Unvented Cylinder vs Vented: Mains Pressure Hot Water
If your showers are weak upstairs and there's a cold tank in the loft, you have a vented system and gravity is the reason.
If your showers are weak upstairs and there's a cold tank in the loft, you have a vented system and gravity is the reason. An unvented cylinder is fed directly from the mains and delivers hot water at close to mains pressure everywhere in the house. It's a genuinely transformative upgrade in the right property — and pointless in the wrong one.
How they differ
A vented cylinder is fed from a cold storage tank above it, so outlet pressure depends on the height difference. An unvented cylinder is sealed and fed from the mains, with expansion accommodated by an air gap or external vessel, and pressure and temperature safety devices as required. The loft tank disappears entirely.
What an unvented cylinder needs
Adequate incoming mains flow and pressure, a compliant discharge route from the safety valves to a safe point, and space for the cylinder. Installation and servicing of unvented systems requires specific qualification, and the safety devices should be checked annually — that annual check is not optional maintenance.
When it's the right upgrade
Multiple bathrooms, a desire for a strong shower without a pump, or a loft conversion that needs the tank gone all point towards unvented. Where incoming mains flow is genuinely poor, an unvented cylinder will not solve the problem and an accumulator or a pumped vented arrangement may serve better.
Compared with going combi
A combi removes the cylinder too, but caps hot water delivery to whatever the mains supplies at that moment. An unvented cylinder stores volume, so two showers running together is comfortable until stored capacity is used. For larger households that difference usually decides it.
When staying vented is the right call
If the incoming mains flow rate is modest, an unvented cylinder won't deliver the strong showers people expect from one, and the vented cylinder's low running risk stays an advantage. Test the mains flow rate before promising anything: a weak supply makes the upgrade a waste of money regardless of how good the new cylinder is. Where the mains is good, a loft you want cleared of tanks and a household wanting equal pressure at every outlet without a pump are what make the upgrade pay off.
Common questions
Do unvented cylinders need annual servicing?
Yes. The expansion vessel, pressure reducing valve, temperature and pressure relief valves all need checking annually to keep the system safe and to maintain guarantee cover.
Can I fit an unvented cylinder if my mains flow is poor?
It won't perform. That's why we measure flow rate in litres per minute first and, where it's low, discuss the alternatives honestly rather than proceeding anyway.
How much space does one need?
More than the cylinder footprint alone — allowance is needed for the safety devices, pipework and access for servicing. We confirm this at survey.
Is it better than a combi?
For multiple-bathroom households, generally yes on simultaneous demand. For a one-bathroom home with good mains flow, a combi is often the simpler and cheaper answer.
Is an unvented cylinder installation notifiable?
Unvented hot water installations require someone competent in that specific area and the work should be notified where Building Regulations require it.
Can the old vented cylinder be reused elsewhere?
Occasionally, if it's in good condition, but most vented cylinder removals happen because the old unit is at the end of its life anyway.
Will an unvented cylinder definitely fix weak showers?
Only if the cause is the cylinder rather than the mains supply. We measure flow before quoting so you're not paying to solve the wrong problem.