiency, but at a variable COST, yet with only a marginal efficiency impact vs a fixed flow - all because those cloudy/morning times simply replicate the concept of maximizing heat transfer to the point of diminishing returns which you've already well summarized.

So respectfully, your analogy of slowing down water to create a large "perceived" feeling of heat is not at all what my variable speed concept is doing. It is simply replicating the same heat transfer efficiency observed at the times I use the manufacturers recommended flow rates for its rated maximum BTUs.

Please read through what I'm suggesting more carefully and if you still disagree, I'd like to hear exactly by which methods of physics you're taking issue with. I'm willing to admit I could be wrong - but everything you've said so far seems to only support the premises I'm using to come to that conclusion.

 

[Dirk]

Caveat: I know what I know from a few internet sites and a few articles hear and there. I'm not an HVAC engineer, nor do I play one on TV. But I am enjoying the discussion!

I think you're missing one glaring detail, and why optimizing the TD is not the path to ultimate efficiency. By efficiency I'm only talking about real world: how much does it cost us each day to bring the pool up X degrees, by running the pump Y RPMs faster than we would otherwise, for Z hours.

I didn't explain the drip tube scenario well enough, in terms of TD. It was an exaggeration. I understand you guys are not trying to get the water warmer than it should be at the returns. If I am understanding, you're trying to adjust the flow through the panels to always maintain a constant TD, the same TD when you're running full bore at noon. To do so, you'd have to reduce the RPMs in the morning, so that the water stays in the panels longer. That, in essence, is the same reasoning that the "tubers" use (which is why I brought it up). That, I think, is what you have wrong. By lowering the flow to maintain the TD (and/or to save energy costs), you are putting less warm water into the pool. You're not moving enough water to affect the pool temp. The TD of the water you're exchanging is way less important than the volume you need to exchange.

Look, I don't know how to do the math. I'm sure there is some to be applied. But my guess is, even at 0.25°, you're better off pushing a whole lot of it into the pool, as opposed to slowing it down and warming it up a bit more. That's the physics I keep referring to. It's why adding panels gets you a warmer pool. Not because the water in any one panel is warmer than when you had less panels, it's because you are moving MORE warm water into the pool.

And there's another problem, there's not really a great way to prove any of this. You can't just run your pool one way, and then the other, and compare the results. There are way too many variables. That's why the guys that think their pool is getting warmer from their drip tubing are hard to convince otherwise. A very large part of the heat gained during the day is from sources other than the solar heater: one is the direct energy from the sun. Ambient air temperature, too. And the biggies: the night time ambient temperature and the humidity affecting evaporation. Yet another factor is residual heat, all affected by the previous list, along with some number of the previous days the pool is subjected to all these variables. Huh? In other words, you won't be able to establish any sort of a controlled experiment. Every day these variables will be different, and every day will be affected differently by the residual heat from one or two previous days and all their variables!

I still submit that the way to reduce electrical costs will not be to reduce flow, but to reduce runtime, especially during peak $ hours. So, for example, instead of running solar at reduced flow for four hours in the morning, save three hours of that electricity and run full bore during the fourth hour. You'll get more heat into the pool from that fourth hour, and that one hour will cost you less than the four hours would have. (I don't actually know the best cut off point, that would depend on your energy costs.) I hope I'm explaining that well enough. I'm not saying that reducing flow to maintain a constant TD won't save energy costs, of course it will. And I'm not saying that less flow won't heat your pool some amount. It will. I'm just saying that it won't get your pool warmer than running at optimum flow rate for a length of time that costs the same in electricity as running longer would at less flow.

Regarding running water through the panels at "SWG speed": I'm gunna say the same thing. Yes, it may only cost you 70 watts to pump that water to the 2nd story roof, but that won't raise the pool temp as well as saving the cost of doing that for three hours, and then spending it on just one hour, at 210 watts to get to your optimum flow rate. (I'm fudging the math, obviously, to make the point.)

Do either of you have a way to test the results of your efforts? And compare those results to the "standard MO" of solar heating?

I didn't try to figure all this out. No hand in front of the return. I didn't mess with the filter gauge, as some do, to figure out flow. I read 5GPM in the Heliocol installation manual, multiplied that by my eight panels, to get my 40GPM optimal flow rate, and then just dialed in my VS pump with my FlowVis. I check the Vis a few times a season to make sure a dirty filter has not affected the flow rate. And that's it. I let the heater start when it wants to in the AM, and it shuts off when the pool is warm enough, or it's 4:00PM.

