Thermal Store Calculator — Buffer Tank Sizing

Heat pumps should run for minimum cycles of 10–20 minutes to operate efficiently. If the heat demand is less than the heat pump output (e.g. mild weather or few zones calling), the surplus energy needs somewhere to go. A buffer tank absorbs this surplus, preventing short-cycling that damages the compressor. Use our Heat Pump Sizing Calculator to size the heat pump correctly first.

Take the larger of two figures. First, the anti-cycling volume: Buffer volume = (surplus kW × minimum run time × 60) ÷ (4.186 × ΔT). Second, a minimum-volume floor that depends on the heat-pump type — about 25 litres per kW for an air-source pump (BS EN 14511 guidance, to cover defrost), or about 10 litres per kW for ground source (Kensa). BS EN 15450:2007 gives a wider 12-35 L/kW design range. Worked example: an 8 kW air-source pump with 6 kW demand, a 20-minute minimum run time and a 10°C ΔT gives an anti-cycling volume of about 57 litres, but the 25 L/kW air-source floor (8 × 25 = 200 litres) governs, so you would fit a 200-litre buffer.

A buffer tank is a simple volume of water added to the heating circuit to prevent short-cycling. A thermal store is more sophisticated — it stores heat from multiple sources (heat pump, solar, wood burner) and can supply both heating and domestic hot water via plate heat exchangers.

The buffer tank operates at the heating system temperature, typically 35-50°C for a heat pump. The ΔT across the buffer is usually 5-10°C. Higher flow temperatures and larger ΔT allow more energy storage in a smaller vessel, but reduce heat pump efficiency (COP).

Yes. A thermal store with multiple coils or connections can accept heat from a heat pump, solar thermal, wood burner, and even off-peak electric immersion. Priority controls determine which source charges the store first, typically: solar → heat pump → backup immersion.