Unvented Cylinder Service in London

What happens during an unvented cylinder service

An unvented cylinder holds mains pressure hot water in a sealed steel vessel. Nothing about that is dangerous while the controls work, and the service exists to prove they still do, because almost all of them fail silently. A seized relief valve looks identical to a healthy one until the day it is needed.

A proper service follows the same order every time, and an engineer who skips straight to the vessel without isolating and draining down is not doing the job. This is what we work through on a standard visit.

If you have never watched one being done and want the longer version before booking, what happens in an annual unvented cylinder service walks through the same visit from the householder side: what gets isolated, why water has to come out of the vessel, how long the cupboard is out of use and what you are handed at the end. The list below is the working order we follow.

• Check the system as found: incoming main pressure and flow at the nearest cold tap, stored water temperature, and whether the tundish is dry or weeping.
• Inspect the cylinder body, the insulation and the floor beneath for staining, rust or any sign of a slow weep from a boss or an immersion flange.
• Isolate the cold feed, break the vacuum and drain enough water to work on the vessel and the valve set safely.
• Test the expansion vessel. On an external vessel that means checking the charge behind the schrader valve and recharging it to suit the incoming main. On a bubble top or internal air gap cylinder it means re-establishing the air gap through the drain down procedure.
• Exercise and test the pressure reducing valve and the expansion relief valve in the cold water combination valve, and confirm the pressure reducer is still holding the cylinder at its design pressure rather than passing mains pressure straight through.
• Manually lift and reseat the temperature and pressure relief valve, confirm it opens freely, discharges cleanly into the tundish and closes without weeping afterwards.
• Clean the strainer in the inlet group. In London this is the single dirtiest component on most cylinders and a blocked strainer is regularly the reason hot water flow has gone weak.
• Check the immersion heater or heaters, the immersion thermostat and the thermal cut out. The cut out is a one shot safety device and a tripped one means something else has already gone wrong.
• Check the cylinder thermostat and the motorised valve that feeds the coil from the boiler, and confirm the stored temperature is being held where it should be.
• Follow the discharge pipe from the tundish to its termination: correct fall, correct material, no reduction in size, and a safe discharge point outside.
• Refill, vent, bring the cylinder back up to temperature, check every joint that was disturbed and check hot flow at the taps and the shower.
• Record the findings and hand them over, including anything that passed this year but will not pass next year.

If an engineer services an unvented cylinder without draining any water from it, the expansion vessel has not been tested. Draining down is how the air side is checked. A dry visit is not a service.

Why an unvented cylinder needs servicing every year

A vented cylinder in a loft is open to the atmosphere through a vent pipe, so if anything overheats or over pressurises, the system has a free path to relieve itself. An unvented cylinder has no such path. It is a closed steel vessel connected straight to the incoming main, and everything that keeps it safe is a mechanical component with a spring, a seat, a diaphragm or a bulb in it.

Water expands as it heats. In a sealed cylinder that expansion has nowhere to go unless something absorbs it, which is the job of the expansion vessel or the internal air gap. If that absorption is lost, every heating cycle pushes the pressure up until the expansion relief valve opens and dumps water through the tundish. The valve is then doing a job it was only ever meant to do in an emergency, several times a day, and it wears out.

Behind that first line sits the temperature and pressure relief valve, which is the device that stops a cylinder becoming genuinely dangerous if the thermostat and the thermal cut out both fail. It sits closed and untouched for years. Relief valve seats pick up limescale and debris, and a valve that has never been lifted can bond to its seat. Lifting and reseating it once a year is the only way anyone knows it still works.

That is the honest reason for an annual service. It is not a formality and it is not a subscription. It is the only point in the year when the safety chain on a pressurised vessel of very hot water gets physically proved rather than assumed. Manufacturers make the annual service a condition of their cylinder warranties for the same reason.

What goes wrong when the service gets skipped

Skipped services do not announce themselves. The cylinder carries on making hot water while three separate problems build underneath, and the first sign is usually water arriving somewhere it should not be.