Ha! I left out one other minor detail. I installed PV solar panels configured to include the electricity costs of pumping pool water through my panels every day, so don't really give a hoot about the efficiency of it all. Heating my pool doesn't cost me anything. Point was (other than being snooty!): if you really want to control pool energy costs, go solar. It'll pay for itself in a few years and you can heat your pool up any ol' way you care to.

 

[Dirk]

Let's see if @mas985 is bored enough to slog through this thread. He knows way more about this than I do. And he's not shy, he'll tell me what I have wrong.

 

[mas985]

If I run the pump at 30gpm, I can then get it back to around a 2 degree temperature differential so that I am lowering the cost of running the pump while only marginally reducing the efficiency of heat transfer - since I'm simply lowering flow to equilibrium right around the point of diminishing returns that you correctly referenced.

If the peak temperature differential is 4F during the heat of the day and then tapers off to 0.25F at some point later in the day, then that tells me that the BTU/hr is 1/16th the peak value and in that case, it probably doesn't make sense to waste any energy trying to extract a very small amount of heat. Why not just turn the pump off instead of lowering the RPM since energy efficiency seems to be a priority?

Also, I don't think there has been a discussion on panel height and vacuum release height. These are critical parameters that will limit the minimum RPM. So what is the current filter pressure, panel height and VRV height (relative to the filter pressure gauge)?

 

[Dirk]

If the peak temperature differential is 4F during the heat of the day and then tapers off to 0.25F at some point later in the day, then that tells me that the BTU/hr is 1/16th the peak value and in that case, it probably doesn't make sense to waste any energy trying to extract a very small amount of heat. Why not just turn the pump off instead of lowering the RPM since energy efficiency seems to be a priority?

Uhg. For whatever reason, I needed 1,493 more words than Mark did to say the same thing. (Yes, I actually counted them.)

 

[TaylorN]

If the peak temperature differential is 4F during the heat of the day and then tapers off to 0.25F at some point later in the day, then that tells me that the BTU/hr is 1/16th the peak value and in that case, it probably doesn't make sense to waste any energy trying to extract a very small amount of heat. Why not just turn the pump off instead of lowering the RPM since energy efficiency seems to be a priority?

Also, I don't think there has been a discussion on panel height and vacuum release height. These are critical parameters that will limit the minimum RPM. So what is the current filter pressure, panel height and VRV height (relative to the filter pressure gauge)?

Thanks for chiming in @mas985. and thanks for bringing him into the conversation @Dirk!

I totally get if the heat is 1/16th of peak, then its probably pointless to waste the 70watts to run water through the solar panels. But what happens when the temperature differential is only 1/2 of peak? Its still worth running the water through the panels, but is it worth running at 1,000watts when we could be running at 400-500watts and achieve the same 4F temperature differential?

I understand we could simply run at 1,000watts and the temp differential might be at 2F when its 1/2peak heat, but I still don't see how that is more efficient. Its a compromise. If I was planning on jumping in the pool that afternoon then I would for sure let it rip at 1,000watts to be the most thermally efficient, but for most days when the pool goes unused, we are just focused on maintaining temperature for the days that follow, and save money on electricity on the unused days. So I am not necessarily looking to run at low SWCG rpms (25gpm), but somewhere in between that and 45gpm without having to think about it and maintain it manually. I literally look out the window some days when the weather isn't perfect and the pump is running high speed through the solar panels. I cringe and end up manually turning off solar heating. Maybe I saved a $0.50-$2 that day, but how many days per year could I save money. Might save $50-$100 a year if the controller was smart enough to de-tune the rpms a little bit. They have variable speed air-conditioning and heating. I imagine that operates with a similar concept.

 

[matthewsunshineflorida]

The thing is, insisting that volume is way more important than ΔT demonstrates you're simply not understanding the application of heat transfer formulas and what they mean for solar panels. Let me take a step back and try to demonstrate this concept in two different ways that you will hopefully either recognize as water tight (ha) or otherwise will need to demonstrate mathematically (or experientially) specifically where I'm going wrong.

