What Is an Unvented Hot Water Cylinder, and Does It Need Servicing?
An unvented hot water cylinder stores hot water at mains pressure, fed straight from the cold mains. There’s no cold water tank in the loft. That gives you stronger showers and lets several taps run at once, but because the cylinder is under pressure, it relies on safety valves to control temperature and pressure.
Yes, it should be serviced, and most manufacturers ask for this once a year. It must be installed by someone qualified to fit unvented systems, usually called “G3 qualified”, after the part of the Building Regulations that covers them.
Vented vs unvented: what’s the difference?
| Vented cylinder | Unvented cylinder | |
|---|---|---|
| Water supply | Gravity-fed from a cold water tank in the loft | Fed directly from the cold mains |
| Water pressure | Depends on the height of the tank. Often weaker upstairs | Mains pressure throughout the house |
| Loft tank needed | Yes | No |
| Safety devices | Open vent pipe to the tank | Expansion vessel or air bubble, pressure and temperature relief valves, tundish, discharge pipe |
| Installation rules | Standard plumbing | Building Regulations G3. Qualified installer, notified to Building Control |
| Servicing | Occasional checks | Annual service usually required by the manufacturer |
Unvented cylinders are common in newer homes, in homes with system boilers, and in homes with heat pumps.
Why unvented cylinders have extra rules
Water heated under pressure holds a lot of stored energy. Unvented cylinders are designed with several independent safety devices so that pressure and temperature can never build up uncontrolled:
- a pressure reducing valve that controls the incoming mains pressure
- an expansion vessel (or an internal air bubble) to absorb the extra volume as water heats up
- an expansion relief valve and a temperature and pressure (T&P) relief valve
- thermostats and a high-limit cut-out on the heat source
- a tundish and discharge pipe, which let you see when a relief valve has opened and carry the water safely away
That’s why Building Regulations require unvented systems to be installed by a competent, suitably qualified person, and the installation has to be notified to Building Control. It isn’t a DIY job, and neither is adjusting the safety valves.
What an unvented cylinder service involves
Exactly what’s done depends on the manufacturer’s instructions, but a typical annual service includes:
- checking and recharging the expansion vessel pre-charge, or the air bubble
- manually testing the relief valves so they open and reseat properly
- checking the pressure reducing valve and cleaning the line strainer
- checking the tundish and discharge pipework is clear and routed correctly
- checking the thermostats and the high-limit cut-out
- looking for leaks and corrosion, and checking the immersion heater if you have one
- filling in the service record, which is often needed for the warranty
Warning signs to act on
Water running through the tundish. The tundish is the small funnel with a visible gap, usually on the pipework near the cylinder. Water passing through it means a relief valve is opening. It can be a small drip or a steady flow, and either way it isn’t normal. Common causes are a lost expansion vessel charge or a faulty valve. Book an engineer.
Very hot water coming from the discharge pipe, or steam. Turn off the heat source to the cylinder (the boiler or immersion heater) and call an engineer straight away.
Other signs:
- hot water pressure changing noticeably
- banging or noises from the cylinder
- dripping from the relief valves
- the cold mains pressure dropping when hot taps are running
Is an unvented cylinder right for your home?
An unvented cylinder can work well if you want mains-pressure showers and you don’t want a loft tank, but it depends on:
- Mains water pressure and flow. The incoming supply needs to be strong enough. A good installer will measure it before recommending one.
- Space for the cylinder and a suitable route for the discharge pipe outside.
- Your heat source. Unvented cylinders suit system boilers and heat pumps. Combi boilers don’t use a cylinder at all.
FAQs
Can I service an unvented cylinder myself?
No. The safety devices need testing and setting up by a qualified engineer. You can keep an eye on the tundish, and report any water or noises you notice.
Does a boiler service include the cylinder?
Not automatically. The cylinder is a separate appliance with its own service routine. Ask for both to be done at the same visit.
How long do unvented cylinders last?
It depends on the water quality, the cylinder’s material and how well it’s maintained. Check the manufacturer’s warranty terms, which often depend on an annual service.