Hot water
Thermal store or unvented cylinder — which should I choose?
3 min read · Last updated · Reviewed for accuracy before publishing
Both deliver mains-pressure hot water — the difference is what they store and how they're installed. An unvented cylinder stores your hot tap water directly, under mains pressure, heated by the boiler; it's simple and suits busy households, but it's a pressurised vessel, so Approved Document G treats installing one as notifiable building work needing at least two independent safety devices, and Heatrae Sadia recommends annual maintenance. A thermal store stores heating water and warms your tap water on demand through a heat exchanger, and takes several heat sources easily. For a standard gas home with high demand: usually unvented. For a multi-source or renewables setup: often a thermal store.
The unvented cylinder case
Stores hot water at mains pressure — Vaillant says an unvented cylinder takes water straight from the mains, which means better pressure. It's the mainstream choice and well understood. The trade: it's a pressurised vessel, so installing one is notifiable building work (or done by an installer in a competent person scheme), with at least two independent safety devices in addition to the thermostat, and Heatrae Sadia recommends annual maintenance. That's not a drawback, just a requirement done properly.
The thermal store case
Comes into its own when heat comes from more than one place — a heat pump and a boiler, solar thermal, a stove back-boiler. Gledhill says a store can take a number of different heat inputs and give mains-pressure hot water, and that installers do not need a G3 qualification to install one. The trade: a hot tank is kept ready for demand, so insulation matters, and sustained flow is limited by the heat exchanger rather than a full tank of hot water.
How we decide with you
It comes down to your heat sources now and planned, your simultaneous hot-water demand, and space. One heat source and busy bathrooms points to unvented; a heat pump or renewables mix points to a thermal store. We survey, explain both honestly with your real numbers, and never push the more expensive option for its own sake.
“Busy bathrooms and one heat source: unvented. Heat pump, solar or a stove in the mix: thermal store. Your setup decides — not the brochure.”
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Sources
- 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)
- Heatrae Sadia — MEGAFLO Eco SystemFit product guide (installation by a competent installer under Building Regulation G3; annual maintenance recommended by a competent person; in an unvented system pressure and flow rate depend directly on the incoming supply, with suggested minimum supply of 1.5 bar and 20 litres per minute, and poor flow possible at those values if several outlets run at once; accessed 19 Oct 2026)
- 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)
- Vaillant — What is a hot water cylinder? (a vented cylinder is fed from a loft tank and relies on gravity for pressure; an unvented cylinder takes water straight from the mains, "which means better pressure"; a cylinder helps homes with two or more bathrooms and high continuous demand; accessed 19 Oct 2026)
Datum's engineers install and service boilers, air conditioning, heat pumps and plumbing across South Hertfordshire and London. Guides are written from real jobs, not brochures.