| By :
Francis Lowe
The pump is the heart of your pool water system, circulating water so that it can be filtered, heated, cleaned and chemically treated. Without a pump the pool will quickly become 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. However, this supposes that the original was the correct size in the first place. As such it is therefore worth going through the correct sizing procedure anyway. 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. Before you can find the flow rate, you need to know the volume of your pool. 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 Water 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. Ideally this will be about every 6 hours, but can vary according to bather load. If we assume turnover required is six hours then the flow rate in hours is the volume divided by 6. This figure must then be divided by 60 to get gallons per minute. Once we have established the required flow rate, we need to determine the head loss, which is a measure of the resistance that the water is subjected to as it is circulated around the pool. This is very difficult to do, and is only theoretically possible on new builds. There are simply too many unknowns and too many variables. 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. For most domestic pools, it is accepted that the average figure for "head" is 10 metres or 33 feet. 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.
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