Unvented cylinder problems: low pressure and noises
Weak hot water, or a cylinder that bangs, hums or hisses? Here is what usually causes it and what you can safely check before you call.
Low hot water pressure
An unvented cylinder runs at the pressure of your incoming cold main, reduced by a valve on the cold inlet. Common causes of weak hot water are:
- Low pressure or flow on the incoming water main.
- A blocked strainer (filter) in the cylinder’s cold inlet valve.
- A faulty pressure reducing valve.
- An isolation valve that has been left partly closed.
- Scale in shower heads, tap aerators or mixer valves.
What you can check: compare hot and cold at the same tap, and see whether every tap is affected or only one. If the cold is weak too, the problem may be the incoming supply. If only one tap or shower is weak, clean its aerator or shower head.
Noises from the cylinder
- Banging or rumbling while heating – often scale on the immersion heater or heating coil.
- Humming or vibration – usually from the inlet valves or a pressure reducing valve.
- Knocking when a tap is turned off – water hammer in the pipework.
- Hissing, or water running into the tundish – a safety valve is letting water out. This needs an engineer, and must never be blocked.
No hot water at all?
A failed immersion heater, a stuck motorised valve or a tripped high-limit thermostat can all stop the hot water. See our repair page for the common faults.
Repair or replace
Most valve and immersion faults can be repaired. If the cylinder itself is leaking or the heating coil has failed, replacement is usually the better option. A new unvented cylinder is fixed, fully installed from £1,639, including removal of the old one.
Arrange a visit
Work on an unvented cylinder’s safety devices must be carried out by an unvented-qualified engineer. Send your postcode, the cylinder make and model from its label, and a short video of the problem. Fixed-price work is confirmed after an assessment – from photos or a short video where possible, otherwise at a visit.