Hot water
Unvented cylinder installation
Mains pressure hot water, strong enough to run two showers at once. Fitted and serviced by G3 qualified engineers, because by law it has to be.
- G3 qualified
- Gas Safe 951167
- Annual servicing
- 12-month guarantee
G3 is a legal requirement, not a preference
An unvented cylinder stores hot water under mains pressure. That is what gives you the shower, and it is also why the installation is regulated. Building Regulations Part G3 requires the work to be done by someone holding the G3 qualification, and the system needs the correct safety devices: an expansion vessel, a pressure reducing valve, a temperature and pressure relief valve, and a discharge route to a safe place.
Whether they hold G3. It is a specific qualification on top of general plumbing, and a cylinder fitted without it is both unlawful and genuinely unsafe. Ours hold it, and you are welcome to ask to see the card.
Where they work well, and where they do not
- Good fit: homes with two or more bathrooms, where a combi would struggle the moment two outlets run together.
- Good fit: replacing an old vented system, where you get to reclaim the loft tank space as well as the pressure.
- Good fit: newer estates already plumbed for one, where the cylinder is simply due for replacement.
- Poor fit: anywhere the incoming mains cannot deliver the flow. We measure this first, because no cylinder fixes a weak supply.
- Poor fit: small flats with one bathroom, where a combi does the same job with no stored water and no cupboard taken up.
Gas boiler installed by our engineers in a Kent homeOur own work · Gas Safe 951167
Servicing, and the tundish
Unvented cylinders need an annual check. The expansion vessel gradually loses its air charge, and once it does, expanding hot water has nowhere to go, so a safety valve opens and dumps water through the tundish and out of the discharge pipe. That is the dripping outside pipe people ring us about.
It is a straightforward fix when caught, and an expensive one when ignored, because the valve seat erodes and the whole assembly ends up needing replacing. Kent's hard water does not help: scale builds on the immersion and the coil, which is the usual reason hot water stops lasting as long as it used to.
Not sure which system you have? Our guide to vented and unvented cylinders explains how to tell them apart. We also cover boiler installation and annual servicing.
Common questions
What is an unvented cylinder?
A sealed hot water cylinder fed straight off the mains, rather than from a tank in the loft. Because it runs at mains pressure you get a much stronger shower and can run more than one outlet properly. There is no cold water tank and no vent pipe.
Do I need a special qualification to fit one?
The installer does, yes. Unvented cylinders are covered by Building Regulations Part G3 and must be fitted by someone holding the G3 qualification. It is not a general plumbing job. Our engineers hold it, and it is worth asking anyone quoting you whether theirs do.
Do they need servicing?
Annually, and it matters more than people expect. The expansion vessel loses its charge over time and the pressure relief valve needs checking. A neglected unvented cylinder is the one that starts discharging water out of the tundish.
Why is water running from the pipe outside?
That is the tundish discharge, and it means a safety valve is releasing. Usually the expansion vessel has lost its air charge or the pressure reducing valve has failed. It is not an emergency in the burst-pipe sense, but it should not be ignored, so ring us.
Is my mains pressure good enough?
That is the first thing we check, before recommending anything. An unvented cylinder needs adequate incoming flow, and if the mains cannot deliver it you will get a worse result than the system you already have. We measure rather than assume.
Unvented or combi?
A combi suits smaller homes with one bathroom. An unvented cylinder suits bigger homes, or anywhere two showers might run at once, because it stores hot water rather than heating it on demand. We will look at how many bathrooms you have and how you actually use them.
Thinking about mains pressure hot water?
Tell us how many bathrooms you have and what your current hot water is like, and we will tell you whether an unvented cylinder is the right answer.