Heating · Glossary
Buffer Tank
A small thermal store (typically 25-100L) that decouples a heat pump from emitters, providing cycle protection and defrost reserve.
A buffer tank is a small unvented hot water vessel placed between the heat pump and the heating circuit. Its job is hydraulic separation and short-cycle protection: when zone valves close (e.g. all rooms reach setpoint), the buffer absorbs the heat pump's minimum modulation output for several minutes, preventing rapid on/off cycling that would shorten compressor life and tank wear.
Sizing depends on heat pump capacity and emitter circuit design. Rule of thumb: 10-25L per kW of heat pump output, so an 8kW Daikin Altherma might pair with a 50L buffer. Some manufacturers (Vaillant aroTHERM plus, Mitsubishi Ecodan PUZ-WM, Viessmann Vitocal 250-A) include sufficient internal water volume to skip the buffer entirely if the emitter circuit volume is large enough.
Buffers also provide defrost reserve: when the heat pump pauses to defrost the outdoor coil (typically 5-10 mins every hour or two in cold humid Cornwall mornings), the buffer keeps emitters warm. See our heat pump installation page.