Attaching temp sensor in PVC pipe.

[Velvet0164]

Funny stumbling on this tonight as I'm working on the same thing.

While our pool is all disconnected I thought it would be a good time to make some changes. We're heavy into home automation and I wanted to get some temperature monitoring inside on the wall mounted tablet. For now it's just for the nice factor, but once it's in place we can do more with it down the road. I have a NodeMCU with a waterproofed DS18B20 sensor. I'm running "Hubduino" (Hubitat), which is based on ST_Anything (SmartThings). (I'm in the process of moving from SmartThings to Hubitat - about 75% done). Right now I just have this running on a breadboard and it works great. I am about to start soldering it all into a PCB and installing it in a waterproof outdoor box. The only thing I have left to figure out is how to get the sensor into the pipe without worrying about springing a leak.

You guys have hit on all of the ideas I have come up with along with the same concerns:
A PVC threaded tee and use a cable gland on that thread, but will a regular cable gland withstand the water pressure?
A PVC threaded tee and a thermowell and just shove the DS18B20 down in there, but how will that interfere with temp readings?

I've not been able to find this particular sensor already in a threaded enclosure. My current installation is using a driver specifically for the DS18B20 so I'd like to stick with it.

 

[setsailsoon]

Christopher,

Try clicking here for the DS 18820 with a 1/2" threaded enclosure. It's available in multiple lengths so it can be placed in the flowing fluid in multiple different pipe sizes.

Chris

 

[Velvet0164]

Well, I guess I need to relearn how to search. I've been researching this for a few days and have yet to find that. Thanks for the link!

 

[Dirk]

Just the thread I was looking for. I'm installing these type of sensors (well sonoff sensors with a TH16) and was thinking of having one on the discharge pipe and one coming out of the solar collectors to monitor the difference in temp. Once the temperature is no longer rising (cloud/rain etc) then it will shut off.

I admit I’m not following all the chat about the custom circuits, but solar pool heater controllers don’t sample water temp at two locations to turn on and off the heater. There are two temp sensors, though. One measures pool water temp. Mine is located between pump and filter (not up by the panels). The other sensor is on the roof, to measure ambient air temp (not water temp). The purpose of that one is to report the heat hitting the solar panel, not to measure the temp of the water inside. The controller circuit regulates the heater based on two parameters: if the pool water temp is below the user’s desired water temp, and if there is enough heat striking the panels to heat the water higher than its current temp.

I’m not sure how one would use the water temp exiting the panels to control heating efficiently. I think it would be too slow, and actually be counter productive. My system reacts to a cloud passing overhead. Contrary to common misconceptions, the water exiting the panels is not supposed to be hot. If it is, the flow is too low to heat the pool efficiently. The temp of the water exiting a solar panel array should only be a degree or two warmer than the pool. It’s counter intuitive, but that’s the way they’re supposed to work (there is some physics involved that are a bit beyond me, but it’s something to do with: a large amount of warm water will increase your pool temp better than a smaller amount of hotter water).

 

[Katodude]

So here is a dumb question, do any of the VS pumps, IntelliFlo etc. Have a temperature sensor in them? How does the freeze protection work do they all use an external temp sensor? It would be cool if you could read the temperature from something that is already in your system.

 

[setsailsoon]

I admit I’m not following all the chat about the custom circuits, but solar pool heater controllers don’t sample water temp at two locations to turn on and off the heater. There are two temp sensors, though. One measures pool water temp. Mine is located between pump and filter (not up by the panels). The other sensor is on the roof, to measure ambient air temp (not water temp). The purpose of that one is to report the heat hitting the solar panel, not to measure the temp of the water inside. The controller circuit regulates the heater based on two parameters: if the pool water temp is below the user’s desired water temp, and if there is enough heat striking the panels to heat the water higher than its current temp.

