Solar heating for swimming pools – the pros and cons

Solar Pool Heating – the pros and cons
Heating a swimming pool using solar energy has always been a good idea, free heat from the sun is always preferable to a large gas or electric bill, but just how good are the solar heating products available?

If you really want the best solar heating there is then you need the vacuum tube type heater. Prices start from about £3,700. But this article is aimed at solar heating for smaller pool so we will not go in to further details here but follow this link for the Thermecro Evacuated Tube Solar Heating System

For the typical domestic  owner of a small to medium size pool who is  on a reasonable budget there are two main options. The rubber matting type and the new solar pods type. The biggest problem that faces these type of solar swimming pool heaters is that they don’t have a thermostat. You cannot say “I want my pool to be 28C, I’ll just set this dial to 28C” and sit back and two days later the pool is at 28C and it stays at 28C all season long. With solar heating you get what the sun gives you. If it decides to shine then you get 28C but if it decides to stay behind the clouds you get 20C or 15C. If it is important to you that the pool water stays at a constant temperature then solar heating alone is not for you. You will need a back up source of heat and that will mean paying for it.

Nature likes equilibrium. If something hot is surrounded by something cold the hot thing will give up its heat until they are both the same temperature.  Given enough time a large mass of water, like your swimming pool, will reach the same temperature as its surroundings.  This is a major problem in the Middle East where pool water can naturally reach 35C. This is too hot and most pools will have a chiller on them to cool them down. No such problems in the UK where left alone a pool may get to 22C, 25C in a long spell of good weather, but typically 18C to 20C is what you would expect. For most people 20C is too cold but 28C is quite pleasant.  8C might not seem very much to have to warm the water by but it if the ambient temperature is lower than 28C, and that is probably 95% of the time in a British summer, then you are constantly having to add heat to the water while nature is constantly trying to take it back.

The problem we have with solar heaters is that when the sun comes out the weather gets warm anyway.  So is the heat gain in the pool down to the solar heater or would it have warmed up anyway? We have carried out experiments on two pools, side by side and in full sun, one with a solar heater and a solar pool cover and one without. We sampled the temperature every hour and we found, not surprisingly, that the temperature fluctuated with day and night. From a daytime high of 23C the pools could drop to 18C at night but then back to 23C the following day. What we did find was that the pool with the solar heater and solar cover was always warmer than the other pool, usually by about 3C but more in sunny weather. In sunny weather the solar heated pool reached 29C in the middle of the day whilst the non-heated pool was 24C.

During the experiment the weather did turn cool and we had rain. At these times the pools fell back to 18C to 22C but in those conditions no one wanted to use the pool anyway. When the sun came back the temperature of the water rose to 26C and that coincided with a desire to swim.

solar heat graph

Comparison of heated and non heated pools


So we think that, provided you accept the limitations in this climate, solar heating it is worth investing in. We would sum up the pros and cons as follows


The pool temperature will never be constant
The water will cool considerably at night and in a cloudy spell of weather


The heat you do get is free
Most people don’t want to swim in cloudy weather anyway, by the time the weather improves the water has warmed up again.

solar pool heating

Solar Matting on a roof Solar Pod