Unvented Cylinder Problems and the Safety Chain
Most unvented cylinder problems sit in one of five parts: the cold inlet group, the expansion vessel, the temperature and pressure relief valve, the tundish or the discharge pipe. Check them in that order, because each feeds the next. None is a householder repair, and no relief device should be capped or bypassed.
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A dripping valveWhat is the unvented safety chain?
An unvented cylinder holds mains-pressure water, so it relies on a chain of safety parts. Water enters through the inlet group, where a strainer, pressure reducing valve and check valve control it. As it heats, the expansion vessel absorbs the extra volume. If the vessel cannot cope, the expansion relief valve on the inlet group opens. If temperature or pressure rise too far, the temperature and pressure relief valve opens. Either way the water leaves through the tundish and discharge pipe.
- Cold inlet group: pressure reducing valve, strainer, check valve and the expansion relief valve.
- Expansion vessel: absorbs the extra volume as water heats.
- Temperature and pressure relief valve: the last-resort release.
- Tundish: the visible break in the pipe where a discharge shows.
- Discharge pipe: carries the water to a safe place.
Which unvented cylinder fault do I have?
Water at the tundish is the commonest sign, and its pattern separates the faults. Constant discharge points at a valve, discharge only on heat-up at the vessel. TPRV and expansion vessel repair covers both. A cylinder that will not heat, trips its cut-out or runs weak is on unvented cylinder fault repair, and the order for no hot water starts at the supply on hot water cylinder diagnosis.
If steam or very hot water is coming from the discharge pipe, switch the heat source off at the isolator and keep well clear until an engineer attends.
What do the regulations say about unvented cylinders?
Two sets of rules apply, and they are not the same thing. Part G of the Building Regulations covers the installation: a competent installer, notification and safety devices beyond the thermostat. The Water Supply (Water Fittings) Regulations 1999 separately require temperature relief on unvented storage over 15 litres, so that stored water cannot exceed 100 degrees Celsius. Regular servicing is how the chain is kept working. See unvented cylinder service check and, for fitting, unvented cylinder installation.
Take the cylinder apart
Guides on this topic
Questions people ask
What are the common problems with unvented cylinders?
A dripping relief valve or tundish, a flat expansion vessel, scale in the strainer or tundish and a tripped thermal cut-out.
Why is my unvented cylinder dripping?
A passing valve seat or an expansion vessel that has lost its charge. Constant dripping suggests the valve, dripping only when heating suggests the vessel.
Can I adjust the relief valve myself?
No. Relief valves, tundish pipework and expansion devices must never be adjusted, capped or bypassed.