Hot water
What is a thermal store, and how does it work?
3 min read · Last updated · 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 and comes out hot, at mains pressure. Gledhill, a thermal store manufacturer, says the stored water is not fed to the taps and that a store can take several heat inputs, such as a boiler, solar thermal or a solid-fuel appliance. 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 plate heat exchanger and is heated as it passes, so it arrives at the tap hot and at mains pressure, without ever being stored. Gledhill says the stored water itself is never fed to the taps.
Turn the tap off and nothing is 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. Gledhill says a store's full potential comes from using a number of different heat inputs, including solid fuel appliances, solar thermal, a conventional boiler and an immersion heater.
It also gives mains-pressure hot water. Gledhill, as a manufacturer, says that with no standing water there is no risk of Legionella, and that installers do not need a G3 qualification to install one.
The honest trade-offs
A thermal store keeps a tank of water hot ready for demand, so the quality of its insulation matters, and continuous hot-water output is limited by the heat exchanger rather than a tank of stored hot water. Approved Document G still sets hot-water safety rules for it — for example, a hot water system with a storage vessel must prevent the stored water exceeding 100°C — and it defines primary thermal stores as vented or unvented. 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.
“A thermal store earns its keep when heat comes from more than one place. For a straightforward gas household it is usually the wrong kind of clever.”
Ask us which suits your home →
Sources
- Gledhill — Five things worth knowing about thermal stores (the stored water is not fed to the taps: mains water is heated through a plate heat exchanger; a store can take several heat inputs and give mains-pressure hot water; "installers do not need to have a G3 qualification to install a thermal store"; manufacturer's statement that with no standing water there is no risk of Legionella; accessed 19 Oct 2026)
- GOV.UK — Approved Document G: sanitation, hot water safety and water efficiency (an unvented hot water storage system should have at least two independent safety devices in addition to its thermostat; installing one is notifiable building work, notified to building control beforehand unless the installer is registered with a competent person scheme; a primary thermal store is a store of heat that heats domestic hot water through a heat exchanger and can be vented or unvented; accessed 19 Oct 2026)
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