A volumiser is a small insulated vessel — typically 20–100 litres — piped in series into a heat pump's heating circuit to increase total system water volume. Unlike a buffer tank it has only two connections, provides no hydraulic separation and causes no mixing; it simply adds thermal mass so the compressor cycles less often and the defrost cycle has a guaranteed energy reserve.
How does a volumiser work?
A volumiser adds water volume to a heating circuit without changing its hydraulics. All system flow passes straight through the vessel — in one connection, out the other — so there is no blending of flow and return and no second circuit to balance. The extra litres act as thermal inertia: the heat pump takes longer to satisfy the water content, so each compressor run is longer and starts are fewer, and when the unit reverses for a defrost cycle there is enough warm water in circulation to melt the evaporator ice without chilling the rooms. Because the vessel is in-line rather than parallel, the flow temperature reaching the radiators is exactly the temperature leaving the heat pump — a key efficiency advantage over poorly piped buffers.
Why do heat pumps need minimum system volume?
A compressor lives longest on long, steady runs, and an air-source unit needs a reserve of heat it can borrow during defrost. Manufacturers therefore specify a minimum open water volume — commonly in the range of 10–20 litres per kW of heating capacity, with the exact figure stated in each unit's installation manual. Modern radiator circuits with zone valves, or microbore pipework, often hold too little water to meet it. The symptoms of a volume-starved system are rapid start-stop operation (short cycling) and defrost faults; as a reference point, EHPA design guidance and manufacturer field data consistently link frequent cycling to measurably reduced seasonal efficiency and compressor life.
Volumiser vs buffer tank: what is the difference?
| Feature | Volumiser (2-port, in-line) | Buffer tank (4-port) |
|---|---|---|
| Connections | 2 | 4 |
| Adds system volume | Yes | Yes |
| Hydraulic separation | No | Yes |
| Mixing / flow temperature penalty | None | Possible if flows are unbalanced |
| Extra pump needed | No | Usually yes (secondary side) |
| Typical size | 20–100 L | 100–1,000 L |
The rule of thumb: if the only problem is missing litres, fit a volumiser; if pumps or multiple generators also need decoupling, fit a buffer tank. The trade-offs are compared in detail in Buffer tank vs volumiser: what is the difference?
Where should a volumiser be installed?
Most manufacturers specify the heating return, just before the heat pump inlet. Return-side placement keeps the vessel at the coolest circuit temperature, which minimises standing losses, and guarantees the defrost reserve sits immediately upstream of the unit where it is drawn from. Flow-side installation is possible and is sometimes preferred where the volumiser doubles as an air separation point, but a return-side vessel is the default. The vessel must never be valved off from the heat pump — it has to remain in the permanently open part of the circuit.
When is a volumiser enough on its own?
When the system has one heat pump, one pump set, and a distribution circuit that stays sufficiently open — for example underfloor heating with most loops uncontrolled, or radiators without motorised zone valves. In that situation an inverter heat pump modulates down to match the load and the volumiser quietly covers the volume shortfall. Once zone valves can shut off large parts of the circuit, minimum flow — not just minimum volume — becomes the problem, and a 4-port buffer or low loss header arrangement is the safer engineering answer.
Frequently asked questions
Is a volumiser the same as a buffer tank?
No. A volumiser is a two-port in-line vessel that only adds water volume, with no hydraulic separation and no mixing. A buffer tank is piped in parallel with four connections and decouples the generator circuit from the heating circuits as well as adding volume.
What size volumiser does my heat pump need?
Enough to bring total system volume up to the manufacturer's stated minimum — commonly in the region of 10–20 litres per kW of capacity. Calculate the water content of pipework and emitters, subtract it from the required minimum, and cover the shortfall; 20–100 L vessels handle most homes.
Does a volumiser reduce heat pump efficiency?
No — used correctly it usually improves seasonal efficiency. Because all flow passes straight through, there is no mixing penalty on flow temperature, while longer compressor runs and reliable defrost operation reduce cycling losses.
Should the volumiser go on the flow or the return?
The heating return, just before the heat pump inlet, is the standard position. It runs cooler, so standing losses are lower, and the defrost energy reserve sits exactly where the unit draws it from. Always follow the specific manufacturer's piping diagram.