Most heat pump installations benefit from a buffer tank, and systems with zoned circuits, low water volume or cold-climate defrost cycles effectively require one. Only open systems without zone valves that meet the manufacturer's minimum water volume and are driven by a modulating inverter heat pump can safely run buffer-free. Typical sizing is 20–50 litres per kW of heat pump capacity.
The three problems a buffer solves
1. Short-cycling. A compressor that starts and stops every few minutes wears out years early and wastes electricity on every start. When zone valves close or thermostats are satisfied, system water volume shrinks; the buffer keeps enough volume in play for run times of 10 minutes or more.
2. Defrost energy. In air-source units, defrost reverses the cycle and pulls heat from the heating water. Manufacturers specify a minimum system volume for safe defrost — often 10–20 L per kW. Without it, rooms cool noticeably and the unit can trip on low flow temperature.
3. Hydraulic separation. The heat pump wants a constant, generous flow; thermostatic valves throttle emitters unpredictably. A buffer (or low-loss header) lets each side run its own flow — mandatory when combining the heat pump with solar thermal or an existing boiler.
When you can safely skip the buffer
- The whole house is open-zone underfloor heating (no zone valves, or one large open reference zone),
- the heat pump is a modulating inverter unit able to ramp down to the building's minimum load,
- pipework volume alone meets the manufacturer's minimum system volume, and
- your climate makes defrost events rare or mild.
Meet all four and a buffer adds cost and standing losses without proportional benefit. Miss any one and the buffer earns its place many times over in compressor lifetime alone.
What size buffer for a heat pump?
| System type | Buffer sizing | 8 kW example |
|---|---|---|
| Open underfloor, inverter HP | 12 – 20 L/kW (or none) | 100 – 150 L |
| Radiators, zoned circuits | 20 – 50 L/kW | 200 – 400 L |
| Cold climate, long defrosts | ≥ 30 L/kW | 300 L + |
| Bivalent (HP + boiler/solar) | Per hydraulic design | 300 – 500 L |
Solimpeks Solibuffer vessels cover 50–1,000 L with ErP-classified insulation; the 50 L model suits minimum-volume defrost duty in compact systems.
The efficiency debate, honestly
Badly integrated buffers can cost efficiency — mixing flow and return water raises the temperature the heat pump must produce. That is an argument for correct piping (two-pipe vs four-pipe configuration chosen deliberately, sensors placed per the scheme), not against buffers. A cycling compressor loses more than a well-piped buffer ever will.
Frequently asked questions
Can the buffer tank also provide my domestic hot water?
Not directly — buffer water is not potable. Combination stores solve this with a hygienic coil or internal tank, letting one vessel do heating buffer and DHW duties with heat pump-friendly large exchanger surfaces.
Does an inverter heat pump remove the need for a buffer?
It reduces the cycling argument when the unit can modulate below the building load, but does nothing for defrost volume or hydraulic separation. Zoned systems still want the buffer.
Two-pipe or four-pipe buffer connection?
Two-pipe (buffer in the return, in series) preserves flow temperature and suits volume/defrost duty; four-pipe fully separates circuits but mixes temperatures. Follow the heat pump manufacturer's hydraulic schemes.