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Heat Pump Buffer Vessel Calculator
Check if your heating system needs a buffer vessel to prevent heat pump short cycling. Calculate minimum system volume and recommended buffer size.
Rated heat output
Auto-estimate uses ~6 litres each (UK panel-rad average) — confirm real volume if known
Override auto-estimate
Approx 25 litres per manifold
Number of heating zones
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How We Calculate This
This calculator checks whether your heating system has sufficient water volume for the heat pump and recommends a buffer vessel size if needed.
Minimum volume check
These litres-per-kW figures are a rule of thumb for a first estimate only — always confirm the heat pump manufacturer’s stated minimum system volume, since MCS MIS 3005-D requires you to design to manufacturer data and the true minimum is set by the defrost cycle (and rises at lower minimum flow temperatures). BS EN 15450 gives a 12–35 L/kW range for buffer storage.
- Fixed speed HP: ~15 litres per kW (rule of thumb)
- Inverter HP: ~10 litres per kW (rule of thumb)
- System volume: Sum of radiator volumes + UFH + pipework (auto-estimate uses ~6 L/radiator — confirm by drain-down or manufacturer rad data)
- Buffer size: Deficit between the minimum volume and the actual system volume
The “worst-case cycle” figure is the shortest possible cycle — the time to raise the system water by 5°C at full rated output with no heat emitted. A modulating inverter at low load runs roughly 2.5–5× longer than this, so treat it as a conservative lower bound, not the expected run time.
Frequently Asked Questions
Heat pumps need a minimum water volume in the system to prevent short cycling (frequent on/off). Short cycling damages the compressor, reduces efficiency, and shortens lifespan. A buffer vessel adds thermal mass to the system, allowing the heat pump to run for longer, more efficient cycles.
There is no single regulated figure — the binding minimum is the heat pump manufacturer’s stated minimum system volume, which MCS MIS 3005-D requires you to design to. As a rule of thumb, inverter models are often sized at about 10 litres per kW and fixed-speed at about 15 L/kW, and BS EN 15450 gives a 12–35 L/kW range for buffer storage. So a 10 kW inverter ASHP is commonly taken to need roughly 100 litres of total system volume (radiators, pipework and any buffer combined) — but always confirm against the manufacturer’s volume-versus-flow-temperature table.
The real driver of the minimum volume is the defrost cycle: when an air source heat pump reverses to clear ice from the outdoor coil, it draws heat back out of the system water. Enough thermal mass must be available to run that defrost without the system temperature collapsing or the unit locking out. The colder the minimum flow temperature you run at, the more stored volume you need — manufacturer tables typically quote a much larger minimum volume at a 15°C minimum flow than at 25°C. Use the manufacturer’s volume-versus-flow-temperature data where available rather than a generic litres-per-kW figure.
Inverter heat pumps can modulate output down to 30-40% of rated capacity, reducing the need for a buffer. However, if the system has zone valves that can close off large sections, the remaining open volume may be insufficient. A small buffer is still often recommended.
Short cycling is when a heat pump starts and stops frequently, typically running for less than 5-10 minutes per cycle. This is caused by insufficient thermal mass in the system. It increases compressor wear, reduces COP, and can trigger lockout errors on some models.
A 2-port buffer (series connection) maintains the heat pump flow rate through the buffer and is recommended for weather compensation systems. A 4-port buffer (parallel connection) decouples the heat pump circuit from the heating circuit, allowing different flow rates. Most domestic installations use 2-port.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.