A buffer tank is an insulated water storage vessel installed between a heat generator and the heating circuit. It stores technical (non-potable) heating water, decouples heat production from demand, prevents heat pump short-cycling, and provides the water volume needed for defrost cycles. Typical sizing for heat pumps is 20–50 litres per kW of heating capacity.
What does a buffer tank actually do?
A heating system rarely needs exactly the amount of heat the generator produces at any moment. The buffer tank absorbs that mismatch. Four jobs matter most:
- Preventing short-cycling. Compressors live longest with long, steady runs. A buffer adds system volume so the heat pump starts less often — a major factor in both efficiency and lifespan.
- Guaranteeing defrost energy. Air-source heat pumps periodically reverse to melt ice off the evaporator, drawing heat from the heating circuit. The buffer guarantees that energy is available without cooling the rooms.
- Hydraulic separation. With a buffer, the generator circuit and the emitter circuits can run different flow rates — essential when combining a heat pump with solar thermal or a boiler.
- Storing renewable surpluses. Solar thermal output peaks at midday; demand peaks in the evening. The buffer bridges the gap.
Buffer tank vs hot water cylinder: not the same thing
A buffer tank holds heating water that circulates through the system and is never drawn from a tap. A domestic hot water cylinder holds potable water and must meet drinking-water hygiene requirements — enamelled or stainless surfaces, corrosion protection anodes. Combination stores (such as tank-in-tank designs) put a potable vessel inside a buffer volume to do both jobs. See magnesium anode for how potable tanks are protected.
How big should a buffer tank be?
| Application | Rule of thumb |
|---|---|
| Heat pump, underfloor heating with open zones | 12 – 20 L per kW |
| Heat pump, radiator system / zoned circuits | 20 – 50 L per kW |
| Solar thermal combi system | 50 – 100 L per m² of collector |
| Biomass boiler | 55 – 100 L per kW (often mandated) |
An 8 kW heat pump with radiators therefore typically pairs with a 200–400 L buffer such as the Solibuffer series. Oversizing costs standing losses; undersizing costs compressor cycles — sizing to the actual hydraulic design beats any single rule.
Does every heat pump need a buffer tank?
No — and this is one of the most debated questions in modern hydronics. Systems with large open underfloor circuits and inverter-driven heat pumps can sometimes run buffer-free. But whenever zone valves can close off circuits, radiators hold little water, or defrost reliability matters in a cold climate, a buffer remains the safe engineering answer. The full analysis is in Do I need a buffer tank with a heat pump?
Frequently asked questions
Does a buffer tank waste energy?
A well-insulated buffer loses surprisingly little — modern ErP class B/C vessels hold standing losses to roughly 1.5–2.5 kWh per day for a 300 L tank, usually far less than the efficiency lost to compressor cycling in a buffer-free system that needs one.
Can one tank serve heating and domestic hot water?
Yes — combination or hygienic stores do exactly this, either with an internal potable tank or a stainless corrugated coil in which drinking water is heated instantaneously. This avoids legionella-prone stored volumes.
Where is the buffer installed — flow or return?
Both configurations exist. Return-side buffers mainly add volume for cycling and defrost; flow-side (hydraulic separator) buffers fully decouple generator and emitter circuits. The hydraulic scheme, not habit, should decide.