What Size Pool Pump Do I Need?
The pump is the heart of your pool water system, circulating water so that it can be filtered, heated, cleaned and chemically treated. A pool without a pump is useless. It is important then that when you either replace or install a pump for the first time that it is sized correctly. It is essential that the pump is neither too big nor too small, as there can be problems associated with both, such as insufficient filtration or the creation of too much pressure on the pipe work or filter installation. The aim is to install a pump which will filter your entire pool water volume every 6 to 8 hours, depending on the swimming load.
If replacing an existing pump, it is tempting to just install the same size as the original. For this to work it depends on the original being correct in the first place. Consequently, it is worth doing the correct sizing exercise regardless.
The factors that need to be considered can be summed up as “flow rate” and “head”.
The flow rate is the amount of water moved in a given time, usually expressed in gallons per minute. To work out your required flow rate you first need to work out your pool volume. For these formulae, take the measurements in feet. To work out the average depth, add the deepest depth to the shallowest depth and divide by two.
Pool Volumes.
Rectangular Pool: Length x Width x Average Depth x 6.25 = Gallons
Round Pool: Radius x Radius x Average Depth x 20 = Gallons
Shaped or Oval Pools: Avg Width x Avg Length x Avg Depth x 6.25 = Gallons
When you have worked out the pool volume, set a turnover time. It is recommended that this is every 6 hours, but can be varied depending on the bather load. If we assume turnover required is six hours then the flow rate in hours is the volume divided by 6.To express this in gallons per minute, divide this further by 60.
Having worked out the flow rate that we need, we then have to look at the head loss, which is a measure of the resistance, for example friction in pipe work, that the water will encounter on its journey around the circulation system.
This is very difficult to do, and is only theoretically possible on new builds. There are very many variables and too many unknowns. For example, to be 100% accurate you would need to know exactly how many feet of pipe you have, exactly how many joints, where they are, who manufactured them, what they are made of, etc,etc.
In reality, it is only possible to make a good estimate. It is accepted in the industry that for most pools an average “head” is 10m or 33ft. This will usually be reasonably accurate for most pools where the pump is situated no more than 30 feet from the pool, and on the same level. Take this 10m figure as a starting point and adjust upwards or downwards if there are any unusual features, such as the pump on a lower level.
Once we have calculated the required flow rate, and estimated the head, then we need to look at the pump curve charts provided by the pump supplier.
On the vertical axis the scale is measured in “Feet Head” or “Metres Head”, whilst the horizontal scale is “Flow Rate”. At the point where your two readings cross, go to the next pump curve up. That is then the pump you require.
