Hot Water Cylinder Emergency Repair

First, is it a vented or an unvented cylinder?

What to do next depends on which type you have. A vented cylinder is usually bare or foam-lagged copper in an airing cupboard, fed by gravity from a cold water tank in the loft. It runs at low pressure, so a leak is usually a steady drip or seep rather than a spray. The risk is water finding the immersion wiring or the ceiling below.

An unvented cylinder runs at mains pressure. You can spot it by the sealed white or steel casing, a small expansion vessel nearby, a group of valves on the incoming cold pipe, and a tundish: a short open funnel on a discharge pipe. Fitting one is controlled by requirement G3 in Part G of the Building Regulations, and the safety valves and controls should only be worked on by someone qualified for unvented systems.

Where hot water tanks usually leak from

Work from the fittings towards the body. The cheap, fixable faults come first, and water on a warm cylinder often runs down well below where it starts.

• Compression joints and the drain cock at the base. Often a slow drip that has gone on for weeks.
• The immersion heater boss. The fibre or rubber washer under the element hardens and weeps, sometimes after an element change.
• The tundish on an unvented system. Cold water dripping into it usually means the expansion vessel has lost its air charge or the pressure reducing valve is letting by, so the expansion relief valve opens.
• Hot water or steam at the tundish. The temperature and pressure relief valve has opened, which it is set to do at around 90 to 95°C. The controls have let the water overheat: usually a failed cylinder thermostat and cut-out, a stuck motorised valve or a faulty immersion thermostat. That needs a visit tonight.
• The cylinder body. A pinhole, green staining or a damp patch low down on a copper cylinder usually means corrosion, and the cylinder needs replacing.

If hot water or steam is coming from the tundish, switch off the boiler and immersion now and leave the cold supply on so the cylinder can cool. Do not touch the relief valve or try to drain the cylinder.

What we do on the visit

The engineer makes the area safe first: electrics near the leak isolated, the heat source off, water contained. Then the cylinder is dried off and each joint and valve watched in turn, with dry tissue run round the joints to catch a weep before it runs.

On an unvented system we check the expansion vessel charge against the cylinder maker’s figure, test the pressure reducing valve, and look at why the relief valve lifted before replacing anything. Replacing a dripping valve without fixing the pressure behind it just moves the problem. Our guide to a tundish overflowing explains what each type of discharge means.

If the body has failed we drain down, cap the pipework so you still have cold water, and quote the replacement.

What cylinder repairs usually cost

The £60 call-out covers getting an engineer to you at any hour, and the repair price is agreed with you on site before any work starts. See our call-out costs for how that works.

Across London the market for small cylinder repairs, such as a new drain cock, immersion washer or relief valve, is typically in the low hundreds of pounds including parts. Recharging or replacing an expansion vessel sits in a similar range. Replacing a whole cylinder is a different scale of job: a like-for-like vented copper cylinder commonly runs to around a thousand pounds fitted, sometimes more, and an unvented replacement usually costs well over a thousand.