Heating · Glossary
Volumiser
A small (12-25L) inline thermal vessel used as a lightweight alternative to a full buffer tank where minimum heat pump system volume is the only concern.
A volumiser is a smaller, simpler cousin of a buffer tank. Where a buffer provides hydraulic separation between the primary (heat pump) and secondary (emitter) circuits, a volumiser sits inline on the primary loop and exists solely to add water volume.
Most monobloc heat pumps specify a minimum system water volume — often 30-50L for an 8kW unit — to ensure the compressor does not short-cycle and the defrost cycle has reserve heat. If your radiator and pipework circuit already contains enough water, no buffer or volumiser is needed. If it falls a few litres short, a 25L volumiser is the cheapest fix.
Common in Cornwall retrofits where existing radiators are oversized (a happy accident in many older houses heated by oil) and only a small extra volume is needed to meet the manufacturer's minimum. See our heat pump installation page.