The expansion vessel is where it starts. The vessel holds a charge of air behind a rubber diaphragm, and that air migrates through the rubber slowly over months and years. As the charge falls, the vessel takes in more water and absorbs less expansion, and eventually it is full of water with no cushion left at all. That is a waterlogged vessel. The symptom is a tundish that drips every time the cylinder reheats, or a relief valve that has begun to weep constantly. People often assume the valve has failed and replace it, and the new valve does exactly the same thing within a fortnight, because the valve was never the fault.

The relief valves are next. An expansion relief valve that has been passing water for months has a scaled, scored seat and will not close properly again even once the vessel is recharged. A temperature and pressure relief valve that has never been exercised can seize shut, which removes the last safety device on the cylinder without anyone knowing. Both of these are cheap parts caught at a service and a bigger job once they have failed.

Then there is scale, which settles on the hottest surfaces first: the immersion element and the boiler coil. A scaled immersion runs hotter to push the same heat into the water and eventually trips its thermal cut out. A scaled coil transfers less heat, so recovery after a bath stretches out. Neither is sudden, and both are the direct result of nobody looking.

The worst case is not a leak. It is a cylinder where the thermostat has failed high, the thermal cut out has already tripped and been reset by somebody without asking why, and the temperature and pressure relief valve is seized. At that point there is nothing left between a heating element and a vessel of water above its boiling point. That combination is rare precisely because unvented cylinders get serviced. It is also exactly what the service is for.

Never cap, plug or isolate a dripping tundish or relief valve to stop the noise. That discharge is the cylinder relieving itself and blocking it removes the safety device rather than the fault. Turn off the immersion, turn the boiler hot water off and get it looked at.

The tundish tells you more than anything else on the cylinder

The tundish is the small open funnel in the pipe leaving the relief valves, usually mounted high on or near the cylinder. It exists so that a discharge is visible rather than hidden inside a pipe. It is the cheapest diagnostic on the whole system and most people have never been told what it means. If yours is already wet, the tundish dripping page works through each cause in order, and there is a separate page on a leaking pressure relief valve for when you can see which valve is doing it.

A tundish that is bone dry is good news. A tundish that drips only while the cylinder is heating and stops when it reaches temperature is almost always an expansion problem: a lost vessel charge, a failed diaphragm, or an internal air gap that has dissolved away. A tundish that runs continuously whether the cylinder is heating or not usually means a relief valve is being held off its seat by debris or scale, or that the pressure reducing valve has failed and is letting full mains pressure into the cylinder.

A tundish discharging hot or steaming water is a different matter and is not a maintenance issue. That is the temperature side of the relief valve operating, which means the stored water has gone well past its normal working temperature. Switch off the immersion heater and the hot water on the boiler, leave the discharge alone and call an engineer straight away.

We check the tundish and its discharge pipework at every service, because the pipework fails inspection more often than the valves do. Common faults are a discharge pipe reduced in diameter along its run, one that rises instead of falling continuously, one terminated inside a cupboard, and one run in a material that will not survive near boiling water.

Hard water and what it does to a cylinder in London

Most of London sits on hard water, and a hot water cylinder is where that shows up first. Calcium comes out of solution as the water heats, and it deposits preferentially on the hottest surface in the vessel. In a cylinder that is the immersion element and the primary coil.

On an immersion element the scale forms an insulating crust. The element keeps producing the same heat but can no longer get it into the water, so the element surface temperature climbs. Elements scaled like that fail early, and before they fail they often trip the thermal cut out on the immersion thermostat, which presents as hot water that stops working and then mysteriously works again after somebody presses a button.

On the boiler coil the effect is quieter. The coil still works, just less well, so recovery time stretches out. If the cylinder used to reheat while you had breakfast and now takes most of the morning, and the boiler itself is fine, a scaled coil is a likely explanation. Beyond a certain point a scaled coil cannot be recovered and the cylinder needs replacing, which is one of the arguments for keeping the service up.