I’m not sure how one would use the water temp exiting the panels to control heating efficiently. I think it would be too slow, and actually be counter productive. My system reacts to a cloud passing overhead. Contrary to common misconceptions, the water exiting the panels is not supposed to be hot. If it is, the flow is too low to heat the pool efficiently. The temp of the water exiting a solar panel array should only be a degree or two warmer than the pool. It’s counter intuitive, but that’s the way they’re supposed to work (there is some physics involved that are a bit beyond me, but it’s something to do with: a large amount of warm water will increase your pool temp better than a smaller amount of hotter water).

Dirk,

Mine works the same way. Temp sensor is located near the outlet but is on the surface. With Intellicenter it adjusts when to open the flow to the panels and then close to bypass based on the difference of the surface of the panel outlet and the water temp. I can adjust the "kick in" and "kick out" temp difference from 2 to 9 deg. Every system has specific amount of heat response based on the volume, velocity etc. so having the adjustment really is nice. Mine seems to work best with 6 deg temp difference to kick in and 3 deg to kick out. Pretty cool to watch the valve open and close when the clouds obstruct the sun then start the gas heat when I have the heater choice in the Intellicenter set to "solar preferred". When either solar or gas valve is active it also kicks the pump to high speed.

Chris

 

[jseyfert3]

I’m not sure how one would use the water temp exiting the panels to control heating efficiently. I think it would be too slow, and actually be counter productive. My system reacts to a cloud passing overhead. Contrary to common misconceptions, the water exiting the panels is not supposed to be hot. If it is, the flow is too low to heat the pool efficiently. The temp of the water exiting a solar panel array should only be a degree or two warmer than the pool. It’s counter intuitive, but that’s the way they’re supposed to work (there is some physics involved that are a bit beyond me, but it’s something to do with: a large amount of warm water will increase your pool temp better than a smaller amount of hotter water).

It should be just as possible to use the exit water temp as well, if you have accurate temp measurements.

As to the low temp difference, that's relatively simple to explain. Solar water panels gain heat via radiation from the sun, right? Well they loose heat via radiation to the sky and convection to the surrounding air. The hotter something is relative to something else, the faster it looses heat. For convection this is proportional to the temp difference between the object and the air. However, for radiative heat loss, it's not proportional to the temp difference (in this case the temp difference between the panel and the sky, the latter of which varies but is probably between 0 and 20 °C for most pool cases). Instead, the radiative heat loss has temp to the 4th power in the equation, meaning radiative heat loss is exponentially increases with higher temps, unlike convective heat loss which proportionally increases with higher temps.

Put simply, low flow means hot panels. Hot panels mean heat lost to the air and sky via convection and radiation rather than heating the pool water.

Additional "nerd notes": Domestic hot water panels, which have to heat water to some decent temp like 160 °F, will almost universally be glazed (covered with glass) to minimize convective heat loss as much as possible (and in very cold climates like where it freezes may even be vacuum insulated to eliminate convective heat losses entirely), and will have special coatings on the collector tubes that maximize radiative heat gain from the sun while minimizing radiative heat loss to the sky, but these methods are expensive and so pool panels have neither of these since they need to be cheap and don't need to output hot water to do their desired function of heating pool water.

 

[setsailsoon]

So here is a dumb question, do any of the VS pumps, IntelliFlo etc. Have a temperature sensor in them? How does the freeze protection work do they all use an external temp sensor? It would be cool if you could read the temperature from something that is already in your system.

Kato,

My V-Green 1.65 has an internal freeze protection that kicks the pump on at 39 deg and runs for an hour. Then it checks again. When temperature increases above 39 deg. the pump reverts to the schedule on the internal timer settings unless one of the digital inputs is activated. I think I can also activate freeze protection from my Intellicenter but didn't spend a lot of time on it since it's not needed here.

Chris

 

[Dirk]

So here is a dumb question, do any of the VS pumps, IntelliFlo etc. Have a temperature sensor in them? How does the freeze protection work do they all use an external temp sensor? It would be cool if you could read the temperature from something that is already in your system.

