# Thermal store or unvented cylinder — which should I choose?

*The Datum team · Heating, cooling & plumbing engineers · Datum · Hot water*

Both deliver strong, mains-pressure hot water — the difference is what they store and how they're regulated. An unvented cylinder stores your hot tap water directly, under mains pressure, heated by the boiler; it's simple, gives excellent flow for busy households, but it's a pressurised vessel so it needs a G3-qualified install and an annual safety check. A thermal store stores heating water and warms your tap water on demand through a heat exchanger; it avoids G3, takes several heat sources easily (heat pump, solar, boiler), but can have higher standing losses and a flow rate capped by the exchanger. 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 for powerful, consistent flow — ideal for two bathrooms running at once. It's the mainstream choice, well understood, and efficient with modern insulation. The trade: it's a pressurised vessel, so it must be fitted by a G3-registered engineer with the correct safety devices, and it needs a yearly check. 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. All can charge the same store. It gives mains-pressure hot water without storing it, so no G3 and lower legionella risk in your tap water. The trade: a hot tank around the clock means more standing loss, and sustained flow is limited by the heat exchanger rather than a full tank.

## 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.
>
> — The Datum team, Heating, cooling & plumbing engineers

## Related questions

### Which gives better water pressure?

Both give mains-pressure hot water. An unvented cylinder can sustain a higher continuous flow for simultaneous outlets; a thermal store's flow is capped by its heat exchanger. For very high simultaneous demand, unvented usually edges it.

### Which is cheaper to run?

A well-insulated modern unvented cylinder generally has lower standing losses than a thermal store kept hot around the clock — but if the thermal store lets you use free or cheap heat (solar, a heat pump running efficiently), that can more than offset it. It depends on your heat sources.

### Do I need a thermal store for a heat pump?

Not necessarily — heat pumps commonly run with a dedicated cylinder. But a thermal store shines when you're blending a heat pump with other sources like solar or a boiler. We'll advise based on your actual setup.

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