An unvented cylinder stores drinking water under mains pressure; a thermal store does the opposite, storing heating water and making hot water instantaneously through a plate heat exchanger. For a heat pump, choose the unvented cylinder — a thermal store has to be held at 65–80 °C to give a decent shower, and a heat pump loses roughly a third of its COP between 45 °C and 65 °C.
What is a thermal store and how is it different?
A thermal store holds heating water, not drinking water. Cold mains passes through a coil or an external plate heat exchanger inside or beside the store and comes out hot, on demand, at mains pressure. An unvented cylinder is the mirror image: the drinking water is what is stored, and the heat source passes through a coil.
That inversion has three consequences. The stored water in a thermal store is never drunk, so it is outside drinking-water hygiene rules. There is almost no stored hot water to run out of — but also no reserve, so output collapses once the store cools. And the store must sit well above the delivery temperature at all times, because a heat exchanger needs a temperature difference to work.
| Unvented cylinder | Thermal store | |
|---|---|---|
| What is stored | Drinking water at 50–60 °C | Heating water at 65–80 °C |
| How hot water is made | Stored, drawn off directly | Instantaneous, through a heat exchanger |
| Working store temperature | Can run at 48–55 °C | Must stay above about 65 °C |
| Heat pump compatibility | Excellent | Poor |
| Multiple heat sources | Two coils maximum in practice | Easy — solid fuel, solar, boiler, immersion |
| Legionella exposure | Stored volume must be pasteurised | Under 3 litres of hot drinking water in the pipework |
| Governing standard | EN 12897 | EN 12977-3 test methods for stores |
| Discharge and safety group | PRV, expansion vessel, T&P valve, tundish | None on the store if it is open-vented |
Which is more efficient?
The unvented cylinder, in nearly every domestic case, and the reason is store temperature rather than insulation. Standing loss is proportional to the difference between the store and the room, so a thermal store held at 75 °C in a 20 °C cupboard loses about 1.4 times as much as the same vessel at 60 °C, and roughly twice as much as one at 48 °C.
On top of that, every heat source has to work harder to reach the higher temperature. A condensing boiler returning water at 65 °C cannot condense at all and drops 5–8 percentage points of efficiency; a heat pump at 65 °C achieves a COP near 2.5 where it would manage 3.5–4.0 at 48 °C.
Thermal store or unvented cylinder for a heat pump?
Unvented cylinder. This is the clearest answer in the whole tank category, and it comes down to arithmetic. A plate heat exchanger sized for a shower needs roughly 10–15 K of approach temperature, so a 45 °C shower demands a store at 58–62 °C minimum and 70 °C for comfortable simultaneous outlets. Heat pumps are engineered to run at 35–55 °C; forcing 70 °C means either a high-temperature machine or an immersion heater doing the last stretch at a COP of 1.
The alternative that does work is a heat pump charging a well-insulated cylinder at 50–55 °C, with a weekly pasteurisation cycle for legionella control, and a buffer tank kept separate on the heating side.
When is a thermal store the right answer?
Four cases, and they are real:
- Several heat sources at once. A wood stove with a back boiler, solar thermal and a boiler can all dump into one store without hydraulic conflict. That is much harder with a two-coil cylinder.
- No route for a discharge pipe. An open-vented thermal store carries no unvented safety group, no tundish and no G3 notification.
- Solid fuel or biomass, where the heat source cannot be modulated and needs somewhere to put its output safely.
- Hygiene-critical installations where stored drinking water is unwelcome — the store keeps under three litres of hot drinking water in the pipework, the volume below which German rule W 551 treats a domestic system as low legionella risk.
What about hygiene and legionella?
A thermal store wins on paper. Legionella multiplies in stored water between 20 °C and 45 °C, and a thermal store stores none: the drinking water is heated as it flows. That is why the same instantaneous principle is used in hygienic combination tanks and fresh water stations.
An unvented cylinder manages the risk instead of avoiding it — store at 60 °C, or at 50–55 °C with a weekly pasteurisation cycle, and fit a thermostatic mixing valve to protect against scalding. For a single household that regime is well proven; in hard water it is also easier to maintain, because a thermal store's plate exchanger scales up faster than a cylinder's coil.
Frequently asked questions
Are thermal stores any good?
Yes, in the right place. They handle several heat sources at once, avoid stored drinking water and need no unvented safety group. They are a poor match for a heat pump, because they must be kept at 65–80 °C to deliver a decent shower.
Which is most energy efficient: vented, unvented or thermal store?
A well-insulated unvented cylinder, because it can run at 48–55 °C rather than the 65–80 °C a thermal store needs, and standing loss is proportional to the temperature difference with the room. Vented and unvented cylinders lose almost identical amounts — pressure has no effect on insulation.
How long do thermal stores last?
Typically 15–25 years for the vessel, similar to a good cylinder, because the stored water is a sealed heating circuit that is neither replaced nor oxygenated. The plate heat exchanger is the wear item and may need descaling or replacement every 5–10 years in hard water.
Can I use a thermal store with a heat pump if I add an immersion heater?
You can, but the immersion does the high-temperature work at a COP of 1, which throws away the reason for having a heat pump. A cylinder with a large coil, charged at 50–55 °C, is both cheaper to run and cheaper to buy.