Start from the heat pump's minimum system volume
Check the heat pump manufacturer's installation manual for the minimum volume of water the system must hold. Then count the water already in the radiators or underfloor pipes. A 2013 laboratory study for the Department of Energy and Climate Change found that heat pump radiators are often large enough to hold that volume on their own. Where they are not, it recommends making up the shortfall with a two pipe buffer in the return to the heat pump.
Allow for valves too. Thermostatic radiator valves and underfloor actuators cut the volume when they close, so the water you can count on is what stays open.
Kensa's 10 litres per kW, and where it applies
Kensa Heat Pumps, a UK ground source maker, sizes buffers at 10 litres per kW of ground source heat pump output. For its twin compressor units only one compressor counts: a 16kW Twin is two 8kW compressors, so it needs 80 litres.
That figure is Kensa's, for its own ground source heat pumps, published in 2016. For any other heat pump, use the figure in its manufacturer's manual; the Advance Appliances buffer installation guide says to always follow the heat pump maker's instructions.
Advance buffer sizes
| Model | Capacity | Size (mm) | Heat loss |
|---|---|---|---|
| BFS 20 | 20 litres | Wall hung | 41W, class C |
| AA BFS 100 | 100 litres | 420 high x 560 x 600 deep | 69W, class C |
| ECO BFS 150 | 150 litres | 580 x 1165 | 42W, class A |
| ECO BFS 200 | 200 litres | 580 x 1505 | 42W, class A |
| ECO BFS 250 | 250 litres | 580 x 1755 | 48W, class A |
| ECO BFS 300 | 300 litres | 580 x 2065 | 52W, class A |
Cylinder sizes are diameter x height. The 20 litre buffer adds volume only and has no immersion heater. A 500 litre stainless buffer is also on the data sheet; call Advance on 01543 377723 about it. See the stainless steel buffer range.
Allow for the extra water
A buffer adds to the water you dose with inhibitor and to the expansion the vessel has to take, so the installation guide asks for both to be worked out on the whole circuit. Short of floor space? The 100 litre buffer is box shaped so a cylinder of up to 300 litres can stand on top; see standing a cylinder on the 100 litre buffer.
Still deciding whether you need a buffer at all? Read do I need a buffer store with my heat pump.
- Start with the heat pump manufacturer's minimum system volume, then count the water already in the radiators or underfloor pipes.
- Only count what stays open: closed radiator valves and actuators reduce the volume.
- Kensa sizes buffers for its ground source heat pumps at 10 litres per kW; other heat pumps have their own figures.
- Advance buffers come in 20, 100, 150, 200, 250 and 300 litres, with 500 litres on request.
- Dose inhibitor and size the expansion vessel for the whole circuit, buffer included.
Managing Director, Advance Appliances. 20+ Years experience