What a Hot Water Cylinder Service Should Include

What a hot water cylinder service actually includes

A service is a set of tests, not a clean. The engineer is there to make the silent parts prove they still work, because every protective device on a cylinder behaves identically whether it is functioning or dead. Nothing changes at the tap when a relief valve scales onto its seat. Nothing changes when a thermal cut out goes open circuit. The only way to know is to operate the device and watch what it does.

On a sealed pressurised cylinder, the visit covers the inlet control group on the cold feed, which holds the pressure reducing valve, the check valve, the line strainer and the expansion relief valve. It covers the temperature and pressure relief valve screwed into the cylinder body. It covers the expansion arrangement, whether that is a separate vessel or an air pocket held in the crown of the cylinder. It covers the thermostat and the cut out sitting behind it. It covers the tundish and the pipe leaving it. On an indirect cylinder it also covers the motorised valve and its switch, and on an electrically heated one the immersion element and its wiring.

What you should see happening is physical. Valves being lifted off their seats by hand and watched to see whether they reseat cleanly. A gauge going onto the expansion vessel with the water pressure taken off first. A thermometer reading the stored water rather than an engineer touching the pipe and saying it feels about right. The strainer cap coming off the inlet group so the mesh can be pulled, flushed and put back. Those four actions take most of the visit, and none of them can be done from the doorway of the cupboard.

An unvented cylinder service is the version of this visit that is restricted by law, because the devices being checked are the ones the regulations exist for in the first place.

• Both relief valves exercised by hand and confirmed to reseat with no drip behind them.
• The expansion arrangement read and recharged, or re established on a bubble top cylinder.
• Stored water temperature measured and recorded as a figure.
• The inlet control group checked static and under flow, with the strainer pulled and cleaned.
• The tundish and the full discharge pipe followed by eye to its termination.
• The findings written against the cylinder before the engineer leaves.

How long should a cylinder service take?

Allow between forty minutes and an hour and a half for an unvented cylinder in reasonable condition. That is not padding. Taking the water pressure off an expansion vessel, reading the charge cold, pumping it back and rechecking it is fifteen to twenty minutes on its own. Pulling and flushing the strainer is another ten. Exercising both relief valves and watching each one reseat is another ten, and on a bubble top cylinder re establishing the air pocket means a controlled drain and refill sequence that cannot be hurried.

A twenty minute visit is not a service. It is enough time to look at the cylinder, put a hand on a few pipes, tick a card and go. In a cramped London airing cupboard with the control group facing a wall and a boarded panel over the discharge run, twenty minutes is barely enough time to get access, let alone test anything.

The other end matters too. If somebody is there for three hours they have found something and started repairing it, which is fine, but it should have been said out loud before the work began.

Time on its own proves nothing. An engineer can be in the cupboard for an hour without touching a relief valve. Watch what is done, not how long it takes.

What a thorough engineer does in the first ten minutes

Before any tool comes out, a good engineer builds a picture. They ask when the cylinder was last serviced and whether there is a record card or benchmark book near it. They ask whether anyone has seen water passing through the tundish, and roughly when it happens. They ask whether the reheat has got slower and whether any tap has gone weak. They look at the cupboard floor and the ceiling of the room below for old staining.

Those questions are not small talk. Water through the tundish that tracks the heating cycle points straight at the expansion arrangement before a single gauge is fitted. A reheat that has doubled over a few years points at scale on the coil or a dead immersion element doing half the work. An engineer who starts by asking these things is narrowing the job. One who opens the cupboard door without a word is running a script.

The second thing that happens early is reading the data badge, because the figures printed there govern the whole visit. The badge gives the expansion charge the manufacturer specified and the operating pressure the cylinder was built for. Setting a vessel to a general number rather than the badge figure is one of the clearest signs of a service being done from memory.

The checks that separate a real service from a quick look

Three checks get skipped more than any others, and all three are the ones most likely to be hiding a fault.

The line strainer inside the inlet control group is first. It is a fine mesh that catches grit off old iron mains, scale flakes and fragments of jointing compound, and in London it fills steadily. As it blocks it starves the cylinder of flow, and people spend a year blaming a weak shower on the mains. Clearing it means isolating, unscrewing the cap, lifting the mesh, flushing it and refitting it. Ten minutes, and skipped constantly because nobody asks about it.

The expansion arrangement is second. The single most common fault found on a working unvented cylinder is a vessel that has lost its air charge, and the only way to know is to take the water pressure off and put a gauge on. Tapping the side of a vessel and listening tells you nothing useful. If a vessel takes air and loses it again within minutes the diaphragm has gone and the vessel needs changing rather than pumping up and leaving.

The discharge route is third, and it is the one with real consequences. The tundish exists so a discharge is visible and so there is an air break between a pressurised cylinder and the drain. The pipe leaving it must fall continuously, must be in a material rated for near boiling water, and must end somewhere the discharge can be seen and cannot scald anyone. In converted London flats the usual finding is a discharge quietly plumbed into a hopper or a waste pipe during a bathroom refit, which removes the only warning the cylinder has. Following that pipe takes two minutes and is worth more than most of the rest of the visit.

There is no mystery to what happens in an annual unvented cylinder service. The order is close to fixed, and an engineer who departs from it is usually skipping something rather than improvising.

• Strainer pulled and flushed, not assumed clean.
• Expansion vessel read with a gauge and compared to the badge figure.
• Discharge pipe followed to its termination rather than glanced at by the tundish.

What changes if you have a vented cylinder or a thermal store

Not every cylinder is a sealed pressurised one, and the service changes considerably with the type.