TLDR: The flow itself is not what dictates heat transfer efficiency, ΔT is, because heat lost via radiation is Q/t = x * y * A * T^4. Further, ΔT and GPM are inversely proportional since BTU/hr = GPM x 500 x ΔT. This means targeting a ΔT by modulating flow as a reaction to BTU (roof temp) necessarily also maintains the same heat transfer efficiency at any combination of flow and BTUs, being in a directly proportional relationship. Thus, a 2 degree differential at 60gpm and a 2 degree differential at 30gpm have the same heat transfer efficiency. Therefore, if a manufacturer determines that 60gpm at peak BTUs with an observed 2 degree temperature differential is the equilibrium point at which higher flow creates diminishing returns due to less significant radiative heat loss, that same equilibrium point exists at 30gpm with the same 2 degree differential.

1. Math/Theory:
   
   1. "For water... the heat transfer formula simplifies to BTU/hr = GPM x 500 x ΔT." This means that, for a fixed BTU/hr heater, flow and TD are inversely proportional. You can't just say you feel like volume is more important than ΔT when they are mathematically necessarily inversely proportional. You keep mentioning physics but you aren't using them to come to your conclusions. With no outside influences, 10gpm will have the same heat transfer as 100gpm - that's a fact.
      1. However, we all agree there ARE outside variables, which is WHY manufacturers recommend flow rates. So let's look at them and consider their effects:
         1. Heat lost to air or ground through radiation
            1. Heat transfer via radiation is Q/t = x * y * A * T^4, where x is the stefan-boltzmann constant, y is the emissivity of the PVC, A is the surface area, and T is the temperature of the PVC. Notice anything especially important?
               • If the temperature difference between the PVC and air goes up 50% from 5 degrees to 7.5 degrees, the rate of radiation increases 500% 5^4 (625) to 7.5^4 (3,164)
                 • So at greater ΔTs, the radiation starts to have a profound impact as a percentage of heat carried by the pipes, but this effect is exponential so it is less and less important at lower absolute ΔTs.
         2. Using that same concept, the emissivity of the black PVC eventually cannot absorb any more energy faster than it loses it as it reaches its maximum, expressed as an asymptote.
            • In other words, if you left water in panels on the roof for 2 hours in direct sunlight, you would see the water inside climb temperature quickly into the 100s before eventually slowing down, representing the fact that it's absorbing less and less BTUs of energy as it reaches the maximum amount of heat that can be transferred into the water faster than it simply radiates into the air and roof. The water wouldn't get thousands of degrees and destroy all life on earth - it would simply slow down its heat absorption as it approaches maximum temp. This means even greater heat lost to radiation exponentially, which is what you'd expect from the formula.
            • That is perfectly consistent with your point about using wraps of black tubing to get ineffective low flow amounts of hot water.
   2. So why does solar panel efficiency begin to plateau at certain flow rates, representing a diminishing return?
      1. Because at low temperature differentials, the heat lost through radiation represents a smaller and smaller percentage of the overall heat inside the panels and pipes, since the increased flow reducing absolute ΔT becomes exponentially less important relative to the overall heat transferred.
   3. CRITICALLY, this means the only way heat is lost (radiation) during the transfer is through ΔT!
      
      1. Therefore, the actual variable that affects heat transfer efficiency is ΔT, which is itself affected through a combination of flow and BTUs delivered. The flow itself is not what dictates heat transfer efficiency, ΔT is, because heat lost via radiation is Q/t = x * y * A * T^4. Further, ΔT and GPM are inversely proportional since BTU/hr = GPM x 500 x ΔT. This means targeting a ΔT by modulating flow as a reaction to BTU (roof temp) necessarily also maintains the same heat transfer efficiency at any combination of flow and BTUs, being in a directly proportional relationship. Thus, a 2 degree differential at 60gpm and a 2 degree differential at 30gpm have the same heat transfer efficiency. Therefore, if a manufacturer determines that 60gpm at peak BTUs with an observed 2 degree temperature differential is the equilibrium point at which higher flow creates diminishing returns due to less significant radiative heat loss, that same equilibrium point exists at 30gpm with the same 2 degree differential.
         • It's worth noting that ΔT in the radiation formula is the average differential between the PVC and the air, whereas ΔT in heat transfer is the pool water and heated water. However, I'm sure we can agree that's a reasonable shortcut to their respective relationships since both ΔTs will rise and fall together.
      2. If you're still convinced volume or flow alone matters independent of ΔT, I'd like to know where the formula for that is or at least how you justify that conceptually
2. Experience & Electric Usage:
   
