Unvented Cylinder Problems: No Hot Water Causes & Repair Costs

What an unvented cylinder is — and why it fails differently

An unvented hot water cylinder is a sealed tank fed straight from your incoming mains. It stores hot water at mains pressure, which is why your taps and showers run strong without a pump.

It is not a loft tank. There is no cold-water cistern in the roof feeding it, and the stored water is held under pressure, so safety controls do a lot of work behind a metal cover.

Because it is pressurised, an unvented cylinder is covered by Building Regulations Part G3. By law, installation, servicing and repair of the pressurised system must be carried out by a competent, G3-qualified engineer. That legal line shapes everything below.

First: is it heated by your boiler or by an immersion heater?

Almost every unvented cylinder is heated one of two ways. Identifying yours is the most useful thing you can do before calling anyone out.

• Heated by the boiler — a coil inside the cylinder is fed by hot water from a system or regular boiler, controlled by a motorised valve and a cylinder thermostat. You will have a programmer with a separate "hot water" setting. These are system and regular boilers that use a cylinder.
• Heated by an immersion heater — an electric element sits inside the cylinder, fed from its own switched fused spur (often a pull-cord switch). This is common in electric-only homes, or as a backup on boiler-heated cylinders.

Many cylinders have both: a boiler coil for everyday heating and an immersion as backup. If so, test each path separately. Switching the immersion on for an hour tells you whether the cylinder and element work, independent of the boiler.

No hot water at all — most likely causes, in order

Work down this list. The order roughly reflects how often each one is the culprit.

1. Tripped overheat (high-limit) thermostat. A latching safety cut-out has fired and needs a manual reset. Extremely common on both immersion and boiler-heated cylinders.
2. Tripped MCB/RCD or blown spur fuse (immersion). Check the consumer unit and the fuse in the immersion's fused spur. A repeatedly tripping RCD points to a failing element.
3. Failed immersion element or immersion thermostat. Elements burn out and furr up over time; the thermostat can stick open and stop calling for heat.
4. Seized motorised valve actuator (boiler-heated). The valve that should open to send heat to the cylinder has stalled. This looks identical to a dead thermostat — see the test below.
5. Cylinder thermostat or coil fault (boiler-heated). A failed cylinder stat never calls for hot water; a scaled or split coil heats poorly or not at all.
6. The boiler itself. If the boiler is in fault or lockout, neither heating nor hot water work. See no hot water from your boiler.

How to reset the overheat (high-limit) thermostat

The high-limit stat is a separate latching cut-out, usually under a small cover near the immersion or cylinder thermostat. Resetting it on the electrical/immersion side is one of the very few jobs a homeowner can reasonably do. If you are not confident, leave it to an engineer.

1. Isolate the power. Switch off the immersion at its spur, or turn off the cylinder's circuit at the consumer unit. Do not work on it live.
2. Remove the cover. Unclip or unscrew the small plastic lockout cover over the thermostat pocket.
3. Press the reset. Look for a small red button (sometimes a recessed pin or hole marked RESET). Press firmly until you feel or hear a click.
4. Refit and restore power. Replace the cover, switch the power back on, and give it 30–60 minutes to heat.

Hot water gone but heating still works (or vice versa)

If your radiators heat fine but the water is cold, the boiler is working — the fault is on the cylinder side. The usual suspects are a seized motorised valve, a failed cylinder thermostat, or (on a cylinder with an immersion) a dead element. See no hot water but the heating still works.

The reverse — heating off but hot water on — points the same way: a zone valve or wiring-centre fault rather than the boiler.

The test that stops a thermostat being wrongly condemned

On an S-plan or Y-plan system, a seized motorised valve actuator behaves exactly like a dead cylinder thermostat — no hot water, with the boiler firing for heating. Engineers tell them apart with a simple live check.

With the programmer calling for hot water, an engineer measures for 230V at the valve's actuator. If 230V is present but the valve is not opening, the actuator has seized and the valve (or its replaceable head) is the fault — not the thermostat. This is a live mains test for a qualified electrician or heating engineer, not a DIY job.

Hot water, but poor pressure or flow

If the water is hot but the flow is weak, the problem is usually on the cold mains-control side of the cylinder.

• Failing pressure reducing valve (PRV). This sets the pressure feeding the cylinder. If it fails or its inlet filter blocks, hot-water flow can drop across the property.
• Waterlogged expansion vessel. If the vessel loses its air charge, expansion has nowhere to go and pressure behaviour goes haywire.
• Low incoming mains. Unvented systems generally need around 1.5 bar minimum incoming pressure and roughly 20 litres/min flow to perform properly. If your whole mains is weak, the cylinder cannot fix that.

Water too hot or scalding

Scalding-hot water usually means the thermostat is set too high or has failed stuck closed. The correct storage setting is around 60–65°C.

