Pentair performance curve for dual speed, low head limit

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Dynamic head loss is proportional to the square of flow rate and flow rate is directly proportional to RPM. So when operating at 1/2 speed, the head loss goes does by a factor of 4 (i.e. 1/4th of high speed). So you will notice that the maximum head of the 2 HP at full speed is about 80' while the low speed is 20' (4:1).
 
The OP won't have 20' of head on low speed. That is like having 80' of head on high speed. Not likely on typical pool plumbing.


Here are some operating points on typical pool plumbing with the OP's pump:

2" Plumbing High Speed: 74 GPM @ 54'
2" Plumbing Low Speed: 37 GPM @ 13.5'

1.5" Plumbing High Speed: 58 GPM @ 56'
1.5" Plumbing Low Speed: 29 GPM @ 14'
 
No, that is not how you determine head loss. It is much more complicated than that. Plus each component in the plumbing system adds head loss included fittings, valves, filters, heaters, SWGs, and anything that touches the water.

I have some articles in my sig that might help.
 
consoleman said:
He did say he has 20' of head, but he may be misunderstanding head (aperantly i am too). I thought you just measure the amount of linear feet you have from the skimmer to pump + drain to pump.... something along those lines.
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There are two types of head, static and dynamic.

If you pump water from a lower level to a higher level, that is static head. For most pools, static head is zero because the water returns to the same level it came from. However, if you pump from a lower pool to a higher pool, the difference in elevation is the feet of static head.

Dynamic head is the amount of pressure required to move water through the system (pipes etc.) due to friction with the system components (pipe walls etc.). The amount of pressure needed depends on water flow rate, pipe diameter, pipe material, fittings, length of pipe and anything that restricts water flow. The pressure needed can be measured in feet of head, PSI or other pressure measurements.

Total head is the sum of static and dynamic head.

Length of pipe is just one factor that determines the amount of dynamic head.

You could have a 1,000 foot pipe and a 100 foot pipe moving the same GPM and have the same feet of head (head loss) if the pipes were sized correctly.

For example, if you had 100 GPM moving through 100 feet of 2” schedule 40 PVC, you would have a head loss of about 16.5 feet.

To move 100 GPM through 1,000 feet of pipe at the same head loss, you would have to increase the pipe inside diameter to about 3.32 inches.

Moving 100 gpm through 100' of 2" pipe or 100 gpm through 1,000' of 3.32" pipe creates the same head loss as pumping from one pool to another pool that was 16.5' above the first pool.

To measure total feet of head, you can put a pressure gauge immediately before the pump and immediately after the pump while the pump is running and the difference in pressure is the total feet of head. You can convert gauge measurements units to feet of head. For example, 1 psi is equal to 2.31 feet of head.
 
Thanks y'all. This makes sense, didn't realize how "dynamic" the dynamic head calculation is. I'm considering a motor swap on my 1.5HP Superflo, to put in a 1HP EE 2-speed (Pentair model 356630s) so that I can run it at 1725 RPM for a longer period and save some money. When I saw that curve I became concerned that I wouldn't get the GPM needed at low RPM for my desired 2 turnovers per day. Obviously it's more complicated than that.

I only know my system by what I can see above ground (part numbers, labels). No idea how much pipe, how many elbows, etc. It sounds like I shouldn't even try to assume how many feet of head I have. On a clean filter my PSI is 15. I've seen other posts (some replies from mas985) that mention PSI could be an indication of dynamic head, but I haven't seen any way to calculate based on that. Straight line from my pump pad to the skimmer is 50ft, for reference.
 
desired 2 turnovers per day.
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That is another thing we should discuss. Turnover is really irrelevant. There is no reason to have a specific number of turnovers. So knowledge of flow rate isn't really that important either. You might want to read this:

Pool School - Determine Pump Run Time

I run a 1/4 per day in the winter and 1/2 turn in the summer (mainly for solar).
 
"studies have shown that a typical pool really only needs about 4 hours of run time for proper chlorination, circulation and cleaning"

Based on what? I'm not challenging it, but is that not based on how much water needs to be filtered, mixed, run through a SWG or Chlorinator, etc? Effectively a turnover amount?

With my size pool and pump...I probably am turning over every 4 hours anyway. Would like to extend that to 8 or 12 hours of runtime at lower speed and get some efficiency.

EDIT: To be clear, in the summer I was running my single-speed pump for 12 hours. Probably getting 2.5 or 3 turnovers in that time. Water has been crystal clear, very happy with it. NOT happy with my electric bill however. So I'm looking for maybe 2 turnovers (could go less if it remains clear) at a lower speed.
 

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Based on what? I'm not challenging it, but is that not based on how much water needs to be filtered, mixed, run through a SWG or Chlorinator, etc? Effectively a turnover amount?
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You can start by reading the run time study in my signature. For dozens of pools, they saw no difference in water quality after about 2 hours of run time. The only true minimum for run time is to get enough chlorine in the pool and circulate it. It doesn't take much time to do this if you are adding liquid chlorine by hand (~30-60 minutes). For an SWG and pucks, the time is longer and dependent on the size of the SWG or feeder but it really has nothing to do with turnovers.

But more importantly, the forum has been advocating this for years and many people are now following these guidelines with much success so there is a ton of empirical evidence to support this position. Some on the forum have run times as little as 30 min per day with great success.
 