   1. While I fully acknowledge there are too many variables day to day to be precise, the results of my testing are so clearly conclusive that it would be all but impossible to explain away with outside variables:
      1. April/May
         1. I think this is a good reference point because in April/May, the pool often didn't get to temp (90... I like it warm) - which means I could track the temperature gain by day and average it
            • With fixed solar, I had my controller run the pump for 60gpm whenever there was heat on the roof
            • With variable speed, my controller logs showed it averaged, for example, somewhere around the following: 30gpm for 4 hours, 40gpm for 1.5hr, 50gpm for 1.5hr, 60gpm for 1hr
            • I went back and forth daily for weeks and there was no measurable difference in the average temps reached between these two methods
            • However, via flowvis and my klein multimeter
              • To run the pump at 60gpm for 8 hours was around 750w x 8hrs x 30 days = 180kw, so at 15 cents, that's $27/mo
              • To run the pump variably for 8 hours, it's 180w x 4 + 280w x 1.5 + 550w x 1.5 + 750w x 1 = 81kw, so at 15 cents, that's $12/mo
              • Less than half the cost with no measurable difference in temperatures means IF there is a difference, we would have to agree it's quite small, right?
                1. But you might say "perhaps what that means is we shouldn't turn on solar until there's a lot more heat on the roof in the first place - maybe the morning isn't really doing anything"
                   1. This would be strangely wasteful since solar controller manufacturers seem to all agree it's worthwhile to run solar when there's only a 5 or so degree ΔT between the water and roof. However, I checked for this too, and regularly found that in the morning temps, solar did regularly increase the pool temp a couple of degrees before noon.
                      1. 180w x 4hrs is 720w to go up 2 degrees, which actually costs slightly LESS per degree than running the 60gpm 750w since that typically goes up 1-2 degrees per hour.
                         • In other words, I appear to have it dialed in such that I get an almost identical cost per degree throughout the day instead of wasting a bunch of electricity in the morning - pretty cool!
      2. Summer
         1. This is particularly interesting because obviously it doesn't take a lot of heat to get the pool to temp, so solar doesn't run nearly as long. This means I often don't even get to temps that call for above 50gpm before I hit each day's set temp.
            • For instance, on August 11th, solar turned on at 9:24am, the water went up 1 degree before increasing speed to 50gpm around 11am, and turned off at 12:30 after hitting its set temp. An hour and a half of under 300w, an hour and a half of 550w, for a total of 1,275w. If I had run it at 60gpm, I would be at 2,250w - almost TWICE the power for the same result.

So imagine if you're @TaylorN, paying an average of say 35 cents - his cost would go from $63 to $28/mo for my typical May day. We're not talking massive savings here, but less than HALF is not nothing - all for no observable loss in heating efficiency either in the theory or in practice! I don't understand the pushback here.

 

[JamesW]

Assuming 100,000 btu/hr of available solar energy hitting the panels.

100,000/(GPM X 60 X 8.34) = temperature rise from inlet to outlet.

TR = 200/GPM

GPM = 200/TR

If the panels specify a total flow of 40 to 80 GPM, the Temperature Rise from inlet to outlet is:

Assuming an inlet temperature of 82 degrees:

GPM......TR.......Outlet......Average

40..........5............87...........84.5

50..........4............86...........84.0

60..........3.3........85.3.........83.6

70..........2.9........84.9.........83.5

80..........2.5........84.5........83.3

Y = Temperature Rise.

X = GPM.

Y = 200/X

As the sun hits the solar panels, the temperature of the outside of the panels increases and the heat transfers from the outside to the inside based on the temperature differential.

As you can see, the temperature rise from inlet to outlet is not significantly different enough to change the efficiency of the heat transfer.

The difference is only 2.5 degrees from 40 GPM to 80 GPM.

If you look at the average temperature in the panels, the difference is even smaller at about 1.2 degrees from 40 GPM to 80 GPM.

As the water flows, the temperature differential will reach an equilibrium between the inside and the outside and you will get a steady transfer of heat.

plot Y = 200/X, y from 2.5 to 30 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

 

[matthewsunshineflorida]

As you can see, the temperature rise from inlet to outlet is not significantly different enough to change the efficiency of the heat transfer.

As the water flows, the temperature differential will reach an equilibrium between the inside and the outside and you will get a steady transfer of heat.

