Sizes a buffer vessel or volumiser against a heat pump's minimum system water volume for defrost and anti-cycling duty. Enter the actual system volume directly, or compute it live from pipework, emitters and plant.
Tender-stage sizing check. Always verify against the heat pump manufacturer's own installation manual.
2-port vs 4-port configuration is advisory only and does not change the sizing result. For design guidance only. Always verify with a qualified engineer.
About this heat pump buffer vessel calculator
This free heat pump buffer vessel and volumiser sizing calculator checks whether a wet heating system holds enough water for a heat pump’s defrost and anti-cycling duty, and if not, sizes the buffer or volumiser needed to make up the shortfall. You enter the heat pump output and either type the actual system volume in directly or have it computed live from the pipework, emitters and plant. The minimum-volume basis follows the BS EN 15450:2007 12–35 L/kW band. It is aimed at UK heat pump designers, M&E engineers and installers at tender or design stage. Everything runs in your browser. Nothing is uploaded.
How the buffer vessel is sized
The minimum required system volume is heat pump output (kW) × minimum-volume basis (L/kW), the basis coming from the BS EN 15450:2007 band: presets of 25 L/kW for air source and 12 L/kW for ground source, or a custom manufacturer figure. The deficit = required − actual system volume, and the recommended vessel is the next standard size up from a 25/40/50/80/100/200 L ladder. When computed rather than entered, the actual volume uses the same method as the System Volume & Water Content calculator: each pipe tier is V = (π ÷ 4) × di² × L summed across the DN sizes, plus the emitter and plant contents. The 2-port volumiser versus 4-port buffer choice is advisory only and does not change the sizing.
Frequently asked questions
Why does a heat pump need a minimum system volume?
A heat pump needs enough water in the circuit to run a reverse-cycle defrost without the flow temperature collapsing, and to avoid short-cycling on low load. If the pipework and emitters do not hold that volume, a buffer vessel or volumiser is added to make it up. Which is exactly the shortfall this tool calculates.
Why is ground source 12 L/kW and air source 25 L/kW?
Air source units defrost and are more prone to short-cycling, so they sit at the higher end of the BS EN 15450 band (a 25 L/kW default here); ground source units do not defrost and take the lower end (12 L/kW). Always use the manufacturer’s own figure where the installation manual states one. Enter it via the custom basis.
Should I fit a 2-port volumiser or a 4-port buffer?
A 2-port volumiser simply adds volume in-line with no hydraulic separation, and is the safer default when the only problem is insufficient minimum volume. A 4-port buffer (or low-loss header) also separates the heat-pump primary from the system secondary, but mismatched flows cause internal mixing that raises the return temperature. Only justify it by a genuine separation need. The choice is advisory here and does not change the litres required.