Hot water
What is a thermal store, and how does it work?
3 min read · Reviewed for accuracy before publishing
A thermal store flips the usual cylinder idea around. Instead of storing your hot tap water, it stores a tank of hot 'heating' water, and when you turn on a tap the incoming cold mains passes through a heat exchanger inside the store and comes out hot, instantly, at mains pressure. Because the water you actually use is never stored — only heated as it flows — a thermal store doesn't fall under the unvented (G3) pressure rules, and it happily takes heat from several sources at once (a boiler, a heat pump, solar thermal, even a wood burner). That flexibility is its whole point.
How it heats your taps
The store keeps a body of water hot. Your mains cold water runs through a coil or plate heat exchanger sitting in that hot water and is heated as it passes — so it arrives at the tap hot and at full mains pressure, without ever being stored. Turn the tap off and nothing's sitting in a cylinder cooling down.
Why people choose one
It's the natural fit when you have (or plan) more than one heat source — a heat pump plus a boiler, solar thermal, or a stove with a back boiler — because they can all feed the same store. It gives strong mains-pressure hot water, and because it isn't storing pressurised hot water it avoids the annual G3 unvented check. It also lowers legionella risk in your tap water, since that water isn't stored warm.
The honest trade-offs
A thermal store keeps a tank hot around the clock, so standing heat loss can be higher than a well-insulated modern cylinder, and continuous hot-water flow is limited by the heat exchanger's output rather than a tank of stored water. It's a specialist choice — brilliant for the right, multi-source setup, unnecessary for a standard gas combi household. We'll tell you honestly whether your home is one of the right ones.
Ask us which suits your home →
Datum's engineers install and service boilers, air conditioning, heat pumps and plumbing across South Hertfordshire and North London. Guides are written from real jobs, not brochures.