Exactly my point!! I'm not saying I'm getting more heat transfer, I'm saying I'm spending less to get roughly the same heat transfer. The math supports this all around. The effect of ΔT on heat lost through radiation (the only way heat is lost from the panels to the pool) is to the FOURTH power so I agree with Dirk that it's increasingly significant at too low flow rates with too high BTUs - but that's only to the extent it causes ΔT to climb higher than it is when measured at the equilibrium point manufacturers recommend at their rated BTUs. ΔT has less and less effect on the overall heat transfer the lower the absolute ΔT is, but more significant (^4) as it climbs. That's why ΔT is what matters, not flow on its own - demonstrated with the formulas already referenced.

 

[Dirk]

I totally get if the heat is 1/16th of peak, then its probably pointless to waste the 70watts to run water through the solar panels. But what happens when the temperature differential is only 1/2 of peak? Its still worth running the water through the panels, but is it worth running at 1,000watts when we could be running at 400-500watts and achieve the same 4F temperature differential?

I suppose I'm not explaining it well enough, or burying it too well in a pages-long post. Though I'm not sure how else I can explain it. And again, I don't know how to do the math (but neither do you). Monkeying with the TD will not matter, and will not significantly change the temperature of your pool, without sufficient flow. How much water you push through the panels is waaaaay more important than the temperature differential.

Let's look at @JamesW's charts. They support your conclusion that running the pump at a lower flow rate does not significantly impact the temperature differential. That's fine. But as I was trying to point out with the drip tube story, temperature differential (TD) doesn't heat your pool! Otherwise, the drip-tubers would be on to something. They can get a huge TD, for next to nothing, by running one tube with a little pump. That example is extreme, but it is what you're attempting to do. Raising the TD by lowering the flow to save electricity will get you a higher TD, but THAT WON'T HEAT YOUR POOL EFFICIENTLY! (Maybe all caps will help?)

What James left off, chart-wise, and what you continue to ignore, is what is actually heating your pool. THE FLOW RATE! You want to move as much water as you can afford to (and as the panels can handle). A TD of 0.25% will heat your pool, if you can move enough pool through that TD!

Just as James' charts' curves are exponential, I expect the physics I'm harping on is as well. As you decrease flow, the actual rise in pool temperature for a given amount of sun on the panels will decrease exponentially. It won't matter if decreasing the flow saves electricity because the amount that electricity use increases the pool temp will be insignificant. You won't be saving electricity, you'll be wasting it.

So, again, if you want to save some electricity costs on days you don't plan to swim, start the pump later, but run it at optimum flow (for your solar panels) when you do. The math for that should be pretty simple, though maybe @JamesW can help. I'll try:

Say you want to spend 70W for four hours to put a little heat in your pool. That's 70 x 4, or 280Wh (watt hours). If I understood you, that 70 is the difference between running water thru your panels while the swg is on, vs swg alone, right? And it costs you 1000W for solar at optimal flow, right? (Does that include SWG at the same time? I'll assume it does.) And 230 for swg alone? So that would be a difference of 770W. With me? So it costs you 770W to run your solar optimally (since you'd be running swg anyway). So: instead of running your solar for four hours at SWG speed (which is doing nothing but wasting 70W per hour), you could run your solar optimally for 21 minutes (60 x 280 ÷ 770 = 21), for the same cost in electricity. (Might have to check my math on that.)

21 minutes at optimal flow will raise your pool temperature more than four hours of "swg-flow," but will cost the same! Actually, it'll do better than that, because if you run that 21 minutes at the end of that four hour time span, the heat on the panels will be at its hottest, so the TD will be all that much better. The same is not true at swg-flow-rate, because even though the TD will increase over that four hour span, you still won't have enough flow to make much difference. You'll be wasting that TD just as you'd be wasting the electricity.

As I mentioned, you'd have to have 4 straight weeks of the exact day time and night time temperatures, and run "your way" for 2 weeks and "my way" for 2 weeks to see who's right. Get started on that, won't you?

 

[mas985]

Thanks for chiming in @mas985. and thanks for bringing him into the conversation @Dirk!

I totally get if the heat is 1/16th of peak, then its probably pointless to waste the 70watts to run water through the solar panels. But what happens when the temperature differential is only 1/2 of peak? Its still worth running the water through the panels, but is it worth running at 1,000watts when we could be running at 400-500watts and achieve the same 4F temperature differential?

