Let’s look at the basics first.  In order to raise the temperature of a body of water, you need to add heat.  The amount of heat you need to add is simple to calculate if we ignore the loss of heat from the pool.  We have included a calculator on this site to enable you to perform your own calculation Calculator.    The calculator uses the basic scientific calculation that you need 1Kw of heat to raise the temperature of 1 cubic metre of water by 1°C in I hour.  The complication to this calculation is that as soon as you start to heat the water, heat is lost through the side walls and floor of the pool and more significantly from the surface.   In order to heat the pool you therefore need to add at least the equivalent amount of heat to match these losses plus a margin of additional heat in order to raise the temperature.

Traditionally, gas and oil heaters are sized so as to heat the pool initially at the start of the season in around 2 days of continuous running.  It is therefore easy with this much capacity for the heater to maintain the target temperature during the season with only a few hours a day running.  As air source heat pumps have increased in popularity over the last few years, the methods of calculating the heat pump size has become more critical as these output a lot less heat the gas or oil heaters.  To give a direct comparison, air source heat pumps typically take 10-14 days to heat the pool up at the start of the season and then have to run for 24hrs a day to maintain the temperature.

To make the selection of pool heaters easier, we have put together a table of recommended heaters based on the pool size.  The table can be viewed here – Pool Heater Sizing Table