# Which hot water cylinder do I need — vented, unvented, or a thermal store? *The Datum team · Heating, cooling & plumbing engineers · Datum · updated 2026-10-19 · Hot water* There are three broad routes. A vented cylinder is fed from a cold tank in the loft, so Vaillant says it relies on gravity rather than mains pressure — cheap and simple, common in older homes. An unvented cylinder is fed directly from the mains, which Vaillant says means better pressure, but it needs a good incoming supply and a competent installer. A thermal store heats water on demand and suits multiple or renewable heat sources. Which you need comes down to your incoming mains pressure and flow, how much hot water you use at once, your heat source, and space. ## What is the difference between vented, unvented and thermal store cylinders? A vented cylinder is fed from a loft tank at lower gravity pressure: simple and inexpensive. An unvented cylinder is mains-fed for stronger pressure at every outlet, but needs a good incoming supply and a competent installer, with at least two independent safety devices. A thermal store heats on demand and suits heat pumps plus solar or a boiler. Which one suits you depends on your mains supply and your heat sources, which we check at the survey. ## What size hot water cylinder does my household need? Size is set by simultaneous hot-water demand: bathrooms, occupants and habits. Vaillant says a cylinder helps most in homes with two or more bathrooms and a high demand for continuous hot water, but the honest size comes from how you actually use hot water and your reheat times, not a chart off the internet. Oversizing wastes standing heat; undersizing leaves you cold mid-shower. ## Does my mains pressure decide which cylinder I can have? Largely, yes. In an unvented system, Heatrae Sadia says pressure and flow rate depend directly on the incoming water supply, and for its Megaflo Eco SystemFit it suggests a minimum of 1.5 bar and 20 litres per minute, with flow possibly poor at those values if several outlets run at once. We measure yours at the survey. If it's weak, we'll be straight that a vented cylinder, an accumulator or a different approach makes more sense than an unvented one that never performs. > Your incoming mains decides more than any brochure — an unvented cylinder on a weak main never delivers its promise, and we measure before we recommend. > > — The Datum team, Heating, cooling & plumbing engineers ## Related questions ### What size hot water cylinder do I need? It's driven by bathrooms and occupants and how much hot water you draw at once, not house size. One bathroom needs less; a busy two-bathroom family needs more. We size it to your real usage and reheat times at the survey. ### Is an unvented cylinder better than a vented one? For pressure at the taps, usually yes — Vaillant says it's fed straight from the mains and needs no loft tank. But it needs a strong enough incoming supply and a competent installer: Approved Document G treats installing one as notifiable building work, and Heatrae Sadia recommends annual maintenance. Where the mains is weak, a vented system or an accumulator can be the better honest choice. ### Can I replace a vented cylinder with an unvented one? Often yes, if your incoming mains flow and pressure are good enough — which we measure first. If they aren't, we'll tell you straight rather than fit an unvented cylinder that underperforms. ## Sources & verification - [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)](https://www.vaillant.co.uk/advice/heating-tips/home-advice/what-is-a-hot-water-cylinder/) - [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)](https://www.heatraesadia.com/-/media/themes/heatrae/literature/installation-manuals/megaflo-eco-systemfit-installation-manual.pdf) - [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)](https://www.gov.uk/government/publications/sanitation-hot-water-safety-and-water-efficiency-approved-document-g) - [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)](https://www.gledhill.net/five-things-worth-knowing-thermal-stores/) --- Canonical page: https://www.datumclimate.co.uk/guides/which-hot-water-cylinder-do-i-need