An open vented copper cylinder fed by a tank in the loft has no pressurised safety chain at all, so there are no relief valves to exercise. The service is shorter and the emphasis moves: the immersion element and its wiring, the stored temperature, the loft tank and its ball valve, the insulation, and any weeping around the boss or the seams. If somebody charges you for a full unvented service on a vented copper cylinder, they have not looked at what is in front of them.

Thermal stores and Pulsacoil type units, which turn up constantly in purpose built London blocks, are different again. The stored water in these is heating water rather than the water you wash in, and your hot water is made on demand through a plate exchanger or an internal coil. So the service goes to the controls and the boards, the immersion elements and their thermostats, the plate exchanger and whether it has scaled on the domestic side, the pump, and the fill level of the store. The relief valve routine from an unvented cylinder does not apply. An engineer who has only ever serviced unvented cylinders will make a poor job of a thermal store, and the polite way to find out is to ask which they have worked on before.

What should be written down at the end

The paperwork is where a thorough service proves itself, and it is the part most easily judged by someone who is not an engineer. You want figures, not ticks. A record with nothing but a signature and a date is worth nothing to the next engineer and nothing to you.

The record should say what the expansion charge read cold and what it was set back to. It should say which relief valves were exercised and whether each one reseated cleanly. It should give the stored temperature as a measured figure. It should say what condition the strainer was in. It should confirm the discharge route was inspected and note anything wrong with it. And it should list anything left outstanding with a plain recommendation and a rough sense of urgency.

That record matters because a cylinder declines slowly. One year of readings tells you almost nothing. Three years tells you the vessel is losing its charge faster each year, or that the reheat has crept up, or that the same valve has needed attention twice. That pattern is what decides whether to keep servicing a cylinder or start planning to replace it, and you only get it if somebody wrote numbers down.

Ask for the record before the engineer leaves, not a week later. A service recorded from memory in the van is a service you cannot check.

Signs the service was not done properly

Some of these show on the day and some in the following weeks. None require any technical knowledge.

On the day, watch for a visit that ends without the cold feed ever being isolated, because almost nothing on the inlet group can be checked with the water still on it. Watch for a cylinder never touched with a thermometer, and for a panel never opened to see where the discharge pipe goes. Watch too for parts being quoted before the cylinder was opened up, because nobody knows which valve set or which vessel you have from a photograph.

In the weeks after, the clearest sign is water passing through the tundish on a cylinder that was serviced recently. That symptom points at the expansion arrangement or the reducing valve, which are exactly the two things the visit was supposed to test. If it starts after a service, the service missed the fault you were paying to have found. The same goes for a relief valve that has been capped, plugged or nipped up to stop a drip. That is not a repair, it is removing a safety device, and any engineer who does it should not be back in the property.

• The cold feed was never isolated.
• No thermometer, no gauge, no tool ever went near the safety devices.
• No paperwork, or paperwork with ticks and no figures.
• Water through the tundish beginning shortly after the visit.
• A drip stopped by capping or tightening a relief valve rather than finding the cause.

What a cylinder service will not include

A service tests and records. It does not replace parts and it is not a descale, and being clear about that in advance saves an argument at the door.

If the expansion vessel has failed, the reducing valve has drifted, a relief valve will not reseat or an immersion element is dead, those are repairs. A decent engineer tells you on the day which is worth doing while the water is already off and which can wait until next year. Fitting a vessel on the same visit is normal. Being told the whole cylinder needs replacing by somebody who has not shown you why is not.

A service also will not rescue a cylinder already weeping from the body. Stainless cylinders fail at welds and at the immersion boss, and once the vessel itself leaks there is nothing serviceable left in it. No sealant belongs in a hot water vessel. At that point the conversation is about a replacement, and the honest version of it includes why the old one failed, because a cylinder sitting on a flat expansion vessel for years did not die of old age.

One last point on cost. A service is priced as labour, and the length of the visit is driven by access and by the expansion arrangement more than by anything else. Nobody can sensibly quote for parts before seeing which control group and which vessel you have, so a parts figure arriving before the visit was invented.

Anything found to be broken rather than merely due for a check belongs in an unvented cylinder repair, priced and agreed on its own, rather than folded silently into the service and appearing at the end as a larger number than you expected.

Nor does it postpone a hot water cylinder replacement that is already due. A service records the condition of the vessel, it does not improve it.

It is also not the moment to revisit hot water cylinder sizes, which is a design question belonging with a replacement rather than with maintenance.

How to book one and actually get the thorough version

Say what you have when you book. Tell them whether it is a sealed pressurised cylinder, an open vented copper one with a loft tank, or a thermal store, and read the name off the badge if you can reach it. That one sentence decides who gets sent and how long is allowed for the visit.

Clear the cupboard first. Cylinders in London flats live behind coats, boxes and a boarded panel, and every minute spent emptying it is a minute not spent testing. Leave the record card out and know where your stopcock is.

Then ask two questions at the door. What will you check on the expansion arrangement, and what will I get in writing. An engineer who answers both without hesitating is going to do the job. One who talks around them is planning a quick look, and the door is a much better place to learn that than next winter.

Ask directly whether the person attending is a G3 qualified plumber and ask for the registration number, because on unvented work that is the difference between a service and a look.

• Say which type of cylinder you have when you book.
• Empty the cupboard and expose the control group and the discharge pipe.
• Leave the record card or benchmark book out.
• Ask what will be checked on the expansion arrangement and what you will be given in writing.