HEATING ABOVE GROUND SWIMMING POOLS.There is nothing - nothing - to compare to being able to swim at home in your own pool but, this being the UK, the problem is that an unheated pool is rarely warm enough for comfort, so there are many days during the summer when it's too cold to swim.
The first stage in heating any pool is to fit a solar cover made from UV stabilised bubble plastic. It is reckoned that 70 percent of heat losses from a pool are by evaporation from the surface, and the solar cover not only stops evaporation heat losses but also retains heat at night through insulation. Cheaper pools come with blue 200g covers; ours lasted a season and a half, so I have replaced it with a 500g clear cover, which is the most expensive but which lets sunlight into the water (heating it) while giving far more insulation than a lighter cover. Solar cover prices vary hugely according to their size, weight and type, so I won't give typical prices - you can find them on Ebay.
Next consider building a windbreak around the pool. This not only keeps cool winds off the pool, it also shelters you from the wind - even if the water is warm, the moment you stand up in a cool wind you'll feel cold. You could plant a hedge or build a wooden fence but I built a fence using polyester covered sheet steel that absorbs heat from sunlight and radiates it out, making the area around the pool warm. The sheets are available from agricultural suppliers at roughly £20-£25 for an 4'x10' sheet.
The two measures outlined above will make the pool and the surrounding area warmer (the pool water one day last year hit 84 degrees F with no other heating) but there will still be many days when the water is too cold to swim even during June-August (I can stand temperatures down to 68 degrees but at that temperature you won't want to lounge in the pool for long!).To raise the temperature higher.you'll need some form of heating.
The cheapest pool heaters are electric. A 3KW heater will be fine and a straightforward DIY installation (simply plug 'em in) for a pool up to 12' in diameter but for larger pools you'll need a higher wattage, and that means it cannot be supplied from a 13 amp socket but will have to be wired via a circuit breaker direct to the fuse box, which means paying a qualified electrician (courtesy of building regs). Electric heaters are not the cheapest method of heating a pool, even if you use them at night on an Economy Seven tariff, though they are relatively painless to install and convenient. The prices of electric heaters vary according to quality, and 3KW heaters cost from roughly £100 upwards, a 6KW (15' pool) from £250 and 9KW (18' pool) from £300 - prices all very approximate intended as a rough guide only.
If you have central heating you can tap into the system via a heat exchanger. This is cheaper to operate than an electric resistance heater but if the pool is any distance from the house there will be heat losses from the hose and - with gas prices set to rocket (oil and electricity will probably catch up) - it is not necessarily the cheapest method of heating a pool, though at £200-£300 plus installation costs it will be the best option for some with permanant pools (as opposed to pools that are drained and packed away for the winter).
Swimming pool boilers fuelled by mains gas or oil are available but are expensive to buy at around £1,000 upwards - ordinary central heating boilers can be used in conjunction with heat exchangers to separate the pool water from the innards, which can be attacked by pool chemicals, especially if the water is allowed to become acidic. Boilers that run on propane/LPG are available; they're not cheap to buy and neither are they cheap to run, though if you already have LPG central heating they're well worth considering. Both oil and gas tanks are subject to building regulations, and installation can be costly. Nevertheless, a boiler system will be a good option for some with permanant pools.
Solar power is the future for pool heating, and solar panels are a good way of tapping into it. However, as we all know, the sun does not always shine in the UK and so you don't always have the power to get the pool heated when you want! It is generally reckoned that you need 50% of the water surface area in solar panels in the UK to keep the water at a comfortable temperature; they need to be angled toward the sun to gain the full potential. An 18' diameter pool would need around 125 square feet of collector area - 3 of the 2'X20' or 4'x10' panels we see advertised on Ebay. In favour of solar panels is their low running cost - a few pence an hour to run the pool pump. If you have the space and can build a frame to support the panels at the correct angle, this is the cheapest method of heating a pool - against which if the sun don't shine the water won't heat much.
I've left the best till last - the heat pump. This transfers heat from the air to the pool water, so it works when the sun disappears behind a cloud but, because it is the sun that warms the air, they are in effect solar heaters. They draw quite a lot of electrical energy - 2.5KW upwards - but put far more than that into the water, depending on air temperature. Published figures suggest that at an air temperature of 68 degrees F a typical smaller heat pump drawing 2.5 KW from the mains will put over 10KW into the water - more when it's hotter, less when it's cooler. Most will give a heat gain equivalent to 3.5 times the electrical input at 50 degrees F - but at that temperature the air's probably going to be too cold for most to consider swimming!
One downside of heat pumps is it takes a huge current to get their compressors started and they MUST be wired via a circuit breaker directly from the fusebox with a suitably high fuse (professional electrician again - good old building regs!). Some larger units draw so much starting energy that the mains can't cope and for these you'll need a 'soft start' (£300+), though check before you buy because some units have a soft start built in.
Heat pumps aren't cheap and most cost on a par with boiler systems, though it is possible to find units that output 10-12KW (pools up to 8,000 gallons) for under £1,500.
Me? I've ordered a heat pump.
If you have any questions or helpful comments on heating above ground swimming pools please feel free to contact me