I studied this a bit when I couldn't get my freeze protection to do what I wanted, but some of it has slipped away. I recall there are two things going on, but can't say if my Pentair pump has one or two sensors (it definitely has at least one). My freeze protection kicks in at 35° and the pump runs at 900 RPM. I can't seem to override either of those numbers. I believe the pump's temp sensor is measuring air temp inside the pump's electronics, and kicks in to protect the electronics (and/or the pump itself, not sure), not necessarily the pool's plumbing. You have to be mindful of this if your pump is in a shed or indoors somehow, and not subject to the same outdoor temps your plumbing is. The other freeze protection, based on water temp, I think is a function of my automation system, available because I installed a water temp sensor in my plumbing. Something like that. Anywho, you wouldn't want to use the pump's sensor for monitoring air or water temp for solar, as that is not what it's designed to measure.

 

[Dirk]

It should be just as possible to use the exit water temp as well, if you have accurate temp measurements.

As to the low temp difference, that's relatively simple to explain. Solar water panels gain heat via radiation from the sun, right? Well they loose heat via radiation to the sky and convection to the surrounding air. The hotter something is relative to something else, the faster it looses heat. For convection this is proportional to the temp difference between the object and the air. However, for radiative heat loss, it's not proportional to the temp difference (in this case the temp difference between the panel and the sky, the latter of which varies but is probably between 0 and 20 °C for most pool cases). Instead, the radiative heat loss has temp to the 4th power in the equation, meaning radiative heat loss is exponentially increases with higher temps, unlike convective heat loss which proportionally increases with higher temps.

Put simply, low flow means hot panels. Hot panels mean heat lost to the air and sky via convection and radiation rather than heating the pool water.

Additional "nerd notes": Domestic hot water panels, which have to heat water to some decent temp like 160 °F, will almost universally be glazed (covered with glass) to minimize convective heat loss as much as possible (and in very cold climates like where it freezes may even be vacuum insulated to eliminate convective heat losses entirely), and will have special coatings on the collector tubes that maximize radiative heat gain from the sun while minimizing radiative heat loss to the sky, but these methods are expensive and so pool panels have neither of these since they need to be cheap and don't need to output hot water to do their desired function of heating pool water.

Yah, what he said! And while all that sounds right, I think there is a secondary thing going on, too, that might also apply to other forms of pool heaters (like gas). Again, I don't know the science, but it has to do with volume (flow): the amount and rate of the water running through the heater. If you pump a little bit of really hot water out of your returns, that heat will dissipate before it can bring up the temp of the entire body of water. You have to exchange more water, faster, between the heater and the pool, to affect change. Something like that...

Regarding "It should be possible to use the exit water temp as well...": well, experiment away, but I have to think if that was the better way to do it, that's how it would be done... Tracking the air/surface temp on the roof has to be faster than waiting for the water temp to cool off. That's my guess. You want to know, as fast as possible, when the heat on the panels drops, so you can close down the flow through the panels immediately. If you have to wait until the water cools off enough to affect a water temp sensor, then you've already pumped that cooler water into your pool, negating the last x amount of heating. As I said, just a guess...

 

[Rattus Suffocatus]

Try clicking here for the DS 18820 with a 1/2" threaded enclosure. It's available in multiple lengths so it can be placed in the flowing fluid in multiple different pipe sizes.

It's about time someone finally made one of these! If the hole dimension is close I ought to consider replacing my hacked up one on my brew kettle! Honestly 2-3 years ago these did not exist anywhere with a DS1820 part.

IMHO, in any case you want the smallest thermal mass you can get, so the 30 cm... if it will physically work, is the best choice for this. If and when I plumb in a Salt Cell, I'll need to get one of these and plumb it in at the same time.

Measuring light and pool water temperature was how I was going to approach building my DIY system if I had to get fancy about it. I'm going to try running a small bilge pump right off the cell and see how it goes (I have a 100W cell which should be overkill for one small pump). I am tempted to also run a little peristaltic bleach pump off of it at the same time! Again, this can only work if the panels are near the ground as the little 12V pumps don't have a high head capacity.

