What is an unvented hot water cylinder and what am I allowed to do to it?
What the safety devices on an unvented cylinder do, how to read a discharging tundish, and exactly where the line falls between what you may check and what needs a ticket.
- Difficulty
- intermediate
- Time
- 15 min
- Read
- 5 min
- Safety
- restricted
Short answer
An unvented cylinder is a sealed store of hot water at mains pressure, kept safe by an expansion vessel, two relief valves and a visible discharge through a tundish. You may look at it, read the tundish, note the pressure and check the thermostat setting. You may not adjust, test, drain or replace any of the safety devices — that requires the unvented qualification and building control notification.
Do this first
An unvented cylinder is why modern houses have strong showers upstairs. Instead of a tank in the loft pushing water down with gravity, the cylinder is sealed and fed straight from the main, so hot water arrives at every outlet with the same force as the cold. The price of that is that water can no longer expand upwards into an open vent, and can no longer boil off harmlessly. Everything that keeps it safe is a device, and every device can fail — which is why the regime around them is as strict as it is.
Safety
Step by step
- Learn the control group on the cold inlet.A cluster of fittings on the pipe entering the bottom of the cylinder: an isolating valve, a strainer, a pressure reducing valve that steps the main down to the cylinder's working pressure, a single check valve stopping hot water pushing back into the cold main, and an expansion relief valve.
- Find the expansion vessel.A red or blue steel vessel, usually mounted near the cylinder or on the pipework. Inside is a rubber diaphragm with an air charge behind it, which absorbs the expansion when the stored water heats. Some cylinders use an internal air bubble instead and have no external vessel.
- Find the temperature and pressure relief valve.Screwed directly into the cylinder body. It is the last line of defence and opens if the stored water becomes dangerously hot or over-pressured. It should never operate in normal service.
- Find the tundish and follow the discharge pipe.A small open funnel with a visible air gap, sited where it can be seen — usually just above the cylinder. Everything that relieves discharges through it. From there a metal pipe falls continuously to a safe visible outside point, so a discharge is noticed.
- Read the tundish as a diagnostic.Water running or dripping through it is never normal. It means a relief valve has lifted: most often a failed expansion vessel, sometimes a failed pressure reducing valve, occasionally a control fault letting the cylinder overheat. Note whether it discharges only when the cylinder is heating, and tell the engineer — it narrows the fault immediately.
- Know what you may do.You may look, listen and read. You may check the cylinder thermostat setting, check that the immersion heater switch is on, note whether the discharge is hot or cold, take a photo of the data badge, and turn the cold inlet isolating valve off in an emergency. That is the whole list.
- Know what you may not do.Do not lift, lever, test or 'free off' any relief valve. Do not fit a hose, a bucket or a plug to a discharging tundish. Do not put a gauge on the expansion vessel or add air to it. Do not drain the cylinder. Do not remove the thermostat pocket or the immersion heater.
- Get the annual service done and keep the paperwork.An unvented cylinder needs annual attention by someone holding the qualification: relief valves tested, expansion vessel charge checked, strainer cleaned. The benchmark log book and installation certificate are also what a buyer's solicitor asks for at sale.
Tips
- Take a photo of the data badge on the cylinder. Make, model and serial number are the first thing anyone will ask for and the badge is often awkward to read in situ.
- The discharge pipe must terminate somewhere visible. If yours vanishes into a wall or a hidden drain, nobody will ever notice a fault developing — mention it at the next service.
- The isolating valve on the cold inlet is the one control here a householder can safely use. Know where it is.
- Buying a house with an unvented cylinder? Ask for the installation certificate and the service history. No paperwork means no evidence it was installed by anyone qualified.
If it doesn't work
Water dripping or running from the tundish
Cause: A relief valve has lifted — most commonly because the expansion vessel has lost its air charge and the water has nowhere to expand into — Fix: Note when it happens relative to the cylinder heating, then call an engineer with the unvented qualification. Do not block it, plumb it away or ignore it.
Water hammer or banging when a hot tap is closed
Cause: The expansion vessel has failed, or the pressure reducing valve is letting full mains pressure through — Fix: Both are engineer work on this system. It is also an early warning of the same failure that will start the tundish discharging.
Hot water is scalding, or the discharge is very hot
Cause: The cylinder thermostat or its control has failed and the water is overheating — Fix: Switch off the heat source — the immersion switch and the boiler — and call an engineer the same day. An overheating unvented cylinder is the failure mode the safety devices exist for.
Pressure at the hot taps has dropped but cold is fine
Cause: Blocked strainer in the control group, or a failed pressure reducing valve — Fix: Engineer work — the strainer sits inside the pressurised control group. Worth reporting alongside anything else you have noticed, as both are routine service items.
Questions people ask
Why is this ticketed when I can work on a vented cylinder?
Because a vented cylinder is open to atmosphere — it cannot pressurise and its worst failure is a leak. An unvented cylinder stores water above atmospheric boiling point held liquid only by pressure. If both relief paths fail, the release is explosive. That is why the building regulations treat it as controlled work.
Can I fit an unvented cylinder myself?
No. It is notifiable work under the building regulations in England, Wales and Northern Ireland and under a building warrant in Scotland, and installation requires a specific competency qualification. An uncertified installation is uninsurable, unsellable and genuinely dangerous.
How do I know if my cylinder is unvented rather than vented?
An unvented cylinder has no tank in the loft, has a tundish and discharge pipe, and gives hot water at the same pressure as the cold. A vented cylinder has an open vent pipe running up to a loft cistern and gives weaker hot water upstairs.
Is a megaflo the same thing?
Megaflo is a brand of unvented cylinder, and the name is often used generically. Everything here applies to any unvented cylinder whatever the badge on it.