Hot Water Cylinder Repair, Part by Part
Most hot water cylinder repairs are part replacements: an immersion element or thermostat, a zone valve, a relief or combination valve, an expansion vessel or a leaking joint. The cylinder shell itself cannot be repaired, so a corroded or split shell is the point where repair ends and replacement begins.
What the visit covers
- Element, thermostat and cut-out replacement
- Two-port zone valve repair or replacement
- Temperature and pressure relief valve, combination valve and tundish replacement
- Expansion vessel recharge or replacement
- Remaking joints and connections
Fixed prices
What it costs
- Cylinder troubleshooting and diagnosis
- No call-out fee.
- £180Fixed price
No VAT is charged
There is no call-out fee
Order of work
Diagnosis names the part
Repair should start only once one failed part is proved, as set out on the diagnosis page.
Isolate and prepare
Power and water are isolated, and the cylinder is drained only if the part sits below the water line.
Replace the part and its fittings
New seals, olives or washers go in with the part, because old ones rarely reseal.
Refill, test and recheck
Air is purged, controls are set, and the new connections are rechecked once the cylinder is hot.
Do plumbers fix hot water cylinders?
Yes, but only the parts around the shell, not the shell. A cylinder is a pressure vessel with controls and valves attached, and the attachments are what fail. Elements burn out, thermostats stick, zone valves seize and relief valves wear.
Repair therefore means replacing one named part and testing again. It follows a hot water cylinder diagnosis, because replacing the likeliest part without testing is how a fuse or timer fault turns into a pointless parts bill.
Which hot water cylinder faults are repairable?
The list of repairable parts is short and each has a recognisable symptom.
- Immersion element: no heat on the electric side, or a tripped breaker. The element is replaced with its washer, and a scaled boss is cleaned first. See immersion heater replacement.
- Cylinder thermostat and cut-out: water too hot, too cold or not heating at all. These are usually one combined unit and are replaced together.
- Two-port zone valve: heating works but the coil never gets heat. The actuator head or the whole valve is replaced.
- Temperature and pressure relief valve and combination valve: discharge through the tundish. These are replaced as a set with new nuts and olives.
- Expansion vessel: intermittent discharge and pressure surges at the taps. The vessel is recharged or replaced. The detail is on the TPRV and expansion vessel repair page.
- Joints and connections: weeping at a nut or olive, remade with new fittings. Where the source is unclear, cylinder leak repair shows how it is traced.
What cannot be repaired on a hot water cylinder?
The shell cannot. A seam that weeps, a base corroded through or a split in the vessel wall means the cylinder has failed as a pressure vessel. A sealed coil that has failed internally cannot be repaired either, because the coil is welded inside the cylinder. A householder should not attempt a patch, and a patch does not make it safe.
A cylinder with several worn parts at once also tips towards replacement. If the element, thermostat, valve group and tundish are all due, the sum of the parts can approach a new unit, and the new unit carries a fresh set of parts. That is a judgement a competent engineer should be able to show with test results, not a sales preference.
Is a repair worth it on an older cylinder?
A useful test is whether the failed part is on its own. A single replaceable part on an otherwise sound cylinder is usually worth repairing while the shell and coil are sound, because they are still doing their job. A failing shell on any cylinder is never worth repairing.
Age matters mostly as a pointer to scale. In hard water, scale shortens element life and furs up the tundish and valve seats. Heavy scale is a reason to inspect the valve group and tundish at the same visit, because scale builds there too.
If repair is ruled out, the route to the right replacement is in the cylinder replacement hub.
Questions people ask
Do plumbers fix hot water cylinders?
They replace the failed parts: elements, thermostats, zone valves, relief valves, expansion vessels and joints. The shell itself cannot be repaired, so a leaking or corroded shell means replacing the cylinder.
Can I repair a hot water cylinder myself?
Not safely. Anything inside an immersion heater cover or electrical enclosure is not a householder job, and relief valves, tundish pipework and unvented controls must never be adjusted or bypassed.
How do I know if it is the cylinder or a part that has failed?
Test in order from the supply to the shell. Most faults sit in a part, and the shell is only blamed once everything else has been proved good.
How much does a cylinder repair cost?
It depends on which part has failed and how it is fitted. Finding the failed part is a cylinder diagnosis, a fixed price of £180, and no VAT is charged. The fixed prices are on the pricing page.
Does repair work on unvented cylinders differ?
Yes. Unvented cylinders have a valve group and expansion vessel as well, and those parts are safety devices. They are replaced as sets by a competent installer and never adjusted or capped.