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What size pump do I need?
Last Updated: 02/11/2011
You need enough water flow to "turn over" the entire amount of pool water in 10 hours or so.
This is not based on Horse Power it is based on "Flow Rate", which is expressed in Gallons Per Minute (GPM).
Horse Power is NOT RELEVANT when buying a pump, in fact the number one problem in the pool industry is over sized pumps. 
Buying a pump based on horsepower alone does nothing but waste electricity.
There are two laws making their way from state to state right now called title 20 and title 24. 
These laws effectively outlaw energy wasting pool pumps.

Once you know the desired turnover rate, you can determine your desired flow rate.

Use the following equation to determine the desired flow rate:

Pool Size - number of gallons of water in the pool

Turnover Time - time to move entire capacity of pool through filter

Flow Rate - desired flow rate in gallons per minute

Pool Size

(US Gallons)

divided by

Turnover

Time

divided by60equals

Minimum

Flow Rate

35,000divided by8 hoursdivided by60equals73 GPM
35,000divided by10 hoursdivided by60equals58 GPM
24,000divided by8 hoursdivided by60equals50 GPM
24,000divided by10 hoursdivided by60equals40 GPM
18,000divided by8 hoursdivided by60equals38 GPM
18,000divided by10 hoursdivided by60equals30 GPM

If you have a 25,000 gallon pool and want a turnover rate of 8 hours, then you will need a pump with a flow rate of 52 gallons per minute.

If you have a 35,000 gallon pool and want a turnover rate of 10 hours, you will need a pump with a flow rate of 56 gallons per minute.

For this situation, we will use a 24,000 gallon pool with a turnover time of 8 hours, giving us a MINIMUM FLOW RATE of 50 gallons/minute.

Flow rates are published by pump manufacturers. Match the rate from the chart above with the rate from the manufacturer then buy that size pump. Don't be fooled by horsepower numbers. They are usually inaccurate and misleading. 

If it is in your budget, purchase a multi-speed pump to save up to 75% in energy costs.

 



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