I understand we could simply run at 1,000watts and the temp differential might be at 2F when its 1/2peak heat, but I still don't see how that is more efficient. Its a compromise. If I was planning on jumping in the pool that afternoon then I would for sure let it rip at 1,000watts to be the most thermally efficient, but for most days when the pool goes unused, we are just focused on maintaining temperature for the days that follow, and save money on electricity on the unused days. So I am not necessarily looking to run at low SWCG rpms (25gpm), but somewhere in between that and 45gpm without having to think about it and maintain it manually. I literally look out the window some days when the weather isn't perfect and the pump is running high speed through the solar panels. I cringe and end up manually turning off solar heating. Maybe I saved a $0.50-$2 that day, but how many days per year could I save money. Might save $50-$100 a year if the controller was smart enough to de-tune the rpms a little bit. They have variable speed air-conditioning and heating. I imagine that operates with a similar concept.

One thing that I forgot to mention above is that I have monitored the temperature differential in my panels over the course of the day and it really does not change all that much until the very end of the day and then it rapidly diminishes. The skirts of the TD vs TOD profile are quite steep so there may not be much benefit to running at lower RPM and it might not be for very long.

Also, you never answered my other questions. This is very important because it can make this entire discussion moot as you may not actually be able to run at a significantly lower RPM.

Have you tried to lower the RPM?

Do you start to get air in the return lines at some point or flow stop?

What is the current filter pressure, panel height and VRV height (relative to the filter pressure gauge)?

This is why manufactures may not have this particular feature as it could result in the dead heading the pump if the user makes a mistake in the setup.

Exactly my point!! I'm not saying I'm getting more heat transfer, I'm saying I'm spending less to get roughly the same heat transfer. The math supports this all around. The effect of ΔT on heat lost through radiation (the only way heat is lost from the panels to the pool) is to the FOURTH power so I agree with Dirk that it's increasingly significant at too low flow rates with too high BTUs - but that's only to the extent it causes ΔT to climb higher than it is when measured at the equilibrium point manufacturers recommend at their rated BTUs. ΔT has less and less effect on the overall heat transfer the lower the absolute ΔT is, but more significant (^4) as it climbs. That's why ΔT is what matters, not flow on its own - demonstrated with the formulas already referenced.

Actually, heat can be lost (or gained) through radiation, convection and conduction (depending on panel mounts) by differing amounts. However, radiation is usually the dominant factor depending on sky emissivity.

Also, note that the heat loss is not proportional to inlet vs outlet DT but between panel temperature, which is close to the water temperature, and sky temperature for radiation. This is why halving the DT does not change efficiency all that much. So one solution to your problem could be just to operate at the lowest RPM possible that your system will allow for depending on panel height and VRV location, and accept the loss in heat transfer efficiency. This is what I have done. More than likely, the extra heat gain from from higher flow rates isn't going to amount to much anyway.

I have some solar heat transfer spreadsheets in my signature that will allow you run different scenarios with varying flow rates through the panels.

For example, most manufactures recommend flow rates at 0.1 GPM/sq-ft of panel area so I set up a system with 50% panel/pool area coverage with panel flow rates from 0.2 GPM/sq-ft to 0.5 GPM/sq-ft. and no solar cover. Here are the results for 5/1/2023 with cloudless sky:

Pool Heat Transfer Tools v021

ReadMe Version:,v021 Disclaimer,These spreadsheets are provided as is without any guarantees or warranty. Spreadsheet License,By using this spreadsheet you agree to the following conditions: This spreadsheet is for personal use only and shall not be used for any commercial business without the ...

docs.google.com

Parameter	No Solar	0.2 GPM/sqft	0.1 GPM/sqft	0.05 GPM/sqft
Morning Pool Temperature (F)	81.55	86.19	86.08	85.85
Evening Pool Temperature (F)	84.47	90.15	90.01	89.73
Panel Efficiency	NA	65%	63%	60%

You probably won't notice the difference with reduced flow rates.

Note that this is an iterative solution and assumes weather conditions are identical for multiple days which then reaches an temperature equilibrium. This is why morning temperatures are different for the different scenarios but operating conditions are still identical, except for panel flow rate.

