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How to Heat and Insulate your Outdoor, Inground UK Pool

The insulation of a UK Swimming Pool

Ground Temperatures

The average ground temperature anywhere in England and France lies normally between 10 and 15 degrees C in the summer. Swimming pool water temperature lies normally between 24 and 28 degrees Centigrade - although some people prefer 30 degrees!

This results in a temperature difference of about 15 degrees C. This is a large temperature differential that will result in the loss of substantial volumes of heat.

Heat Loss from a Swimming Pool

Swimming pool heat is lost from the water surface, walls and floor.

Heat loss is at a maximimum when the pool is in use and the summer cover has been removed. Most of this occurs because of the large amount of heat energy that is taken out of the swimming pool water when evaporation occurs from the pools surface.

The evaporation rate is dependent on the pool’s initial temperature, the surrounding air temperature and humidity, and the wind speed at the pool surface. The higher the pool temperature and wind speed and the lower the humidity, the greater the evaporation rate. Though a windbreak— such as trees, shrubs, or a fence—can reduce evaporation, it may shade the pool from the sun, which is a strong natural heat source for the pool.

But if the pool is only used for 4 hours a day for 5 days a week on average then this major heat loss from the pool surface will only occur for about 12% of the time. If the pool is covered with a solar cover then most of the heat loss then occurs through the walls and floor of a conventional swimming pool.

Factors that affect the Heat Loss

Heat lost through the walls and floor is continuous and is dissipated into the surrounding soil and is dependent on three basic factors:

  1. Area of the swimming pool walls and floor
  2. Difference in temperature between the soil and the swimming pool water
  3. The heat conducting properties of the wall and floor construction.

The first factor is fixed by the pool size and depth.

The second factor can be affected by the heating of the pool water to increase its temperature, by the presence of water in the surrounding soil and by any water flow around the pool that can continually remove heat from it.

The third factor depends on the materials from which the pool is built.

If the water table is high and the swimming pool construction materials can become saturated this will cause a reduction in the thermal efficiency and an increase in the heat lost.

How to reduce Heat loss

Heat loss can be reduced by well over 90% if the external surface of the pool that is in contact with the soil is polystyrene. This is because polystyrene consists essentially of miilions of air bubbles encapsulated by a thin layer of hydocarbons. The swimming pool water is then effectively encapsulated in a “thermos flask”, which dramatically reduces heat extraction into the ground.

Covering a pool in-between periods of use is the single most effective means of reducing pool heating costs. As much as 70% of heat loss from a swimming pool is caused by evaporation. A pool cover acts as a vapor barrier that dramatically cuts heat losses from evaporation, resulting in significantly higher pool water temperatures. A high performance solar cover can reduce the water loss by evaporation by as much as 95%.

If there is static water under the floor then convection comes in to play and the water retains the heat like an insulator. If the water is flowing however, then heat is extracted at an alarming rate and insulation is a must. We can provide permanent pumped drainage for your pool if we find that ground water will flow around your pool when we are building it. Even if you do not heat the pool this will increase the normal water temperature quite significantly for a few centimes a day.

The Insulation Benefits of a Bluepool

Our swimming pools and swimming pool kits are all based on the use of hollow polystyrene blocks for the construction of the pool walls. These provide 100mm of dense 30kg / m2 polystyrene in the walls and around the Roman Steps that reduces heat loss by over 95%.

We can provide a 50 mm or 100 mm thickness of similar polystyrene on the floor of the swimming pool beneath the screed. The approximate costs are shown in the following table. These may vary because the cost of buying polystyrene fluctuates a lot because of the direct connection with the cost of crude oil;

Pool Size
50 mm without hopper (HT)
50 mm with hopper (HT)
100mm without hopper (HT)
100mm with hopper (HT)
8 x 4
10 x 5
12 x 6

If the pool floor insulation is combined with a modern high performance solar cover that combines a heat absorbing top surface and a heat reflecting bottom surface the whole pool becomes in effect a giant solar heater that should be comfortable to swim in for nearly all the swimming season -

without the need for any other form of heating!

What are the different types of summer pool covers

Pool covers are designed using UV-stabilized polyethylene, polypropylene, or vinyl to resist deterioration. A cover lasts several years.

Solar covers are reminiscent of bubble packing material, but using a thicker grade of plastic with UV inhibitors. If you want to turn your pool into a giant solar heater this is the type we recommend.

Vinyl covers consist of a heavier material and have a longer life expectancy than bubble covers. Insulated vinyl covers are also available with a thin layer of flexible insulation sandwiched between two layers of vinyl.

pool summer cover types

Solar Swimming Pool Covers

Solar covers heat a swimming pool by converting the sun's light into heat like thousands of tiny greenhouses, instead of reflecting it as water normally does. This heat energy is efficiently conducted into the pool water by the increased area of the covers underneath surface. Tests have shown that 6 degrees of free heat can be gained and the swimming season extended by a number of weeks!

In addition to significant energy savings, solar pool covers do the following:

  • Conserve water by reducing the amount of make-up water needed by 30%–50%
  • Reduce the pool's chemical consumption by 35%–60%
  • Reduce cleaning time by keeping dirt and other debris out of the pool

The high heat absorption covers look this;

summer cover detail photo

The aluminium surface that projects down into the water maximises the contact area available to transmit the heat from the sun into the water and the aluminium colour reflects the light and heat coming up from the pool water back down into it.

Heating of a new or existing pool

Is it possible to add heating to an existing pool?

If you have a pool without heating and would like to extend your pool swimming season by 2 to 3 months each year then it is probably quite easy to add a pool heating system.

The pool heating options

Options Available

The options for pool heating include solar, heat exchanger using an existing oil or gas boiler, direct electric heating, heat pump using the ambient air as a heat sink, heat pump using the ground as a heat sink and finally a heat pump using ground water as a heat sink.

Selection Criteria

In our opinion the criteria that are most important in terms of assessing the relative merit of pool heating systems are the following with the most important ones first:-

  1. Reliability - It is no good having a pool where the temperature varies wildly from week to week - you just end up not using it
  2. Running cost - If the heating is going to increase your energy bill by several hundred per cent - you will end up turning it off
  3. Maintenance cost - This goes hand in hand with reliability - if the heating system is always leaking water you will end turning it off
  4. Capital cost


Solar is very unreliable and if the system quality is not absolutely first class it can soon become very costly and irksome to maintain. After a few years of frost even the best system will start to leak - and emptying the system in winter does not really help because there will always be back-falling areas in the pipework that you cannot drain down. If you want to take advantage of solar power it is better to insulate the pool walls and floor and install a high performance energy absorbing summer cover all as described above.

Heat exchangers and direct electric heaters

Heat exchangers and direct electric heaters will only cost about 30% of the cost of a heat pump but can use 10 x as much electricity or fuel costs. They are very reliable and cheap to maintain but can cost as much as 12 - 15 euros per day to run.

Heat pumps using the air as a heat sink

Heat pumps of this type will use about a euro a day in power - depending on pool size and water temperature. They are very reliable and should last at least 5 - 7 years before requiring renewal and all the parts are replaceable.

Other types of heat pump

Other types of heat pump will be economical to run but much more expensive to install and so we are not considering them further at this point in time

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