How to Size a New Swimming Pool Pump

January 30th, 2012 | Posted by Chris Low in - How To | Pool Equipment

pool pump size, how to size a pool pumpWhat size pool pump do I need? We get that phone call or email – all the time, about how to properly size a pool pump – to the filter size and plumbing runs. The pool pump is the heart of the pool circulation system. Sizing your pool pump is very important to allow the proper amount of water flow, to and from your swimming pool.

The purpose of the pool pump is to pass all the water through your pool filter to remove debris, seen and unseen. The pump rate of flow, as measured in Gallons per Minute (GPM) is one of the main factors. Now, let’s discuss how you can choose the correct pump size for your above ground or inground pool – so that you get the best results.

The first thing you need to know is that not all horsepowers are equal. Every pump has different flow characteristics, and the flow rate will vary by manufacturer and model. Pumps are also made for aboveground pools and for inground pools. The main difference here is that aboveground pumps are not typically self priming, and must be installed below water level. Another big difference is that the internal designs of inground pumps are meant to produce more flow, to overcome longer pipe runs and more bits of pool equipment.

Turnover rate for most pools is designed to be close to 8 hours.  The turnover rate is the amount of time it takes to move all the water in your pool through the filter system. Using 8 hours as our desired turnover rate, you can calculate the rate of water flow (GPM) needed to provide an 8 hour turnover.  The formula you use to determine your flow rate is the following:

Flow Rate = Gallons in Pool / 480

To calculate the number of gallons in your pool, we have another mathematical formula.

For Rectangle pools, use the formula L x W x average depth of pool x 7.5.
For Oval pools, use the formula L x W x average depth of pool x 5.9.
For Round pools, use the formula  L x W x Depth of pool x 5.9.

An example of this for a 16’x32’ rectangular pool would look like this:

16’x32’ x 4.5 (average depth) x 7.5 (gallons of water to a cubic foot) = 17,664 gallons of water in this 16′x32′ pool. After determining the amount of water you have in your pool – divide it by the number of minutes in 8 hours (480 mins.).

Returning to our previous example, we have 17664 / 480 = 36.8 (GPM). Based on this calculation, we need a pump that will produce 37 gallons per minute of flow rate. But wait! We have to account for all of the resistances that our pool pump encounters.

As the water flow is pumped through your pipes, fittings, valves and various equipment – water flow is reduced by friction. This resistance to water flow is what the pump must overcome to produce the needed flow rate for a pool or spa. There are many long drawn out formulas to calculate Total Dynamic Head; it can be a complicated exercise to accurately compute.

If you want to roughly calculate the amount of resistance, measured in feet of head, that your pump must overcome, here’s a rough approximation method that can be used. Add up all of these resistances to find the total resistance in your system.

Pool Filter – 10ft
Pool Heater – 10ft
Pool Cleaner – 10ft
Pool Piping – 1ft per 10 feet
90 degree fittings – 1ft each
Inline Chlorinator or Purifier – 2ft
Directional Valves – 1ft each
Skimmers & Drains, 2ft each
Wall Returns, 2ft each

Add these all up, to calculate the total feet of head measurement that your new pool pump must overcome. Then – refer to the manufacturer’s website, and look for an image or a link to the Pump Performance Curve for the  particular swimming pool pumps you are interested in. Pump Curves are a chart or graph – with feet of head (FOH) on the vertical axis and gallons per minute on the horizontal. Find the point on the curve where the calculated GPM meets the calculated FOH. There are several curves on the graph, representing different horsepowers of the same pump. Choose the hp curve that comes closest to where your GPM and FOH intersect on the graph.

Bigger is not Better! If 2 curves come close, choose the smaller pump. When it comes to pool pumps people tend to think bigger is always better. Indeed, this is not true with pool pumps.  A larger pump will only cost you more money to operate and if too large – it will start to cavitate, as it can’t get enough water to satisfy it’s appetite. Pool pumps that are too large also compromise the filtration and can even damage a filter that is too small for the flow being pumped through it.

Another good bit of information is located on your pool filter tank, or on the manufacturer’s website. Every pool filter has a Design Flow Rate, or the flow rate, (GPM) that is ideal for the filter to operate. If you exceed this number, you risk inefficient filtering and possibly filter damage. You can only move so much water through 1.5” pipe so don’t oversize the pump because you will just giving extra money to your local electrical company and your circulation system will not run efficiently.

Another way to calculate feet of head is to use a vacuum gauge and a pressure gauge (if your pump is working). Put a vacuum gauge on the suction side of the pump and the pressure gauge on the discharge side of the pump. With the pump running (and the filter just backwashed, and baskets empty), take the gauge readings. Each 1psi on the vacuum gauge equals 1.1 feet of head and on the pressure gauge, each 1psi is equal to 2.3 feet of head. Add these together to determine total feet of head.

If this all seems to complicated still – I apologize. I have one other option for you – pick up the phone and call one of our helpful pool techs, who can help answer all of these questions about your circulation system – to correctly size a new pool pump for your swimming pool. Call us at 800-983-POOL

Chris Low
SPP Pool Expert

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