 

[Dirk]

No solar: 2.92°
0.2 GPM/sqft: 3.96°
0.1 GPM/sqft: 3.93°
0.05 GPM/sqft: 3.88°

0.2 GPM/sqft: 3.96° - 2.92° = 1.04°
0.1 GPM/sqft: 3.93°- 2.92° = 1.01°
0.05 GPM/sqft: 3.88°- 2.92° = 0.96°

Great data Mark, thank you. But as you point out, without control conditions the data is what it is. Further, I did a little math and, if I did it right, in early May in CA your solar panels were only adding about 1° over what the sun would have done anyway. Not sure that determines the efficiency, one way or the other, of what the OP is proposing for his setup. Also, not sure how you got four different conditions all on the same day (no solar plus three different flow rates and four different pool "before and after" temps). What am I reading wrong?

Anyway, I'm not sure this is conclusive of anything.

Can you correct me if my underlying understanding of the physics is wrong? I keep claiming that the amount of water exchanged (based on flow rate) is the factor that is more important to consider than temperature differential at the panels.

My theory is that the OP will raise his pool temp more running 21 minutes, around 11:30am, at manufacturer-recommend flow rate (at 2800RPM) vs four hours, 8am-12 noon, at significantly reduced flow rate (1700 RPM). NOTE: The OP didn't state what flow rates those two RPMs produce. I guessed at his scheduling. If I did my math right, those two schedules/RPM settings will cost the same in electricity. One will heat the pool better than the other. I say it's the 21 minutes.

 

[JamesW]

When the sun is shining on the panels with no water flow, the outside of the panels gets very hot.

As water begins to flow, the temperature difference causes heat to flow from the outside to the inside and the outside surface cools down.

At some point, the outside surface temperature stops going down and reaches a stable temperature that is above the temperature of the water.

The heat transfer depends on the temperature difference between the outside surface and the inside surface as well as the conductivity of the solar panel wall.

The outside temperature depends on how much heat is received and lost.

The heat from the outside panel surface is lost due to conductivity as the heat is transferred through the panel wall to the water.

So, the initial heat transfer is large due to the large temperature difference between the outside and the inside and the heat transfer slows down as the outside surface temperature drops.

Eventually the heat transfer levels off as the outside surface temperature stops dropping.

If you increase the flow and drop the internal temp down some, the temp difference will increase and you will get a tiny bit more heat transfer, but this cools the outside some and you end up with a slightly larger temp difference and slightly more heat transfer.

The absolute maximum efficiency that you could get is at infinite flow where the water entering and exiting are at the same temperature.

For practical purposes, the best option is to maintain the flow in the range that the manufacturer recommends with a preferred flow near the lower end with a few GPM for margin.

 

[mas985]

No solar: 2.92°
0.2 GPM/sqft: 3.96°
0.1 GPM/sqft: 3.93°
0.05 GPM/sqft: 3.88°

These are below freezing? Not sure what you did.

This is for Tampa, FL 5/1/2023:

Pool Heat Transfer Tools v021

ReadMe Version:,v021 Disclaimer,These spreadsheets are provided as is without any guarantees or warranty. Spreadsheet License,By using this spreadsheet you agree to the following conditions: This spreadsheet is for personal use only and shall not be used for any commercial business without the ...

docs.google.com

This is what it would be for Pleasanton, CA 5/1/2023

Pool Heat Transfer Tools v021

ReadMe Version:,v021 Disclaimer,These spreadsheets are provided as is without any guarantees or warranty. Spreadsheet License,By using this spreadsheet you agree to the following conditions: This spreadsheet is for personal use only and shall not be used for any commercial business without the ...

docs.google.com

Water Temperature (F) - Morning	69.37	74.18	74.04	73.74
Water Temperature (F) - Evening	72.32	77.96	77.79	77.45

Panel Efficiency

0%

48%

47%

44%

Don't forget, when you change the date, the weather conditions should also be updated as they too will generally change with the date.

Great data Mark, thank you. But as you point out, without control conditions the data is what it is. Further, I did a little math and, if I did it right, in early May in CA your solar panels were only adding about 1° over what the sun would have done anyway. Not sure that determines the efficiency, one way or the other, of what the OP is proposing for his setup. Also, not sure how you got four different conditions all on the same day (no solar plus three different flow rates and four different pool "before and after" temps). What am I reading wrong?

As I explained above, the morning and evening temperatures are effectively long term averages (multiple days). So both the morning and evening temperatures are going to be different as each flow rate will have different equilibrium temperatures for morning and evening.

Can you correct me if my underlying understanding of the physics is wrong? I keep claiming that the amount of water exchanged (based on flow rate) is the factor that is more important to consider than temperature differential at the panels.

