Unvented cylinder service: what is checked and why

An unvented cylinder holds hot water at mains pressure. That gives strong showers on every floor, but it also means the tank has to be kept safe with a small set of valves. A yearly service checks that each one still works. In Bristol, where the water is hard, the main enemy of those valves is limescale.

This guide explains what the service covers, what can go wrong in a Bristol home, and what the rules say about who can touch the system. If you want the work done, our hot water cylinder page explains how a visit runs.

What is an unvented cylinder?

Older homes in Bristol often have a vented system. A cold water tank in the loft feeds a copper cylinder in the airing cupboard, and gravity does the work. Pressure on the top floor is weak because the tank is only a few metres above the taps.

An unvented cylinder is fed straight from the mains. There is no loft tank. The water is held under pressure and heated by the boiler, an immersion heater or both. Because a sealed tank of hot water could be dangerous if it overheated, the law treats it seriously. In England this sits under Part G of the Building Regulations, in the section called G3.

What a service checks

Part	What it does	What we look for
Temperature and pressure relief valve	Opens if the water gets too hot or the pressure climbs too high	Valve opens and closes cleanly, no weeping, no scale on the seat
Expansion relief valve	Lets off pressure if the expansion vessel fails	Same as above, plus a dry tundish
Expansion vessel	A small tank that soaks up the swell when water heats	Air charge checked and topped up to the maker’s figure
Pressure reducing valve	Lowers incoming mains pressure to a safe level	Outlet pressure steady, strainer cleaned
Discharge pipe and tundish	Carries any hot water safely away to a visible point	Route, size and fall are correct, outlet safe and visible
Thermostats and cut outs	Stop the water overheating	Working at the right settings
Immersion element	Backup heating	Signs of scale, slow heating, tripping

The engineer also looks at the cylinder for corrosion, checks the joints for leaks and fills in the service record in the back of the cylinder booklet.

Why hard water matters here

Bristol Water says all the water it supplies is moderately hard to very hard. The rocks across its area are mostly limestone, which dissolves into the water and adds calcium and magnesium. When that water is heated, the minerals drop out as scale.

In an unvented cylinder, scale has three common targets:

• The relief valve seats. A flake of scale stops a valve closing. You then see a drip in the tundish, or the discharge pipe outside runs warm.
• The heating coil. Scale on the coil slows how fast the boiler heats the tank. Recovery times get longer.
• The immersion element. Elements fur up and can fail early. Our guide to an immersion heater that is not working covers the signs.

Bristol Water suggests keeping stored hot water below 60 C to reduce scale. Be careful with that advice on a cylinder. Health and safety guidance on legionella asks for stored hot water to reach 60 C. So we do not turn cylinders down to save on scale. Treating the water or descaling during service is the safer route. Our guide to Bristol’s hard water explains the options.

Signs your cylinder needs attention now

Do not wait for the yearly visit if you notice any of these:

• Water dripping or running through the tundish.
• A warm or wet pipe outside, low on the wall, where the discharge pipe ends.
• Hot water pressure surging, then dropping.
• A banging or ticking noise as the tank heats.
• The cold water pressure suddenly feels much higher than before. That can point to a failed pressure reducing valve.

If hot water is pouring from the discharge pipe, turn off the boiler and the immersion heater, then turn off the cold supply to the cylinder. Call for help. Do not touch the pipe, as it can be scalding.

Bristol homes where unvented cylinders are common

Unvented cylinders are common in larger homes that have had a refit. In Redland and Clifton, tall Victorian and Georgian houses with two or three bathrooms often swapped their loft tanks for an unvented cylinder to get pressure to the top floor. In Bradley Stoke, where building began in 1987, many homes have a cylinder in an airing cupboard. Some of those have since been changed to unvented, and the early ones are now old enough to need parts.

If you live in a Redland villa or a flat made from one, our Redland area page covers what common in that housing.

Who can do the work

Installing an unvented cylinder is notifiable work. The installer must be competent in unvented systems. That is usually shown by a G3 qualification card, and the job is reported to building control or through a scheme that lets the installer self certify. Ask to see the card before work starts.

If the cylinder is heated by a gas boiler, any work on the boiler itself must be done by a Gas Safe registered engineer. You can check their ID card on the Gas Safe Register. Electrical work on the immersion circuit has to meet Part P of the Building Regulations.

What affects the time and price of a service

We do not publish a set price. The cost is confirmed after a look at the job. The main things that change it:

• Whether the expansion vessel needs replacing, not just recharging.
• Whether a relief valve has to be swapped because scale has damaged the seat.
• How easy the cylinder is to reach. A cupboard in a Clifton top floor flat is a different job to a utility room in Keynsham.
• Whether the discharge pipe meets the rules, or needs rerouting.

A short checklist for owners

• Find your cylinder booklet and see when it was last serviced.
• Look at the tundish once a month. It should be dry.
• Know where the cold supply valve to the cylinder is.
• Keep the area around the cylinder clear so it can be reached.

A yearly check is small compared with the cost of a scalded floor or a failed tank. In a hard water city, it is worth keeping on the calendar.