Why do heat pumps need buffer tanks?

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.

How do I size a buffer tank for a heat pump?

A common rule of thumb is 10-15 litres per kW of heat pump output. For precise sizing: Buffer volume = (surplus kW × minimum run time × 60) ÷ (4.186 × ΔT). An 8 kW heat pump with 5 kW demand and 10°C ΔT needs about 43 litres, but the 10L/kW rule suggests 80 litres.

What is the difference between a buffer tank and a thermal store?

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.

What temperature should a heat pump buffer run at?

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).

Can I use a buffer tank for multiple heat sources?

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.

TradeCalculator

Thermal Store Calculator — Buffer Tank Sizing

Size buffer tanks and thermal stores for heat pump anti-cycling protection. Calculate charge/discharge rates and multi-source priority.

Heat Pump Size

Heat pump rated output

Minimum Run Time

Minimum compressor run time (20 mins typical)

Heat Demand

Current / minimum heat demand on system

Flow Temperature

System flow temperature

How We Calculate This

This calculator sizes buffer tanks and thermal stores to prevent heat pump short-cycling. It calculates the minimum volume needed based on the surplus energy that must be absorbed during the minimum compressor run time.

Formulas

Surplus kW: Heat pump output − current heat demand
Buffer volume: (Surplus kW × run time × 60) ÷ (4.186 × ΔT)
Minimum volume: 10L per kW heat pump output
Energy stored: (Volume × 4.186 × ΔT) ÷ 3600 kWh

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.