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What are the Different Options for Heating My Pool?

When it comes to heating your pool’s water, there are two different types of system you can use: primary and secondary.

A primary system allows the bather to set the thermostatic controls at a specific temperature – typically between 22°C and 30°C. The heating system will then operate up to and until the water reaches that desired temperature, irrespective of external factors (i.e. ambient temperature, weather, sunshine, etc.)

A secondary system will raise the water temperature, but is dependent on these external factors. Secondary systems generally cannot guarantee the pool water will reach the desired temperature and, as such, are often installed as a back-up to primary systems.

When used in combination, a primary and secondary system can offer considerable savings on running costs. This is because the secondary system will kick in when external factors are in its favour, and the primary system will take over when they are not.

Air-source heat pumps provide a good example this. When the weather is good and the ambient temperature is relatively high, these heat pumps work very efficiently as a secondary system. However, when the weather is poor and the ambient temperature is a lot cooler, the heat pumps do not work as effectively. To combat this, a gas heater (primary system) can be used to ensure the desired temperature is reached.


Primary Heating Systems

Listed below are some primary heating systems which can be used to heat our swimming pools.

1. High performance fuel oil boiler

In a dedicated swimming pool oil heater, domestic fuel oil is burned, heating fluid in a closed loop to a high temperature. This fluid is circulated through a series of stainless or titanium tubes which come into contact with the pool water, heating it up as a result.


  • The heat source is always available, irrespective of weather conditions.
  • Rapidly heats pools.
  • Can provide an economic investment opportunity, especially if the existing boiler located close to the pool system.
  • Combines perfectly with alternative systems (i.e. renewable energy secondary systems).


  • Relatively high running costs when used as the sole heating source.
  • Powered by fossil-fuels, so a high carbon footprint.
  • Rising cost of the fuel oil.
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2. High performance gas boiler

High performance gas boilers run in the same way as a fuel oil boiler but use natural gas, LPG or butane as the main fuel source.


  • The same advantages as the fuel oil boiler (above).
  • Gas boilers are compact and occupy very little space.
  • Reduced emissions over oil boilers.


  • Relatively high running costs when used as the sole heating source.
  • If there is no gas supply nearby, the connection costs can mount up.

NB: Propane (LPG) and butane solutions provide the same advantages as natural gas if mains gas is not available where you live. However, the running costs are approximately 280% as high so are extremely expensive to run in comparison.

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3. Air/water heat pump.

With fossil fuels continually on the rise, heat pumps are gaining substantial media coverage for their green credentials and highly efficient performance. They are also becoming a more and more popular choice to heat swimming pools with.

How they work

An air-to-water heat pump absorbs heat from the air and transfers it to the pool water. The ambient air doesn’t itself have to be warm – even cool or cold air has heat energy within it which a heat pump can extract. That said, the warmer the air is, the more heat the pump will be able to extract, and the more efficiently it will operate.

Natural energy from the surroundings is drawn into the heat pump by a large fan. This is initially absorbed by the heat pump’s first heat exchanger, known as the ‘evaporator’, which contains a highly conductive cold refrigerant liquid. Another small pump is then used to circulate the energy around the heat pump, allowing the liquid to absorb heat energy from the air. As this happens, the liquid turns from a cold liquid into a cool vapour.

This cool vapour then passes through a compressor which squeezes it and significantly raises the temperature of the vapour. This now hot vapour passes through a pool water heat exchanger where the heat transfers to the pool water, heating it up. As it offloads its heat, the hot vapour condenses back into a cool liquid, before passing through an expansion valve to convert back into a cold liquid, and restarting the cycle.

Why they’re a great choice

The most noteworthy feature of a heat pump is the fact that they can actually extract more heat energy than they consume during their operation. This principle is known as the ‘coefficient of performance’ (COP) and explains why heat pumps can report COPs higher than 100%.

As an example, a heat pump with a COP of 5 means that it can extract 5kWh of heat from the air for every 1kWh it uses in electricity to run. This effectively means it has a COP output of 500%.

The latest generation of air-to-water heat pumps, such as the ones we use in our pools, achieve COPs of between 5.22 and 5.73 (i.e. 522% and 473% efficiency output). When compared to the maximum COP value of 1.09 for modern gas boilers, it’s not hard to see why heat pumps are growing in popularity.


  • Affordable system with a good return on investment compared with fossil fuel heating systems.
  • Full-scale primary heating if the output capacity is +0.35 kWh per m³ pool water.
  • Superb ecological credentials, and suitable for achieving an entirely energy self-sufficient pool with photovoltaic panels.
  • Works using heat in ambient air, so it heats your pool even when the sun isn’t shining.
  • Water can be heated at night and utilise cheaper electricity night rates.


  • It is usually not possible to use a heat pump to heat the water to swim in the winter, as its efficiency diminishes when the temperature drops below 10°C.
  • Its qualities are enormous but difficult to explain to a general audience
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Secondary Heating Systems

1. Solar panels

Solar panels capture the rays of the sun and, using a liquid, store their energy in a primary circuit. They then convert this energy into heat through a heat exchanger, before delivering the heat energy to the pool water. Since they use a natural resource, they are one of the most well-known forms of renewable energy.


  • Frost-free system due to added glycol in the primary circuit.
  • Aesthetic designs.
  • Relatively little area needed in comparison with other solar systems (i.e. 12 to 14m² to heat 50m³).
  • Work with diffused light as well as the sun.


  • Extremely expensive system and take a long time to break-even (at least 12 to 18 years).
  • Expensive plate heat exchanger required to achieve yield during spring and autumn.
  • Despite the high investment, this system only achieves its expected output under optimal conditions (30° slope facing south).
Beautiful XL-Trainer Pool in Woldingham, Surrey - Gallery Image 6

2. EPDM Rubber or PE Plastic Sun Mats

Sun mats can be installed on flat or sloping roofs, as long as there is sufficient space. Most manufacturers advise that 67% of the pool area should be used as a mat area. However, we think this is wrong – from our experience we suggest that 100% is more realistic and 150% is a more optimal area.

The black flow-through mats directly receive the pool water and heat it with their warmed-up plastic covering. A simple measurement and regulation technique is then used to heat the pool water.


  • Affordable heating system.
  • Simple technology.
  • Aesthetic, due to invisible installation on a flat roof.
  • Limited break-even time (3 to 4 years).


  • Not very aesthetic on sloping roofs.
  • Sensitive to storm damage.
  • Only work when the sun is available.
  • Obstruct any repair work needed on the roof.
  • Do not allow pool water temperatures to be regulated.

How to choose the best heat pump

  • Pick a model with titanium heat exchanger. This material offers the best heat transfer, which is vitally important for the heat pump’s working efficiency. It is also resistant to many of the chemicals used to maintain the pool’s water.
  • Ask for COP values 15°C temperature. The best air-to-water heat pumps produce a COP of at least 5.2 at this temperature.
  • Ask for a written guarantee of 5 years. The legal minimum is 2 years but, as components like heat exchangers are very expensive to replace, better manufacturers back their quality with a longer warranty length.
  • Be wary of cheap products. While the initial cost may be tempting, you’ll end up paying out when the product breaks down or becomes less efficient in time.
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