Case in point - last season with careful chemical management I was able to get my pump run times down to 4 hours per day @ 2000rpm on my VSP (~750W). On the very hottest days, I would bump it up to 6 hours per day. That's all my pool needed to maintain proper FC, run the suction cleaner and keep the pool clean & clear.
 
It's splitting hairs, but what they are talking about is still turnover. There's an amount of water, slightly different for each pool, that must flow through the system in order to reach desired quality. It can't be based only on run time since every system and GPM will vary. However for most residential pools, they found that 2 hours is sufficient water movement through the systems. They say physical characteristics have no affect, but I doubt there were any 100k gallon pools in their study (they don't mention sizes). They also don't mention what kind of GPM the average system they studied had. That would be helpful since this study was from 1984 before variable speed motors were available (to my knowledge).

But bottom line appears to be, according to this study, get a small pump and run it as little as possible to chlorinate and mix, and focus efforts on vacuuming, skimming, and brushing.
 
t's splitting hairs, but what they are talking about is still turnover.
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I disagree. They don't really focus on GPM or turnover at all. The focus is strictly on run time. As I said before GPM simply doesn't matter for water quality (i.e. sanitation). Also, what we are talking about here is only residential pools because public pools have a special set of government regulations they must adhere to.

You have to remember, aesthetics and sanitation are two different things. Sanitation requires well distributed chlorine and nothing else. I have done several tests on my own pool where I first tested the water to get a FC baseline. I then poured a full gallon of chlorine into the pool in front of one of the returns while the pump was running on low speed. After 30 min, I tested the water opposite of that return and in several other locations and the FC level was elevated the same everywhere. Given my flow rate of 35 GPM, this is the equivalent of 1/20th of a turnover. So to get a pool well sanitized, it doesn't take much run time at all to do that no matter what the size of the pool. So based upon my experience, 2 hours would be plenty for an 80k pool manually dosed. On high speed, a 160k pool could easily be dosed. The point here is that FC distribution is not going to be the limiting factor on run time.

But there are other factors for run time. A SWG operating at 100% may require more run time to generate enough FC for the pool. But again, that has nothing to do with GPM and is only based on the run time and production rate of the SWG. Larger SWGs can operate at less run time to generate the same FC.

Next, aesthetics can drive run time. Pool skimming and and floor cleaning (non-robot) require pump run time and that is dependent on the amount of debris that drops in the pool and how fast the skimmers work. Do skimmers and auto-cleaners require a certain a minimum flow rate to work properly? Yes they do but these functions are optional and aesthetically driven (i.e. pool owner's tolerance for debris) and not part of sanitation so they do not "require" a minimum turnover. Plus there is no one size fits all for this function. Every pool is going to be different so referencing turnovers is pointless.


But bottom line appears to be, according to this study, get a small pump and run it as little as possible to chlorinate and mix, and focus efforts on vacuuming, skimming, and brushing.
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Exactly! :goodjob:
 
You can keep calling it run time...but it's run time of the pump. And the pump moves water.

In your example, if your was a GPM of 1 instead of 35, it would take longer.

Water has to move through a SWG (or an inline chlorinator) to be chlorinated. If it takes 100 gallons or 10k gallons of super-chlorinated post-SWG water to mix with the pool to reach optimal FC, there's still a water amount needed. And how quickly that water moves through is dependent on the system and how much it can move in a period of time.

I understand I can dump some bleach in my pool and wait for it to naturally dissolve (or maybe find a giant paddle and stir it) and not run a pump at all. But that's not the usual scenario because people have filters and chlorinators that need water movement to function. And those things only affect the water that goes through them.

Your system happens to take 30 minutes to reach optimal FC. Some may take 4 hours, some may take 10 minutes. The difference is the system and the size of the pool. They are abstracting turnover by using runtime instead because it's universal and easier to measure. They could have easily had said "the average pool only needs 1/20th turnover" and it would have been the same thing, just requires more math to figure out how long to run your pump to get there. That's all I'm saying.
 
They are abstracting turnover by using run time instead because it's universal and easier to measure.
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Equal run time does not mean equal turnovers for each of the different pools. Pool size and pump size plus plumbing determine turnover rates and the study included pools of all sizes with pumps of all sizes so even though the run time is the same, the number of turnovers (or fractions there of) was not the same for each pool yet each pool attained the same water quality in the same amount of time.

There is no magic number of turnovers for a pool that anyone should target. That is a myth.
 
I would also add that the concept of "turnovers" in residential pools is fictitious at best. Hydraulic systems being as they are for pool designs, there is a much greater likelihood that the same (smaller) volume of water is being run through the plumbing on average while the vast bulk of the water is mostly stagnant. There is a video that shows a test pool with a main drain running at some insane flow rate (near 800 GPM) and blue dye is introduced just a foot above the drain. The dye gently floats past the drain without ever being sucked into it.
 
Given the size of that drain, the water velocity into the drain would be very slow (< 0.5 ft/sec) so it is not surprising that it is not disturbing the surface water much. Even slower a foot away where the surface is. It can draw from all sides so most is probably coming from other areas. It probably also helps to have a surface current too.

That video is a better illustration for why MDs don't really do much but as far as circulation and distribution, that is more dependent on the return plumbing and the number of eyeballs in the pool. As I mentioned before, I don't see any dead spots in the pool after only about 30 min @ 35 GPM.
 
Assuming an 18" x 18" grate, the velocity is about 0.8 feet per second. That means the amount of water 9.6" directly above the grate is ingested every second.

Maybe it's a legitimate video, but I seriously doubt it.
 

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