Unvented Cylinder Guide: How They Work and What They Need
An unvented cylinder gives you mains-pressure hot water at every outlet without a loft tank, which is why so many homes with more than one bathroom end up with one.
An unvented cylinder gives you mains-pressure hot water at every outlet without a loft tank, which is why so many homes with more than one bathroom end up with one. It is also a pressure vessel, and that is the part homeowners underestimate: it has a layered set of safety devices, it must be installed and serviced by someone holding the specific qualification, and its annual checks are not optional maintenance.
What makes it different from a vented cylinder
A vented cylinder is fed by gravity from a cold tank in the loft, so hot water pressure depends on the height of that tank. An unvented cylinder is fed directly from the mains, so hot water arrives at the same pressure as the cold — better showers, no loft tank, no gravity limitations. In exchange, the cylinder has to accommodate the expansion of heated water safely, which is what the expansion vessel or internal air gap, the pressure reducing valve, the expansion relief valve and the temperature and pressure relief valve exist to do.
The safety devices, and why there are several
The layers are deliberate. A pressure reducing valve limits incoming mains pressure. An expansion vessel absorbs the volume increase as water heats. An expansion relief valve opens if that fails. A thermal cut-out interrupts the heat source if the thermostat fails. A temperature and pressure relief valve is the final protection. Each one covers the failure of the one before it, and all of them discharge through a tundish to a safe termination point. Water at the tundish always means something in that chain has operated.
What the annual service actually checks
The expansion vessel charge pressure, tested with the system depressurised, because a vessel that has lost its charge is the most common cause of relief valves weeping. The operation of the relief valves. The immersion heater and thermostat where fitted. The condition of the tundish and discharge pipework. The setting of the pressure reducing valve. And the anode where the cylinder type requires one. Skipping the service typically shows up as dripping from the outside discharge pipe a year or two later.
Mains flow is the limit, not the cylinder
An unvented cylinder can only deliver what your incoming main can supply. If the mains flow rate at the property is poor, a large cylinder will not fix weak showers, because the constraint is upstream. Measuring static pressure and dynamic flow rate before specifying anything is the difference between an installation that transforms the hot water and one that disappoints. Where flow is genuinely limited, an accumulator or a different approach may be the honest answer.
Sizing it for the household, not the airing cupboard
Cylinder size is driven by how many bathrooms are used simultaneously, the household's habits, and the reheat rate from the boiler or heat source. Undersize it and you run out mid-shower; oversize it and you pay to keep water hot that nobody uses. Recovery time matters as much as capacity: a smaller cylinder with strong reheat can outperform a larger one on a weak heat source.
Legal and competence requirements
Unvented hot water storage is notifiable work under Building Regulations, and installation must be carried out by someone holding the relevant unvented hot water systems qualification. That is separate from Gas Safe registration, which covers the gas appliance rather than the cylinder. Discharge pipework must terminate safely, and a system with the discharge run to an unsafe or invisible location is a genuine defect, not a detail.
Unvented cylinder service or installation
Tell us the cylinder make and when it was last serviced, and we will get the safety devices checked properly.
Questions we get asked
Why is water dripping from the pipe outside?
Usually the expansion vessel has lost its charge, so the relief valve is doing its job and dumping expanded water. It needs testing and re-charging, not ignoring.
Does an unvented cylinder really need annual servicing?
Yes. It is a pressure vessel with layered safety devices, and the checks confirm those devices still work. Many warranties also require it.
Can any plumber work on an unvented cylinder?
No. It requires the specific unvented hot water systems qualification, which is separate from Gas Safe registration.
Will a bigger cylinder improve my shower pressure?
Only if the incoming mains can supply the flow. If mains flow is the constraint, cylinder size will not change what comes out of the shower.
Can I convert from a vented cylinder to unvented?
Often yes, provided the mains flow supports it and the discharge pipework can be routed safely. It is notifiable work and needs the flow measuring first.