Scale also collects in the strainer on the inlet group along with grit from the mains, and in a city with a lot of old iron and disturbed supply pipes there can be a surprising amount of debris in there. A partly blocked strainer restricts the flow into the cylinder, which people experience as weak hot water at a shower while the cold is fine. Cleaning it is part of every service and it is often the moment the customer says the pressure is better than it has been in years. Our article on hard water and hot water cylinders in London goes into what scale does to a cylinder over its life and what actually slows it down.

Hot water cylinder service: unvented, vented and thermal store

People book a hot water cylinder service for three quite different pieces of equipment, and what the visit involves changes depending on which one is in the cupboard.

An unvented cylinder is a sealed steel vessel fed from the mains, with a valve set on the cold inlet, a tundish, and either an external expansion vessel or an internal air gap. This is the type that legally needs a qualified engineer and the type this page is about. Common examples across London include Megaflo, Telford, OSO, Gledhill, Ariston and Joule cylinders, and while the trim differs, the service principles are identical.

A vented copper cylinder is fed by a storage tank in the loft and is open to atmosphere. With no relief valves and no expansion vessel there is much less to service: an immersion and thermostat check, a look at the tank, ball valve and overflow, and an inspection for pinholing on the copper. Anyone competent can do this work.

A thermal store or a combined unit such as a Pulsacoil reverses the arrangement: the stored water is heating water and the domestic hot water is heated instantly as it passes through a plate or a coil. These need servicing too, but the components are different, so make sure whoever attends knows which unit they are coming to.

If you are not sure which you have, look for a tundish and a valve set on the cold inlet, and check whether there is a tank in the loft. Mains pressure hot water with no loft tank points to an unvented cylinder. A photograph of the cupboard before the visit means we bring the right parts. If you are still working out which type you have, the comparison of unvented and vented cylinders covers how to tell them apart in thirty seconds, and there is a separate explanation of how a Pulsacoil works for anyone with one of those in the cupboard.

Whichever type you have, it is worth knowing what a hot water cylinder service should include before anyone attends, so you can tell a proper visit from a quick look and a sticker. The short test is whether water came out of the vessel and whether you were left with a written record of what was checked. If neither happened, you have paid for an inspection rather than a service, and the components that fail silently are exactly the ones that were not touched.

Who is allowed to service an unvented cylinder?

Unvented hot water systems sit under Building Regulations, and the work must be carried out by someone holding the relevant unvented hot water qualification, which the trade knows as G3. That is the only sentence of regulation you need from us.

The practical point matters more. A G3 card means the engineer has been trained specifically on sealed hot water storage: what the valve set does, how the expansion side works, what the discharge arrangement must look like and how the safety chain is meant to fail safe. A very good general plumber without that training can still change a tap, but will not reliably spot a discharge pipe that has been run wrong or an expansion vessel that has quietly gone flat.

It also matters for your paperwork, because a manufacturer or an insurer asking about a cylinder will want a service record with the right qualification behind it, and so will a managing agent. Our page on what a G3 qualified plumber is covers what the qualification allows and what to ask an engineer to show you at the door.

Every engineer we send to an unvented cylinder is qualified for that work. We have twenty engineers across London and the cylinder work goes to the ones who do it every day.

How long an unvented cylinder service takes, and what we need from you

Allow about an hour for a straightforward annual service on a cylinder that is in good order and easy to reach. The drain down and refill is most of that time, because the vessel side cannot be checked properly with the cylinder full and the refill has to be vented and brought back up to temperature before we leave.

It takes longer when access is awkward, which in London it very often is. Cylinders live in airing cupboards stacked with bedding, in low loft spaces, behind boarded in bulkheads in converted flats and in kitchen cupboards in new build flats. Clearing the cupboard before we arrive means we are working on the cylinder rather than moving towels.