If that doesn't work at all or eventually, I'll put an Arduino or something on it to make sure the pool water isn't too hot, and that there is sunlight (a cadmium sulfate cell to determine luminosity or very small silicon solar cell hooked to the A/D converter across a resistor)... I think that would be adequate to turn the pump on and off. My goal is to make it completely solar, which is quite different from a normal commercial system. I do not want to be forced to run the pool pump.

Does anyone know of a device that can give temperature to Alexa from a DS18B20 right out of the box? All of the home control toys look fun, but I honestly don't care to do that.. I just want to monitor temp.... It doesn't appear that the Sonoff (or my WeMos units) will do that unless third-part software is used with some sort of MQTT cloud bridge... Maybe I am misreading that though.

My Hayward VSP does the freeze protection thing at about 40F air temperature. At full speed. It's really annoying and unnecessary in Tuscon, but what can I do except pull the breaker every night? So yes, there is a air temperature sensor in it, but apparently no water temperature sensor that I can tell. Maybe if I had their full automation system which is never going to happen.

 

[Rattus Suffocatus]

Regarding "It should be possible to use the exit water temp as well...": well, experiment away, but I have to think if that was the better way to do it, that's how it would be done... Tracking the air/surface temp on the roof has to be faster than waiting for the water temp to cool off. That's my guess. You want to know, as fast as possible, when the heat on the panels drops, so you can close down the flow through the panels immediately. If you have to wait until the water cools off enough to affect a water temp sensor, then you've already pumped that cooler water into your pool, negating the last x amount of heating. As I said, just a guess...

Think about this for a minute. If the pool water is too hot, you don't want to heat it any more (so a sensor for static water temperature). If there is no sun, you don't want the panels to become a convection cooler. Beyond that since you can't control solar output or the rotation of the earth there is nothing else that matters for a passive solar heating system like this. So this is why I suggest those two. You might be able to argue that running through the panels when it is really cold out might cool instead of heat even with some solar radiation on them, but those times of the year, a switch to turn it off would be pretty effective as well.

All of the research I have done on this seems to point to smaller delta T's through the panels as being more efficient because you do otherwise have re-radiation of heat on uncoated panels. But that's fine and you get as good or better efficiencies that way. So this sensor arrangement seems to support the practical best practices for a setup.

 

[Dirk]

It's about time someone finally made one of these! If the hole dimension is close I ought to consider replacing my hacked up one on my brew kettle! Honestly 2-3 years ago these did not exist anywhere with a DS1820 part.

IMHO, in any case you want the smallest thermal mass you can get, so the 30 cm... if it will physically work, is the best choice for this. If and when I plumb in a Salt Cell, I'll need to get one of these and plumb it in at the same time.

Measuring light and pool water temperature was how I was going to approach building my DIY system if I had to get fancy about it. I'm going to try running a small bilge pump right off the cell and see how it goes (I have a 100W cell which should be overkill for one small pump). I am tempted to also run a little peristaltic bleach pump off of it at the same time! Again, this can only work if the panels are near the ground as the little 12V pumps don't have a high head capacity.

If that doesn't work at all or eventually, I'll put an Arduino or something on it to make sure the pool water isn't too hot, and that there is sunlight (a cadmium sulfate cell to determine luminosity or very small silicon solar cell hooked to the A/D converter across a resistor)... I think that would be adequate to turn the pump on and off. My goal is to make it completely solar, which is quite different from a normal commercial system. I do not want to be forced to run the pool pump.

Does anyone know of a device that can give temperature to Alexa from a DS18B20 right out of the box? All of the home control toys look fun, but I honestly don't care to do that.. I just want to monitor temp.... It doesn't appear that the Sonoff (or my WeMos units) will do that unless third-part software is used with some sort of MQTT cloud bridge... Maybe I am misreading that though.

My Hayward VSP does the freeze protection thing at about 40F air temperature. At full speed. It's really annoying and unnecessary in Tuscon, but what can I do except pull the breaker every night? So yes, there is a air temperature sensor in it, but apparently no water temperature sensor that I can tell. Maybe if I had their full automation system which is never going to happen.