That range is a deliberate balance: hot enough to control Legionella bacteria (which need water stored at 60°C or above), but not so hot it scalds. Do not turn an unvented cylinder up beyond this. If it scalds even at a correct setting, the thermostat has failed and needs a G3 engineer.

Water dripping or pouring from the tundish or discharge pipe

The tundish is the small open funnel in the discharge pipework, usually near the cylinder. It lets you see when a safety valve is releasing water — by design, so a discharge is never normal background behaviour.

Occasional dripping into the tundish almost always means the expansion vessel has lost its air charge, or the expansion/pressure relief valve is failing. The vessel's air charge is normally set just below the PRV's operating pressure so it can absorb expansion.

A constant pour is more serious and usually means a failed temperature/pressure relief valve. Either way it is a G3 repair. Recharging the vessel sometimes works, but often it needs an external expansion vessel or, in stubborn cases, cylinder replacement — see how an expansion vessel has failed on the boiler side too.

DIY safe checks vs the G3 legal line

The bright line here is set by law. Under Building Regulations Part G3, anyone working on the pressurised parts of an unvented cylinder must be a competent person, typically holding a current G3 qualification.

You may reasonably:

• Check the programmer is set to heat water and the time/date is correct.
• Check the consumer unit, RCD/MCB and the immersion's fused spur.
• Switch the immersion on as a test if one is fitted.
• Reset a tripped overheat thermostat once, after isolating power.
• Look for water in the tundish and note any dripping.

You must leave to a G3-registered engineer: replacing elements, thermostats, motorised valves, the PRV, the expansion vessel or any pressure/temperature relief valve; and any work on the sealed pressurised side. Always confirm credentials — see checking your engineer is qualified. Gas, burner, flue, sealed-circuit, gas-valve and PCB work is for a Gas Safe registered engineer only.

2026 UK repair and replacement costs

Indicative UK ranges for 2026, last checked 2026. London and the South East tend to sit at the top of each range. These are general guide figures, not quotes — always confirm on the day with your engineer.

Job	Indicative cost (parts + labour)
Annual G3 service	from ≈£84–£150 inc VAT (often less if several are booked together)
Diagnosis / repair labour	from ≈£50–£100 per hour (higher in London)
Immersion element (part only, 3kW)	£20–£90
Immersion / twin thermostat (part only)	£35–£80
Immersion element or stat replacement (all-in)	£140–£280
Cylinder thermostat replacement (all-in)	£110–£170
Motorised valve (2-port/3-port) fitted	£150–£280
Pressure reducing valve (PRV) replacement	£120–£250
Expansion vessel recharge or replacement	£100–£250+
Whole cylinder replacement (150–210L)	£1,000–£3,000 fitted (more for upgrades/larger units)

For wider context on call-out and parts pricing, see typical UK boiler repair costs.

Will boiler cover or home emergency insurance pay for it?

This is where many homeowners get a nasty surprise. A large share of boiler cover and home-emergency policies exclude unvented cylinders altogether, or will not touch electric immersion heaters because there is no gas appliance involved.

Before you assume you are covered, check the policy wording for these things specifically:

• Is the hot water cylinder named as covered, or only the boiler?
• Are unvented / pressurised cylinders explicitly excluded?
• Are immersion heaters covered, or treated as electrical and excluded?
• Will the provider pay for a G3-registered engineer, or refuse work needing G3?

See what boiler cover doesn't cover for the common exclusions, and central heating cover that may include the cylinder for products that go beyond the boiler. You can also compare boiler cover for 2026 across our selected panel of providers.

How we make money and what we compare: we feature a selected panel of providers, not the whole market, and may earn a commission if you take out a policy through our links — this never affects the price you pay. Regulated boiler insurance (FCA-authorised) works differently from unregulated service or care plans — a service/care plan is not insurance and is not FCA-regulated. Prices are indicative "from" figures, last checked in 2026; always confirm cover, exclusions and price on the provider's own page before buying.

How to prevent it: the annual G3 service

A sensible preventative step is a yearly G3 service. The engineer checks and recharges the expansion vessel, tests the PRV and relief valves, and inspects the controls before they fail.

Many unvented faults — tundish dripping, scalding, sudden cold water — trace back to a vessel or valve that was never serviced. At roughly £84–£150 a year it is usually far cheaper than an emergency call-out or a premature cylinder replacement, and skipping it often voids the manufacturer's warranty.

Keep your benchmark logbook updated at each visit. It protects your warranty and records that the G3 requirements have been met.

This article is general information, not financial or gas-safety advice. We compare a selected panel of providers, not the whole market, and may earn a commission if you buy through our links. Always have gas appliances checked and repaired by a Gas Safe registered engineer; in a gas emergency call 0800 111 999. Prices are indicative UK guides for 2026 — confirm current prices on the provider's own site.