We need to know where your internal stop tap is and we need a drain point, usually the cylinder drain cock or an outside tap. In a flat with no accessible drain, say so when you book and we will bring the kit for it.

If the cylinder is heated by the boiler as well as an immersion, leave the heating system available to us rather than switched off at the isolator, since part of the check is confirming the motorised valve and cylinder thermostat still bring the coil on when they should.

• Clear access to the cylinder and to the valve set on the cold inlet.
• Access to the internal stop tap and to a drain point.
• The boiler and its controls left on and available.
• Any previous service records or the manufacturer handbook if you have them.
• Somebody in the property who can confirm what the hot water has been doing lately, because the history usually points at the fault faster than the inspection does.

Signs your cylinder is overdue a service

A cylinder rarely goes from perfect to failed overnight. It gives signals for months first, and most of them are small enough to live with.

The clearest signal is water at the tundish or the outside discharge pipe. If you have ever noticed a wet patch on an outside wall that has no gutter above it, that is very likely your cylinder discharging. Intermittent damp there, with nothing wrong indoors, is the classic signature of a lost expansion charge.

The second is recovery time. If the cylinder reheats noticeably slower than it did a year ago and the boiler has not changed, look at scale on the coil and at the cylinder thermostat before blaming the boiler.

The third is noise. A cylinder that has started rumbling, ticking or knocking on reheat is worth a visit. Some of that is normal expansion in pipework and some of it is scale on a heat source, and telling those apart takes two minutes at the cylinder.

Beyond that: a shower that has gone weak on hot but is fine on cold, an immersion that trips its cut out and has to be reset, any rust staining on the cylinder body or on the floor around it, and a hot water temperature that has drifted well above or below the low sixties. Any one of those is a reason to bring the service forward rather than wait for the anniversary.

Landlords, agents and blocks of flats

If you let a property with an unvented cylinder, the annual service is part of keeping the installation safe and part of the evidence that you have done so. The service record sits alongside the gas safety record in the same file and gets asked for in the same situations.

For managing agents with several units in one building, servicing the cylinders together in one visit is far more efficient than attending individually, because the isolation, the drain point and the access arrangements are usually shared. Tell us the number of units and the cylinder makes when you book and we will plan the visit around the building rather than around one flat.

A particular risk in flats is the discharge pipework. Conversions and older blocks are where we most often find a tundish discharge terminating somewhere it should not, into a soil stack, into a hopper above a doorway, or occasionally nowhere at all. That matters because a discharge in those circumstances is very hot water arriving unexpectedly, and it is straightforward to put right once somebody has traced the pipe. What a landlord has to keep working, and how quickly, is covered on our landlord hot water responsibilities page.

If we find something wrong during the service

Most services end with everything passing and a note on what to watch. When something fails the check, we tell you what it is, what it will do if left, and whether it needs doing today or can wait.

A lot of it we carry and can sort on the visit: expansion relief and temperature relief valves for common cylinders, immersion heaters and immersion thermostats, cylinder thermostats, strainers and the usual fittings. Recharging or replacing an external expansion vessel is normally same visit work too. Where a part is specific to an unusual cylinder we will identify it properly and come back with it rather than fit something that is nearly right.

Occasionally the honest answer is that the cylinder has reached the end. A vessel that is leaking from the body, a coil scaled past recovery, or a cylinder old enough that parts are no longer made is not worth repeated repair, and we will say so rather than sell you a series of valves. If that is the conclusion we will explain what would replace it and what the work involves. The signs a cylinder needs replacing are set out separately, and where a part has failed rather than the vessel, the unvented cylinder repair page covers the fault by fault detail.

Anything that touches the discharge pipework or the safety devices we will always raise, even when the cylinder is working perfectly, because those are the parts that matter on the day something else fails.

If at any point your cylinder is discharging steam or scalding water, or the outside of the cylinder is hot to touch rather than warm, switch off the immersion heater and the hot water at the boiler, leave the taps alone and get an engineer out. Do not attempt to drain it yourself while it is that hot.