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SIZING THE POOL PUMP


If you're having a pool built it is important for you to insist that your filtering pump be sized properly. And if you don't care about operating cost then what are you doing looking at this web site! The most strict pool filtration guideline says you need to theoretically turn the pool over once per day or more strictly in 6 hours. If you look at a pump curve showing pressure vs flow and figure a pressure that isn't ridiculously high because the pump is oversized, the flow rate will be quite high. For a typical 3/4HP pump it is not unusual to see 80 gpm even with the filter dirty. An average 16x32 pool with an average depth of 5 feet holds 19000 gallons of water. 19000 gallons divided by 80 gallons per minute is 239 minutes or 4 hours. Note that a 2HP pump operating at a higher pressure will deliver maybe 120 gpm. That turns the pool over in 2.7 hours but at an operating cost that is triple. Note you pay triple to run the big pump but you only get 50% more flow. Hmmm. You need the pump on more than 3 hours a day if you want to operate a solar heater. Even if you weren't going solar you normally would want the pump on for longer rather than shorter. It is a lot easier to maintain chemistry with the pump on. Modern day features like waterfalls, ozonators, salt water chlorinators, and even gas heaters require that the pump be on. Stagnating the water for 20 hours a day is a good way to establish a scum line. If the pump is off during the day, the hottest water establishes at the top surface meaning you are maximizing the heat LOSS from the pool and maximizing evaporation and chemical loss. You need the pump on during the day. An oversized pump makes more noise, causes everything to shake violently when it starts up and it pressurizes everything reducing the life expectancy of everything, not just the solar panels. We often run into 2HP pumps on pools plumbed with 1.5" pipes and the pressure on the filter is NOT too high. The reason is the pump has worn itself down operating at such a high pressure over time. The customer is paying 2HP for his electricity and getting the flow of a 3/4HP pump. Have your pool builder show you a pump curve for the pump he is selling you if he is going high on the pump HP and ask him how many pool turnovers are required per day. Do the above calculation right in front of him and ask him to justify the larger HP pump. A 3/4 Hp or 1 HP pump will operate at 12-16 psi on a sand filter, 8-10 on a cartridge filter and 5-15 on a DE filter. He may have a valid justification or at least an argument. Its important that you make him aware that you don't mind skimming the pool at a different time than vacuuming or you don't mind if the water feature isn't on all the time.

There are a few situations where the builder has a reason to use a larger pump than would be required just for filtering. Perhaps there is a built in spa and the spa doesn't have its own jet pump. In this case he makes the pool pump huge so he doesn't have to use 2 pumps. Make him use 2 pumps in this case. If the waterfall between the spa and the pool requires a large flow that normally does have to come from a large filtering pump and if that pump is huge anyway then it may as well also provide the spa jet power when in spa mode. In this case there is no need for a separate spa pump. Now we step outside the normal realm of standard pool design. I say that if you want to have a high volume waterfall operating between the spa and the pool, it should be on a large pump that isn't the filtering pump. That is a challenge to pool builders. I don't build pools but I can see how that could be done quite easily. Challenge your pool builder to give you more than just the standard set up that he's done over and over. Hire a pool builder who will work with you and isn't just marking up subtrades making a big profit and doing nothing.

VARIABLE FREQUENCY DRIVE PUMPS

This is the latest and greatest thing in the pool business. Now everybody is happy. If you want 150 GPM so you can run the waterfall at the same time you vacuum, you can set it up to do that.....as long as you hire the one guy in town that works for Pentair that can program the thing!! My thinking is still that you're better off with one or two single speed pumps but the problem there is you have to apply some design creativity and expertise. Most pool builders don't want to talk to you...or me. They just want to do their trade. It is important for you to understand that your pool builder is not a mechancial engineer. You really should have a pool mechancial system designed but unfortunately that service is not available in today's competitive consumer marketplace. On the other side of this argument is the fact that even the smallest available pool pump (1/2HP) is way more power than you often need so the VFD pump allows you to operate at half or even 1/4 of the energy consumption of a 1/2 HP pump. The only concern we have with these pumps is that we are relying on check valves and positive sealing 3 way valves to isolatre solar from the high pressure of a higher speed function like spa jet operation. We're working on this with Pentair. We may eventually end up providing our own software for the Suntouch and building in a pressure sensor.

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