When a heat pump system can do without one
Advance Appliances sells buffer stores, and many heat pump systems still do not need one.
- A 2013 laboratory study for the Department of Energy and Climate Change found that radiators on heat pump systems tend to be larger, so the water they hold is often enough without a buffer.
- The same study found buffers less relevant with inverter driven heat pumps, which can turn down to about 30% of their rated output, so cycling is less of a problem.
- Kensa Heat Pumps says you do not have to use one, provided enough of the heating system stays uncontrolled, with no radiator valves or actuator heads. It puts that at around 25% of the total system volume.
When a buffer earns its place
- The heat pump is fixed speed. The DECC study found buffers more important here, especially with air source.
- The system holds less water than the heat pump needs. The study recommends adding a two pipe buffer in the return to the heat pump.
- Most of the system can shut off. Thermostatic radiator valves and underfloor actuators cut the volume and the flow as they close, which is why the study says to use TRVs cautiously.
- A wood burner or biomass boiler needs somewhere to store its heat. Advance's data sheet says buffers can be used with these as accumulators.
Check the heat pump manual first
Look up the minimum system volume and minimum flow rate in the heat pump's installation manual. Not all heat pumps connect to a buffer the same way, and the Advance buffer installation guide says to always refer to the heat pump manufacturer's instructions. Do that before deciding, and before you size anything.
If you do fit one
- Add inhibitor for the whole volume, the heating circuit plus the buffer.
- Size the expansion vessel for the expansion of the full circuit, buffer included.
- On a sealed system, fit a 3 bar relief valve and overheat protection in open circuit.
- Leave access for maintenance. Advance recommends periodic checks of inhibitor level and expansion vessel pressure.
The Advance range runs from the wall hung 20 litre buffer, which adds volume only, to stainless steel buffers from 150 litres. For sizing, see what size buffer store do I need.
key takeaways
- Many heat pump systems work without a buffer, especially with inverter driven heat pumps and larger radiators.
- A buffer helps where the system holds too little water, where valves can shut most of it off, or with a fixed speed heat pump.
- Buffers can also store heat from wood burners and biomass boilers.
- Check the heat pump manufacturer's minimum system volume and flow rate before deciding.
- If you fit one, dose inhibitor and size the expansion vessel for the whole circuit.
Reviewed by Geoff Egginton
Managing Director, Advance Appliances. 20+ Years experience