We've had several threads about it, where folks tried to outsmart pool heating physics (drip tubing seems to get a lot of attention). Good luck. I think your little pump will produce hot water, maybe even really hot water, but it won't be of sufficient volume to do a thing to your pool. We've beat this up over the years, and the conclusion (and results) are always the same. I expect your results to be similar. Based on your location and sun exposure, ya gotta have x amount of heat exchange, and ya gotta run y amount of water through it, and that requires z amount of energy (pump flow) to raise your pool water's temp. You can't cheat that. I have a 12K pool, I run my big ol' pump about 2200 RPM to move 40 gallons a minute through about 400sq.ft. of some of the best solar panels available, to move my water temp 5-10° (on a good day)... takes most of the day to do that. And my system is optimized about as perfectly as possible (via flow meter and manufacturer recommendations).

 

[Dirk]

Think about this for a minute. If the pool water is too hot, you don't want to heat it any more (so a sensor for static water temperature). If there is no sun, you don't want the panels to become a convection cooler. Beyond that since you can't control solar output or the rotation of the earth there is nothing else that matters for a passive solar heating system like this. So this is why I suggest those two. You might be able to argue that running through the panels when it is really cold out might cool instead of heat even with some solar radiation on them, but those times of the year, a switch to turn it off would be pretty effective as well.

All of the research I have done on this seems to point to smaller delta T's through the panels as being more efficient because you do otherwise have re-radiation of heat on uncoated panels. But that's fine and you get as good or better efficiencies that way. So this sensor arrangement seems to support the practical best practices for a setup.

I didn't follow all of that. I think you're suggesting measuring light. That might work, but won't be as efficient. My controller often runs water through my panels when it's cloudy, and doesn't at other times when it's sunny. It's about available heat on the roof, not amount of sunlight. Wind is a huge factor, too.

"a switch to turn it off"? "those times of the year"? Not sure what you mean. My controller turns the solar heater on and off every day, multiple times a day. In the summer, it mostly stays on continuously until the pool reaches temp. But in the fringe seasons (spring, fall), the valve to the panels is turning on and off as much as dozens of times a day. Sometimes the heater comes on early in the morning. Sometimes not. The controller is finding the best way to heat my pool given the existing conditions, which change by the minute, not by "times of year."

This is a great academic exercise, but IMO not particularly realistic. It seems you're trying to reinvent something that already exists, and already works as well as it's ever going to. You can buy a solar control system for around $300 or so, which includes the automated valve, so I'm not sure what your end goal is, to build something that cost's less, or costs less to run, but won't work nearly as well (or at all)? But do keep us posted if you come up with something useful...

 

[jseyfert3]

Yah, what he said! And while all that sounds right, I think there is a secondary thing going on, too, that might also apply to other forms of pool heaters (like gas). Again, I don't know the science, but it has to do with volume (flow): the amount and rate of the water running through the heater. If you pump a little bit of really hot water out of your returns, that heat will dissipate before it can't bring up the temp of the entire body of water. You have to exchange more water, faster, between the heater and the pool, to affect change. Something like that...

Yes, pretty much. The flow rate has to do with the efficiency of the heat exchanger inside the gas heater. For similar reasons to what I described for why you want high flow through a solar panel, you'd want high flow through a gas heater as well. Lower water flow in a gas heater would mean lower efficiency of the heat exchanger, and less heat into the pool, in almost the same way that low flow in a solar panel means lower efficiency in the panel, and less heat into the pool. (the lower flow means the heat exchanger is hotter, so less heat ultimately goes into the heat exchanger and more heat goes up the exhaust flue)

The pool itself doesn't care about the actual flow rate, it's specific to the gas heater/heat pump/solar panel efficiencies. I could explain more with some equations later tonight if you're interested, which may make it a bit clearer than my admittedly not very clear answer...

