Category Archives: Swimming Pool Heaters

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

You have got an above ground pool and now you want to heat it

So, you have bought an above ground pool, probably an Intex type pool, that sits on your lawn. You have filled it with water, connected up the filter pump and you and your family are all ready to climb that ladder and step in. But then you hear “it’s freezing cold! I am not going in that!”.
Sound familiar? You need a heater. In this article we will discuss the various options for heating a garden above ground pool. We are talking about the type of pool like the one pictured below, you can buy them from us or from Argos, Toys r us, B&Q that sort of thing. Typically 8ft to 18ft round with a combined paper filter and pump.

If  your pool is in the 8ft to 12ft bracket then brace yourself: The heater is going to cost more than the pool. Infact regardless of how big your pool is, if you want a really good, efficient heater it is going to cost way more than the pool. We will not be discussing solar heating systems in this article, we’ll talk about that in another article.

So let’s start from first principles. If you get a cold drink and a hot cup of tea and leave them both on the table for an hour what happens? The cold drink gets warm and the hot drink gets cold. The liquid warms up or cools down to the ambient temperature surrounding it. It is exactly the same with your pool. The only difference is your pool sits there day and night  so whilst it may warm up some in the day it will cool down at night. So if you do nothing the water will reach the average day/night temperature for the time of year. In a typical british summer that is about 17C (63F), not especially warm.

The first thing you should do it buy a solar cover. Even the cheapest type will help but a good quality 400 micron solar cover will help the sunshine to warm water and also help keep the heat in during the colder nights. Using a solar cover should add at least 2C maybe up to 6C to the temperature. That will bring it up to about 19 to 23C (66 to 73C). 23C is not bad, on a really warm day most people would swim in 23C.

If the solar cover does not warm it enough for you then the next stage involves forking out for heater of some sort. There is a big barrier that affects your choice and that is the fact that from a 13amp plug socket in the wall you can only run a 3kw appliance.  3kw heaters are a very popular choice for that reason – you do not need an electrician to wire it in. You plug them in and connect them up and away you go. This is great but unless your pool is 10ft round or smaller you will not get the pool very warm. A 3kw heater will be OK on a 12ft pool but don’t expect anything over 25 or 26C in normal summers. Intex make a cheap 2kw heater. At £95.00 it is good value but it is not hi-tech, it has no thermostat but it will warm up a small pool. Elecro’s 3kw Nano heater at £215.00 is a much better piece of kit.

If 3kw is not enough to heat your pool, i.e. your pool is bigger than 12ft round then you have two main choices. If you are happy to get an electrician in then get a 6kw or a 9kw direct electric heater. Elecro are by far the best manufacturer of these heaters. 9kw will heat a pool up to 18ft round. Bear in mind that when running a 9kw heater and assuming day rate electricity of 13p per unit it will cost £1.17 per hour to run. They typically need to run for 4 to 6 hours per day to maintain the temperature. In a 150 day season your lecky bill could be over £1,000.00. Every season. You will not save electricity by having a smaller heater, it will cost the same and just take longer to heat the pool.

Don’t fancy the idea of paying £5,000 in electricity over the next 5 years? Then a heat pump is what you need. An Ecowarm EW10 gives up to 9.5kw of heat, but it only draws 1.9kw in electricity. It can plug in to the wall socket, no need for an electrician. At 25p per hour to run your lecky bill for the season should be more like £250.00.  So what is the drawback? Well, the EW10 costs £1,200.00 to buy but with a saving of £750 per season it pays back after 1 year.
“£1,200 for a heater, the pool only cost £300!” I hear you say. Yes, that is quite right. Heating up a large volume of water is not a cheap and easy thing to do.

Further Reading: How much does it cost to heat a swimming pool?

Click the links below to see the products discussed in this article:

Intex 2kw heater

Ecowarm Heat Pumps

Circular Solar Covers

Elecro Nano 3kw Heater


A quick word about heat pumps in the spring

Heat pumps are without question the most efficient way to heat a swimming pool. They suck heat out the air and put it in your swimming pool at up to 600% efficiency.

But they have their limitations and they are particuarly noticable in the spring. This article was written on 19th April 2010. Yesterday the temperature reached 21.4C at about 4pm, perfect weather for your heat pump you might think. Well, yes, but at 6am that morning the temperature was 3.0C. It bearly got above 4.5C all night. That is cold! Too cold for a heat pump.  If it does not will freeze up and wait to defrost then it will be working at a COP of about 2 or 3 so the biggest and best heat pumps like the Nirvana M40 will only put about 8 or 12 kw in the pool.

Cold temperatures are a double whammy for pool owners. Not only will your heat pump struggle but the poor old pool which may be at 20 or 25C by now will only have a sheet of plastic between it and air at 3C or 4C for hours on end. Even with the heat pump on all night the pool will still be colder in the morning than it was the night before.

So if your heat pump is not making much head way in April or early May do not be surprised.  The rule of thumb for best heat pump performance at night is “would you go out in a short sleeved shirt?” By this we mean if the night is warm enough that you could wander around in short sleeves for a while without feeling cold then your heat pump will be fine. If you feel chilly in short sleeves or would not contemplate going out there without a jumper on then your heat pump will not like it much either.

If your heat pump has reverse cycle defrost you can forget most of the above. They will work down to lower temperatures but even at the lower temperatures you still only get 2 to 3 as a COP but since they are mostly used on indoor pools they don’t have the cold air temperatures next to the pool to deal with and should cope easily enough.