A volumiser is an insulated vessel plumbed in series on the return that adds water volume without mixing flow and return; a buffer tank adds volume and hydraulic separation, letting zoned, mixed or bivalent circuits each run their own flow — and, correctly piped, it protects efficiency by preventing compressor cycling. Fit a volumiser when the only problem is minimum system volume, typically 10–20 litres per kW. Fit a buffer when circuits need separating.
What is the actual difference?
Both are insulated steel vessels full of heating water. The difference is how they are connected and therefore what they do to the water temperature.
A volumiser has two connections and sits in series, almost always on the return to the heat pump. All the water passes through it. It behaves like a very large, well-insulated piece of pipe: it adds thermal mass, and it does nothing else. Flow and return never meet, so the heat pump still sees the temperature the emitters actually returned.
A buffer tank has four connections and sits in parallel between two circuits — heat pump on one side, heating circuits on the other. It adds thermal mass and hydraulic separation, letting each side run its own flow rate. The price is that water from the flow and the return share one vessel, so they mix.
| Volumiser | Buffer tank (4-port) | |
|---|---|---|
| Connections | 2, in series | 4, in parallel |
| Adds system volume | Yes | Yes |
| Hydraulic separation | No | Yes |
| Mixes flow and return | No | Yes |
| Typical size | 10 – 20 L per kW | 20 – 50 L per kW |
| Efficiency effect | Standing losses only | Standing losses; mixing only if flows are mismatched |
| Best for | Minimum volume and defrost reserve | Multiple zones, mixed circuits, bivalent systems |
Do buffer tanks reduce heat pump efficiency?
Only when they are piped so that flow and return mix. In that case the heat pump may have to produce water 4–5 °C hotter than the emitters need — at 2–2.5% of COP per degree, a 10–12% penalty. Correct piping avoids it.
But the penalty belongs to the configuration, not to the vessel. A two-port buffer in the return is effectively a volumiser and mixes nothing. A four-port buffer with matched primary and secondary flow rates mixes very little. A four-port buffer with an oversized primary pump and a throttled secondary mixes constantly and is the version that earns buffers their bad reputation. Good stratification inside the vessel — bottom entry, low inlet velocity, tall rather than squat geometry — reduces the effect further.
The alternative failure is worse. A compressor cycling six times an hour wears out years early and wastes energy on every start, and a defrost that aborts because there is no stored heat leaves the house cold. See why does my heat pump keep switching on and off?
Which one do I need?
Fit a volumiser when the system has one open heating circuit, a modulating inverter heat pump, and the only shortfall is the manufacturer's minimum water volume for defrost and run time. This covers most single-zone radiator and underfloor retrofits, and it is the cheapest and most efficient answer.
Fit a buffer when circuits genuinely cannot be run from one flow: several zones with valves that close independently, underfloor and radiators at different temperatures, a bivalent arrangement with a boiler or solar thermal input, or a heat pump whose minimum output exceeds the smallest zone's load. See do I need a buffer tank with a heat pump?
Fit neither when the pipework volume already meets the manufacturer's minimum — a large underfloor screed system often does — and there is nothing to separate.
Flow or return: where does a volumiser go?
On the return, in the overwhelming majority of designs, and this is close to consensus among heat pump installers. On the return the vessel buffers the coldest water in the circuit, so its standing losses are smallest and it does not blunt the flow temperature reaching the emitters at the start of a run. On the flow it delays the emitters warming up and leaks heat at the highest temperature in the system. The exception is a defrost-priority design where a manufacturer explicitly specifies flow-side volume; follow the manual when it says so.
How big should each be?
Work from the manufacturer's minimum system volume, not from a rule of thumb. Most air-to-water units specify 10–20 litres per kW of nominal output including pipework and emitter content, which for an 8 kW unit means 80–160 litres total — often only 50–100 litres of extra vessel. Buffers for zoned systems are usually 20–50 L/kW. Monitoring of high-performing domestic systems shows the best results clustering at 15 litres per kW or more of total system volume, so err upward rather than downward within the manufacturer's range.
Solimpeks Solibuffer vessels cover 50–1,000 litres with ErP class C insulation, from a compact 50 L vessel suited to volumiser duty upward; see how many litres of buffer per kW of heat pump? for the full sizing tables.
Frequently asked questions
Is a volumiser better than a buffer tank?
For a single-zone system, yes. A volumiser adds the water volume the heat pump needs without mixing flow and return, so it avoids the 4–5 °C temperature penalty a badly piped buffer imposes. A buffer is the right choice when circuits need hydraulic separation.
Should a volumiser be on the flow or the return?
On the return in almost all designs. There it stores the coolest water in the system, minimising standing losses, and it does not delay the emitters reaching temperature. Only fit one on the flow if the heat pump manufacturer's hydraulic scheme specifically calls for it.
How many litres does a volumiser need to be?
Enough to bring total system volume up to the manufacturer's minimum, typically 10–20 litres per kW of heat pump output including pipework and radiators. For an 8 kW unit that usually means an extra 50–100 litres.
Can I use my hot water cylinder as a buffer?
No. Cylinder water is potable and must stay separate from the heating circuit. A combination or hygienic store solves this properly by keeping the heating buffer water in the vessel and passing domestic water through an internal coil or tank.