Regarding "It should be possible to use the exit water temp as well...": well, experiment away, but I have to think if that was the better way to do it, that's how it would be done... Tracking the air/surface temp on the roof has to be faster than waiting for the water temp to cool off. That's my guess. You want to know, as fast as possible, when the heat on the panels drops, so you can close down the flow through the panels immediately. If you have to wait until the water cools off enough to affect a water temp sensor, then you've already pumped that cooler water into your pool, negating the last x amount of heating. As I said, just a guess...

AH HA! I figured it out. While a temp sensor at the water outlet would work fine for shutting off the water, it wouldn't know when to turn the water back on! That's why they use an ambient temp sensor near the panels themselves.

You're right, of course there was a reason it was done that way...

That said, my solar setup, which will likely have DIY controls, will have a temp inlet and outlet temp sensor, because I'm a nerd, and I want to calculate and track the actual total heat gain from the solar panels throughout the year (so I have numbers to back up just how effective they are), and you need the flow rate and inlet/outlet temps to do that.

 

[Dirk]

That said, my solar setup, which will likely have DIY controls, will have a temp inlet and outlet temp sensor, because I'm a nerd, and I want to calculate and track the actual total heat gain from the solar panels throughout the year (so I have numbers to back up just how effective they are), and you need the flow rate and inlet/outlet temps to do that.

Nerd on! Just keep in mind: you're punching two more holes into your plumbing, two more seals, two more sensors, two more wires, etc. Like I did with my vacuum breaker, which I mounted just under the eve to keep it out of the sun and to make replacing it much easier than having to climb up on the roof, I'd suggest you install those two temp sensors somewhere other than on the roof. The o-rings that seal those sensors to the pipe will not like it up there, and the less you have on the roof, for both longevity and maintenance, the better.

Besides, if the pipes running across the roof to the panels contribute to the exchange (they must, if even a tiny bit, right?), you might want to pull the temps just before the pipe gets to the roof, and just after...

 

[Dirk]

PS. You'll notice in my signature a FlowVis Flow Meter. I installed that primarily to fine tune the flow through the solar panels. I was able to very easily adjust my pumps RPMs to optimize the heat exchange. And I can keep an eye on flow throughout the year. It does change a bit, based on the filter's condition, so I can quickly compensate for that by goosing RPMs slightly until the filter gets cleaned. Lather, rinse, repeat. And it's turned out to be quite useful for other purposes, too (I optimized my pool vac's flow, etc). Plus it doubles as the check valve for the solar heater plumbing. There are a few (cheaper) ways to determine heater efficiency and flow, but neither is as accurate as the Vis, and the learning curve for the calculation method was way too steep for me. The Vis was "cheaper" in that regard.

 

[Rattus Suffocatus]

You can adjust a solar cell or Cadmium cell to trigger on/off during cloudy times.. It doesn't have to be pitch black. You set it so at a certain intensity it turns on, and then with some hysteresis in place have it turn off at a certain different intensity. So, you can still only run when there is positive heat from the cell with light, the amounts of light are actually measures in W/M^2 in many weather stations. Once you hit a certain intensity you are going to generate a fixed amount of watts of energy (and if you drop out time, then Joules or direct heat output). So doing it by light is an absolutely valid way of doing it with far more resolution than measuring water temperature.

The big thing is you don't want to cool the pool. I am not that concerned with losing 20 minutes a day or whatever when I can be heating.. it's not that precise of a science anyway.

In my case there won't be any extra latent heat from a roof, anyway, so doing it with sun makes perfectly good sense. Especially because I am going to run the pump with a solar panel anyway.

As for turning a switch off when it is too cold outside.. I.e. not enough solar heat output to compensate for the losses from thermal radiation of the panels (there is efficiency loss no matter what and you can get in that situation)... like in the winter... yeah.. flip the darned system off then. Same thing if the water gets too hot which in my case it would in July/August.. It's not a difficult concept really.

My back yard faces north. I can't put the system on a roof. Nor do I want to. I will likely not get as much heat out of this system as I would with running a lot of head loss and the pool pump on a roof like most systems do, but it will cost zero to run. Everything has it's tradeoffs. Since I will be at ground level I don't need much of a pump and whatever I lose to non-ideal control systems, I can easily make up cheaply by running another section of panel or two. Even another $16 pump if need be.

