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A pool in the backyard is a real highlight, but the water is often too cold for a relaxing swim—however, there are various ways to heat it effectively. There are cost-effective DIY solutions and professional heating systems.
Heating Pool Water with the Sun and a Cover
When heating pool water, smart use of space and thorough insulation quickly pay off.
- Sizing solar collectors correctly: Install flat-plate or tube collectors whose surface area is at least half—preferably three-quarters—of the water’s surface; on a clear June day, the system can transfer up to 0.6 kWh of heat per square meter. Mount the elements at a 25–35° angle facing south, lay out the pipe run with an inner diameter of 32 mm, and connect a three-way bypass so that sensors only route the water over the roof when the return flow is more than 3 K (Kelvin) cooler.
- Solar tarp + solar mat as a heating duo: Combine a 400 µm bubble tarp with the absorber: During the day, the mat absorbs radiation and transfers up to 5 K into the circulation system; at night, the floating sheet retains up to 95% of this energy in the pool, while evaporation drops dramatically. This means a short filter cycle in the morning is enough to distribute the heated water evenly.
- Use the cover consistently: Roll the cover back over the water immediately after swimming. A sturdy manual or solar reel makes daily handling easier, and thanks to UV stabilizers, the cover will last four to five seasons. When wind speeds reach 25 km/h or higher, secure the edges with tension straps to prevent cold air from seeping underneath.
- Adjust the pump operation to the sun: Set the variable-speed filter pump to a low speed (approx. 1,000 rpm) between 11 a.m. and 4 p.m. The slower flow rate is sufficient to feed the absorber, saves up to 60% in electricity compared to full load, and reduces noise.
- Vent the collector in the spring: Briefly open the upper header pipe until water flows out without bubbles. Trapped air would reduce heat transfer and create hot spots that cause the plastic to age.
- Install a windbreak: Install a 1.6 m high polycarbonate wall on the weather-facing side. Even a 5 km/h reduction in wind speed cuts convective losses in half; the water barely cools down at night.
Technology for a Constant Temperature
A weather-independent heating system supplements solar heat and keeps the pool at a stable temperature during cool weeks.
- Select a heat pump suitable for the pool’s volume: Calculate approximately 5 BTU (British Thermal Unit) per liter of water; a 40 m³ pool therefore requires about 200,000 BTU ≈ 12 kW of heating capacity. In practice, manufacturers of pool heat pumps specify the heating capacity in BTU per hour (BTU/h).
- Make sure to choose a unit whose manufacturer’s specifications clearly state the maximum volume; otherwise, the pump will run for long periods and operate inefficiently. Heat pumps work most effectively when the air temperature is above 20 °C; therefore, plan to run them preferably during the day and link the controller to the pool cover.
- Use heat-cool models for the height of summer: Reversible units reverse the cooling cycle and lower the water temperature on hot days. This is gentler on children’s skin and prevents algae blooms when the water temperature reaches 30 °C.
- Electric pool heater for small pools: A 3 kW instantaneous water heater is inexpensive to purchase but consumes exactly 3 kWh of electricity per hour of operation; at 0.30 € /kWh, that amounts to 0.90 € per hour. Therefore, the unit is particularly worthwhile for temporary wading pools up to about 10 m³ that are heated only on weekends.
- Connecting a heat exchanger to the home heating system: A stainless steel plate heat exchanger transfers energy from the building’s gas, oil, or air-source heat pump. Connect the primary side with a 60 °C flow temperature, a superglue seal, and its own circulation pump; the exchanger then provides an increase of about 2 K per hour for 40 m³ of water.
- A cover saves energy and chemicals: The cover not only reduces heat loss but also cuts evaporation by up to 95%. As a result, the water stays in the pool longer, eliminating the need to top off with fresh water and add salt to balance the chemistry; you’ll lower both operating and heating costs.
- Retrofit pool shell insulation: If you’re excavating the soil around the pool anyway, attach 50 mm XPS panels to the outside of the walls and floor. The additional insulation significantly shortens heating times and reduces the operating time of the heating source.
