Why do heat pumps need buffer vessels?

Fixed speed HP: 15 litres per kW minimum system volume
Inverter HP: 10 litres per kW minimum system volume
System volume: Sum of radiator volumes + UFH + pipework
Buffer size: Deficit between minimum volume and actual system volume

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.

How big should a heat pump buffer be?

The general rule is 10-15 litres per kW of heat pump capacity. Fixed-speed heat pumps need more (15 L/kW) than inverter-driven models (10 L/kW). A 10 kW inverter ASHP needs at least 100 litres of total system volume including radiators, pipework, and any buffer.

Do inverter heat pumps need buffers?

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.

What is short cycling?

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.

Should I use a 2-port or 4-port buffer?

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.