What are the different options for heating my pool?
There are two different types of system for heating pool water, primary and secondary.
A primary system is a system where the bather sets the thermostatic controls at their desired temperature (typically between 22°C and 30°C), and the heating system will, as long as it was correctly sized, specified and installed, operate up to and until the water reaches that desired set temperature, irrespective of external factors like the ambient temperature, the weather, amount of sunshine etc.
A secondary system will raise the water temperature, but is dependent on outside factors which will impact its performance, such as the weather, ambient temperature, sunshine etc. They generally can not guarantee that the pool water will actually reach the required temperature. A secondary system is generally installed as a back-up to a primary system, as they normally offer considerable running cost savings when conditions are in their favour, with the primary system able to take over and make up any shortfall when they are not. For example, an air-source heat pump (secondary system) works very efficiently and cost-effectively when the weather is good and the ambient temperature relatively high. However, to heat the pool up from cold, or during periods when the ambient temperature is low and a heat pump does not work as effectively, a gas heater (primary system) can be switched on to make sure the desired set temperature is reached.
Primary heating systems:
High performance fuel oil boiler
In dedicated swimming pool oil heaters, domestic fuel oil is burned, heating up a fluid in a closed loop to a high temperature. This fluid is circulated through a series of stainless or titanium tubes which are in contact with the pool water, which heats up as a result.
- Rapid heating of the pool due to the high temperatures in the heating circuit.
- Heat source always available, irrespective of weather conditions, even at extremely low outside temperatures.
- Economic investment, especially if the existing boiler is not too far from the pool system.
- Combines perfectly with alternative systems such as secondary renewable energy systems.
- Relatively high running costs when used as the sole heating source.
- Fossil-fuel powered, so high carbon footprint.
- Rising cost of the fuel oil.
High performance gas boiler
The same working principle as with a fuel oil boiler (above), but using natural gas, LPG or butane as the fuel source.
- The same advantages as the fuel oil boiler (above).
- The gas boiler is compact, occupying very little space.
- Lower emissions than an oil boiler.
- 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 offer 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 this solution is extremely expensive to run in comparison.
Air/water heat pump.
Heat pumps are gaining much coverage in the media for their green credentials and efficient performance, and are therefore also strongly gaining in popularity for heating pools, especially as the costs of fossil fuels continue to rise.
An air to water heat pump works by absorbing heat from the ambient air and transferring 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 though, clearly, the warmer the air, the more heat is available to extract, and the more efficiently the heat pump will operate.
This free natural energy from the surroundings is drawn into the heat pump by a large fan and is initially absorbed by the heat pump’s first heat exchanger, called the ‘evaporator’. The evaporator contains a highly conductive cold refrigerant liquid, like in your fridge at home, which is circulated around the heat pump by a small pump. As the liquid absorbs heat energy from the air which has been drawn across it, it turns from a cold liquid into a cool vapour.
This cool vapour is circulated and then passes through a compressor. As the name suggests, the compressor squeezes the cool vapour, raising its temperature significantly to a hot vapour. This hot vapour then passes through a pool water heat exchanger where heat is transferred from the hot vapour to the pool water, heating the pool water up in the process. As it gives up its heat, the hot vapour condenses back to a cool liquid. It then passes through an expansion valve where it is allowed to expand back to a cold liquid, and so the cycle starts again.
The most noteworthy feature of a heat pump, and the principle feature that has seen their rise in popularity, particularly in the face of ever rising energy costs and greater public awareness of environmental issues, is that they can extract more energy in the form of heat than they consume in the course of their operation. This is referred to as the ‘coefficient of performance’ (COP). This is why heat pumps can report COPs higher than 100%. For example, a heat pump with a COP of 5 means that the heat pump is in theory capable of extracting 5kWh of heat from the air for every 1kWh it uses in electricity to run itself. That corresponds with an output of 500%.