They are actual two sides of the same coin. Lower flow rates through the panels results in higher temperature difference and visa versa. So saying one is more important than the other is not really relevant since they are connected.

My theory is that the OP will raise his pool temp more running 21 minutes, around 11:30am, at manufacturer-recommend flow rate (at 2800RPM) vs four hours, 8am-12 noon, at significantly reduced flow rate (1700 RPM). NOTE: The OP didn't state what flow rates those two RPMs produce. I guessed at his scheduling. If I did my math right, those two schedules/RPM settings will cost the same in electricity. One will heat the pool better than the other. I say it's the 21 minutes.

I added two more cases to this one:

Pool Heat Transfer Tools v021

ReadMe Version:,v021 Disclaimer,These spreadsheets are provided as is without any guarantees or warranty. Spreadsheet License,By using this spreadsheet you agree to the following conditions: This spreadsheet is for personal use only and shall not be used for any commercial business without the ...

docs.google.com

You can see running for 4 hours at 0.05 GPM/sqft adds more heat than running 2 hours at 0.1 GPM/sqft even when stopping at noon. In both cases, the same amount of water is pumped but the longer duration makes a bigger difference.

 

[matthewsunshineflorida]

They are actual two sides of the same coin. Lower flow rates through the panels results in higher temperature difference and visa versa. So saying one is more important than the other is not really relevant since they are connected.

This is literally all I am saying @Dirk - they are necessarily inversely related IN the heat transfer formula itself. In order to argue that anyone is wasting electricity and leaving a bunch of heat on the table due to slow flow in the morning vs medium (much less your inaccurate 21 minutes claim), you have to not only imply direct experience is less-than-honest, but also somehow break both the heat transfer and radiant heat loss formulas. You should really read my post because I don't think there's a way to argue what you keep arguing in the face of the physics of this that you keep appealing to but not recognizing - it's really strange.

Additionally, nothing @JamesW and @mas985 have brought to light is a problem for that claim because I didn't even address the rate of efficiency change between flow rates with a fixed BTU - I just demonstrated that the efficiency doesn't change given a fixed TD with flow adjusted proportionally as BTUs go down. So of course it IS possible that 30gpm to 60gpm at the same BTU doesn't even make that big of a difference in delivered heat due to many of the factors @JamesW and @mas985 have laid out - but that also only further emphasizes that it can't make any noticeable difference in delivered heat to go from 60gpm to 30gpm with the same TD and thus approximately the same heat transfer efficiency.

The idea that there could even exist a magic flow number that makes panels work best independent of the actual quantity of heat (BTU/hr) that is needing to be transferred flies in the face of the principles here - and I think that's all but proven...

 

[matthewsunshineflorida]

Do you start to get air in the return lines at some point or flow stop?

Since you've mentioned this a couple times and it's a good point, wanted to add my experience here - I can run my solar all the way down to 900 RPM and still get flow at low pressures with my heater bypass on, but my VRV opens up at about 25gpm for the spa and 35gpm for the pool, so I just have those lower limits set.

Actually, heat can be lost (or gained) through radiation, convection and conduction (depending on panel mounts) by differing amounts. However, radiation is usually the dominant factor depending on sky emissivity.

Convection and conduction are great points too, even if to a lesser degree, but I suspect the same temperature differential patterns would exist in those formulas.

Also, note that the heat loss is not proportional to inlet vs outlet DT but between panel temperature, which is close to the water temperature, and sky temperature for radiation.

Yes I state this very thing in my post (I know it was long):

• It's worth noting that ΔT in the radiation formula is the average differential between the PVC and the air, whereas ΔT in heat transfer is the pool water and heated water. However, I'm sure we can agree that's a reasonable shortcut to their respective relationships since both ΔTs will rise and fall together.

 

[matthewsunshineflorida]

All I've been saying from the beginning is that it cannot (and in my direct experience, does not) make any appreciable difference in delivered heat to run flow lower when the BTUs available are lower (assuming they can properly run that speed) - because the very same methods of heat transfer and loss are present proportionally to one another, causing efficiency to be around the same. Why shouldn't @TaylorN save $30-$50/mo for no appreciable difference in performance if he can? I'm not saying it's worth buying a bunch of hardware, but it's an interesting thought exercise and if you already have what you need - why NOT do it?

Perhaps instead even running it low speed ALL the time is really only slightly less efficient and thus the even better financial move... but at the very LEAST - it certainly ISN'T less efficient to modulate the flow relative to DT.