Hydraulics
Minimum system volume and buffer or volumiser size
- where
- required free system volume (litres)
- manufacturer minimum volume (litres)
- volume guidance per kW (litres/kW)
- heat pump rated output (kW)
- available free volume (litres)
- water in emitters always open (litres)
- water in pipework always open (litres)
- water in the heat pump (litres)
- existing buffer or volumiser (litres)
- extra volume needed (litres)
Explain this equation
Checks whether a heat pump system holds enough circulating water for defrost and to stop short cycling, and how big a volumiser or buffer is needed if not.
The requirement is the larger of the manufacturer's minimum and any litres-per-kW rule you are using. Only water that is always connected to the heat pump counts: emitters with no TRV or zone valve, the pipework to them, the unit's own water and any existing buffer. The shortfall, if any, is the difference, never less than zero.
It is a simple volume budget: requirement minus what is always available.
Sense check: an 8 kW unit needing 40 litres, with 33 litres always open, needs about 7 litres more.
Watch out: water behind closed TRVs or zone valves does not count, because it can be shut off just when a defrost needs it.
Checks there is enough water always open to the heat pump for defrost and to stop short cycling, and sizes a volumiser or buffer if there is not.
Enter your values to see the result.
Good to know
Use the manufacturer’s figure first: for example 40 litres for Ideal Logic Air 8/10 kW and 30 / 40 / 100 litres for Worcester CS2000AWF 4–6 / 8–16 / 18–30 kW. The default 5 litres per kW is the Ideal 8 kW requirement expressed per kW; set it to 0 if you only use the manufacturer minimum. Count only water that cannot be shut off by TRVs or zone valves. The Energy Saving Trust toolkit notes the requirement rises as the minimum flow temperature falls.
Common mistake
Counting the whole system volume. Rooms closed by TRVs or zone valves do not help during defrost.
Limits
A volume check only. Some heat pumps also set a minimum emitter output at the lowest flow temperature; follow the installation manual and MIS 3005-D.
Sources
- Ideal Heating Logic Air heat pump installation manual (design flow rates 22.9 l/min at 8 kW and 28.7 l/min at 10 kW, i.e. 5 K ΔT; minimum free system volume 24 L (4/5 kW) and 40 L (8/10 kW) for defrost)
- Worcester Bosch Technical Bulletin TB 0186 (02/2025): minimum water content for defrost, 30 L (4–6 kW), 40 L (8–16 kW), 100 L (18–30 kW)
- Energy Saving Trust heat pump installer toolkit, 3.3 System volume and frost protection (minimum volume set by the minimum flow temperature; volumiser or buffer when pipework and open emitters are not enough)
- MCS MIS 3005-D Heat pump design standard (heat pump to meet the design heat loss at the design outdoor temperature; room temperatures and outdoor design temperatures; design to BS EN 12831-1)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.