Hot water
Which hot water cylinder do I need — vented, unvented, or a thermal store?
3 min read · Reviewed for accuracy before publishing
There are three broad routes. A vented cylinder is fed from a cold tank in the loft, giving gravity-fed (lower) pressure — cheap and simple, common in older homes. An unvented cylinder is fed directly from the mains, storing hot water at mains pressure for strong flow to every tap — the modern default, but it needs good incoming mains and a G3 install. 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. Size is then set by bathrooms and occupants, not house price.
Vented vs unvented vs thermal store
Vented: fed by a loft tank, lower gravity pressure, simple and inexpensive — fine if you're happy with the pressure and have the tank space. Unvented: mains-fed, powerful pressure at every outlet, the modern standard, but needs a strong incoming main and a G3-certified fit. Thermal store: heats on demand, ideal for heat pumps plus solar or a boiler, avoids G3. Most new installs are unvented — unless your mains is weak or you're running renewables.
Sizing it to your household
Cylinder size is about simultaneous hot-water demand: bathrooms, occupants and habits. As a rough steer, a one-bathroom home suits a smaller cylinder and a two-plus-bathroom family a larger one — but the honest figure 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.
Your mains is the deciding factor
An unvented cylinder needs a decent incoming mains flow and pressure to deliver its promise — 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. Measuring first is exactly why the survey matters more than any online guide.
Size it properly — free survey →
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.