The unvented cylinder annual service, and why it is not optional

Why it matters more than a boiler service

A boiler that is not serviced becomes inefficient and eventually unreliable. You notice. An unvented cylinder that is not serviced becomes progressively less protected, and you do not notice at all – because the protection is only ever exercised when something has already gone wrong.

The safety devices on an unvented cylinder are arranged in layers, each taking over when the one before it fails. The system is designed on the assumption that every layer works. If the expansion vessel has silently lost its charge and the expansion relief valve has seized from never being operated, two layers have gone and the household has no way of knowing.

That is the substantive argument. The warranty condition and the manufacturer’s requirement are both real, and they are secondary to it.

What the service should cover

1. Visual inspection of the whole installationCylinder, controls, pipework, insulation, and any signs of previous discharge.
2. The discharge arrangementsTundish visible and correctly positioned, D1 within its maximum length, D2 larger than D1, continuous fall, termination safe. Non-compliance here is the most common finding on older installations.
3. Expansion vessel chargeSystem depressurised, charge checked against the specified figure with a gauge, and recharged if low. On cylinders with an internal air gap, the gap is recharged instead by the manufacturer’s procedure.
4. Expansion relief valveOperated manually to confirm it opens and, critically, reseats cleanly. A valve that has never been operated can seize; one that has been discharging repeatedly may fail to reseat.
5. Temperature and pressure relief valveOperated and confirmed to reseat. This is the last line of protection and it must work.
6. Pressure reducing valveOutlet pressure checked against the required figure.
7. Cylinder thermostatOperation and setting verified.
8. High-limit thermostatChecked. It is a non-self-resetting device, so if it has tripped there is a reason and it needs finding rather than just resetting.
9. Immersion heater and its thermostatWhere fitted.
10. Strainer or filterCleaned where the manufacturer specifies one.
11. System refilled, vented and checkedAnd left working.
12. Record completedBenchmark or equivalent service record, with findings.

The expansion vessel, which is the heart of it

This is the component most likely to have failed and the one that accounts for most unvented call-outs.

The vessel’s air charge falls gradually over years as air permeates through the diaphragm and past the valve. As it falls, the vessel absorbs less expansion, so pressure rises further on each heating cycle, until it exceeds the expansion relief valve setting and the valve opens. You get water through the tundish and out of the discharge pipe.

Checking it requires the water side to be depressurised first – a reading taken against system pressure is meaningless. That is a few minutes of work and it is the step most likely to be skipped on a rushed visit.

Full detail in expansion vessel failure.

What tends to be found

The discharge pipework findings are worth emphasising. A great many unvented installations were fitted with D2 the same size as D1, or with the tundish boxed in out of sight. Neither of those is a small detail – between them they mean the system cannot discharge safely and nobody would see it if it tried.

How to tell it was actually done

1. You have a completed service record with findings and readings, not just a sticker.
2. The expansion vessel charge is recorded, with the figure found and the figure set.
3. The engineer depressurised the system, which you will have noticed – it is not a five-minute visit.
4. The relief valves were operated, which is audible and produces water at the tundish.
5. You were told about the discharge pipework, whether compliant or not.
6. Any non-compliance was explained, rather than not mentioned.

An unvented service that takes a few minutes and produces a sticker has not checked the vessel charge, because that alone takes longer than that.

Frequency and the warranty

Annually, and every manufacturer requires it. The warranty on an unvented cylinder is conditional on it, recorded in the benchmark book or equivalent, and manufacturers do check when a claim is made.

The practical case is stronger than the warranty case. The expansion vessel genuinely does lose charge each year. The relief valves genuinely do seize when never operated. Those are not theoretical risks – they are the two things we find most often.

If you are buying a house with an unvented cylinder, ask for the service history and the G3 certificate. Their absence is not a deal-breaker but it is worth knowing, because an unserviced cylinder with no paperwork is a cylinder somebody will have to bring back into a known state.