Most people think about their hot water cylinder precisely twice: when it starts running out of hot water, and when it starts leaking. Then comes a decision you’ll live with for the next fifteen years or more — and the first question any good engineer will ask is: vented or unvented?
Here’s what that choice actually means, in plain English.
Vented cylinders: the traditional setup
A vented cylinder is the classic British arrangement: a copper cylinder in the airing cupboard, fed by a cold water tank in the loft. Gravity provides the pressure — which is why older showers upstairs can feel gentle, and why the tank needs to sit as high as possible.
The good: simple, reliable, inexpensive to replace like-for-like, and unfussy about incoming mains pressure. If a part fails, it’s usually cheap.
The less good: modest pressure (especially upstairs), a tank in the loft that needs maintaining and protecting from frost, and showers that often need a pump to feel modern.
Unvented cylinders: mains-pressure hot water
An unvented cylinder does away with the loft tank entirely — it’s a sealed, reinforced cylinder fed directly from the cold main, storing hot water at mains pressure.
The good: powerful showers on every floor without pumps, no loft tank to freeze or contaminate, balanced hot and cold pressures (great for mixer showers), and flexible siting — it doesn’t need to be above anything.
The less good: it costs more to install, it needs annual safety checks of its expansion and relief valves, and — crucially — it’s only as good as your incoming main. If your mains flow is weak, an unvented cylinder can’t fix that.
The test that settles it
This is why we always measure your incoming mains flow and pressure before recommending anything. As a rule of thumb, an unvented system wants roughly 20 litres per minute or better to shine. Plenty of Gloucestershire homes have that; some older supplies don’t. Five minutes with a flow cup tells us whether unvented is a transformation or an expensive disappointment — before you spend anything.
Sizing: the mistake to avoid
Whichever type you choose, size for the household you actually are. A quick guide: allow 35–45 litres per person per day, and think about bathrooms more than bedrooms — two showers running back-to-back is what empties a cylinder. Families upgrading from a 120-litre relic to a properly sized 210-litre unvented cylinder routinely tell us it’s the best thing they’ve done to the house.
When to act
If your cylinder is rusty at the base, your hot water runs out faster than it used to, or your airing cupboard relic predates the millennium — have it assessed before it makes the decision for you, usually in the coldest week of the year. We install and replace both vented and unvented cylinders across Cheltenham and Gloucester, and we’ll test your mains and recommend honestly. Free quotes on 07386 705571.