If you do research there are many people who even claim that solar water heating isn't worth doing at all. I think here in AZ it is but it probably has limited use in most places.

 

[Rattus Suffocatus]

We've had several threads about it, where folks tried to outsmart pool heating physics (drip tubing seems to get a lot of attention). Good luck. I think your little pump will produce hot water, maybe even really hot water, but it won't be of sufficient volume to do a thing to your pool. We've beat this up over the years, and the conclusion (and results) are always the same. I expect your results to be similar. Based on your location and sun exposure, ya gotta have x amount of heat exchange, and ya gotta run y amount of water through it, and that requires z amount of energy (pump flow) to raise your pool water's temp. You can't cheat that. I have a 12K pool, I run my big ol' pump about 2200 RPM to move 40 gallons a minute through about 400sq.ft. of some of the best solar panels available, to move my water temp 5-10° (on a good day)... takes most of the day to do that. And my system is optimized about as perfectly as possible (via flow meter and manufacturer recommendations).

The pump isn't going to produce anything. It only will move water through the panels. If you read about it, you actually don't need much of a pump to do this is you are not compensating for head loss. Volume does matter of course but they do pretty well this way as that is what they are designed for. The pump will work because I am doing it at ground level, and is designed for only about 7' of head but at least 360 GPH at the head level (say 3' or so) that I intend on using it on. So that's 6 GPM... Which will move enough water to make a difference. (You don't need 40 gallons per minute, but it is true that the lower differential of hot water out versus inlet temperature a system is more efficient unless it has reflective coatings, etc on the panels.) I can do this because I am going to put the panels (exposed to nearly 100% sunlight in a day) at ground level. I will have enough of a surplus of electricity that I could run one on each panel if need be up to about 4 of them at once. They are $16 each. Really quite doable. Will it heat as well as using the pool pump?.. no, but I don't really care. And I could reasonably upgrade the system with about 20 gallon per minute pumps for a couple of hundred bucks if need be and still stay 100% solar powered.

You lose a lot more than you think by having the panels on your roof. After having a closed loop geothermal heat system for several years, if I had a significant head distance I'd close the system (you have less head losses) and do a heat exchanger instead. Closed loop horizontal geothermal systems are twice as efficient as open loop vertical ones.

I don't need to get 5-10F a day. 2F/day would likely extend my season a couple of months which is all I would be after. The really sad thing is that a solar cover does as much as most full blown solar systems do. You usually are at best doubling the temperature rise. There is a lot of actual scientific information out there beyond "buy a commercial system". Right mow I am at 54F just with the cover. If I could get 1F a day, in March, that alone would extend the season. (I'd be able to swim in three weeks at that rate assuming nothing else warms it up) If not, I will just deal with it, I intentionally did not put a heater on the pool. I have a detached hot tub for winter...

 

[Dirk]

Sounds like you know way more about this stuff than I do, but I still think you’re mistaken if you think you’re going to heat a pool up with a $16 solar-powered pump (or four of them). I barely understand head, but I think I have this right: the only substantial push is initially getting the water up on the roof, but after that you’re mostly dealing with the friction, as the water coming down from the roof is pulling water up (like a siphon). You’re not moving all that water up there, after all. Also, your calculating with 360 gph of a small pump. Yah, maybe through the fat 10’ length of single hose they measured that spec with! But not when you’re forcing all the water you’ll need to through enough tubing to achieve the exchange you’ll need (x-amount of square inches of surface area facing the sun). Not seein’ it. I think that’ll bring all four of those pumps to their knees!

And when you say you only need 1F to get you three more weeks of season! Are you talking about only raising your pool temp 1 degree? You’re working with some interesting math...

Anywho... I’m just messin’ with ya, as I don’t think there’s a chance that you’ll beat the physics. But I’m wrong a lot, and I’ll be the first to admit it if you find success. So don’t mind me. Build the thing, then come on back and report. I’m sure I can scrounge up a hat to eat if I have to! ;-)