From an ambient air temperature as low as 15°C the output of the latest generation air to water heat pumps, like the ones installed by Compass Pools UK, achieve COPs of 5.22 to 5.73. If you compare this with the theoretical maximum COP value of 1.09 for a modern gas boiler, you get an immediate sense as to why heat pumps are growing in popularity. As the range of high-quality heat pumps for pool applications is widening, capital investment costs have also reduced.
- Affordable system with a good return on investment compared with fossil fuel heating systems. Typical break-even point is sub 4 years.
- Full-scale primary heating if the output capacity is +0.35 kWh per m³ pool water, for example, for +16 kWh for 50m³ pool water.
- Superb ecological credentials, and suitable for achieving an entirely energy self-sufficient pool with suitably sized photovoltaic panels.
- Works using heat in ambient air, so it still heats your pool even when the sun isn’t shining.
- Because it uses heat from ambient air, you can also heat your pool water at night, benefitting from the cheaper electricity night rates.
- It is usually not possible to use a heat pump to heat the water to swim in the winter, as the output of a heat pump, and therefore its efficiency, rapidly decreases when the ambient temperature drops below 10°C.
- The range of qualities is enormous and difficult to explain to the layman?
What advice can we give you in the choice of heat pump?
- Always opt for a model with a titanium heat exchanger. This material offers the best heat transfer, which is vitally important for the heat pump’s working efficiency, and it is also extremely highly resistant to pool chemicals in the water.
- Ask for written confirmation of the COP values at an ambient air temperature of 15°C; the better air to water heat pumps produce a COP of 5.2 or more at this temperature.
- Ask for a written guarantee for 5 years instead of the legal minimum required 2 years. Components like heat exchangers are very expensive to replace, and the better quality manufacturers back their quality on the length of warranty.
- As with any complicated and expensive piece of capital equipment, be very wary of cheap products. Whilst the initial cost may be tempting, you’ll usually pay in terms of poorer efficiency and more, potentially expensive, breakdowns.
Secondary heating systems:
- Calculation example for start-up air/water heat pump 4 to 12 kWh for heating 50 m³.
These devices are capable of producing the same output as under point 3 above for additional heating of the pool. The disadvantage, however, is that you give up having the pool water at a constant temperature. The heat loss from the pool will not always be compensated for.The device will also be constantly working to achieve the required comfort temperature.
- Solar panels on the roof or in the garden.
Solar panels work with a liquid in a closed circuit that capture the rays of the sun and stores them in the primary circuit to transfer via a heat exchanger to heat the pool water.
- Frost-free system due to added glycol in the primary circuit.
- Aesthetic designs.
- Relatively little area needed in comparison with other solar systems. 12 to 14m² to heat 50m³.
- Also works with diffused light, albeit with a lower output.
- Extremely expensive system and a long break-even time, at least 12 to 18 years, taking the investment into account.
- Expensive plate heat exchanger required to achieve any yield in spring and autumn.
- Despite the high investment, this remains a secondary system that only achieves its expected output under optimal conditions, on a 30° slope facing south.
- EPDM rubber or PE plastic sun mats.
Sun mats can be installed on either flat or sloping roofs as long as there is enough space. Manufacturers advise 67% of the pool area as mat area—from experience we suggest 100% as realistic and 150% as an optimal area.
The black flow-through mats directly receive the pool water and heat it with their warmed up plastic covering without any further loss in an extra heat exchanger. A simple measurement and regulation technique heats the pool water.
- Affordable heating system, simple technology, installed and working pool, 4m x 8m.
- Aesthetic, due to invisible installation on a flat roof.
- Limited break-even time, 3 to 4 years, allowing for the investment.
- Unaesthetic when installed on a sloping roof.
- Sensitive to storm damage.
- Does not work with diffused light, only full sun.
- Effective functioning only 2 to 3 months at our latitude.
- Obstructs repair work to the roof.
- Does not allow the pool water temperature to be regulated.