Unvented hot water cylinders

Why this needs a separate qualification

An unvented cylinder is fed straight off the mains rather than from a cold tank in the loft, so the water inside is stored under pressure and above the temperature it would boil at normally. If the safety controls fail, that is genuinely dangerous — which is why Approved Document G3 of the Building Regulations requires the work to be done by someone holding a current G3 qualification.

Gas Safe registration alone does not cover it. G3 is a separate assessment, and a lot of domestic engineers do not hold it. If you are ringing round, ask — and be wary of anyone who is vague about the answer.

What G3 covers, and what we can do

• Annual servicing — this is the one people are surprised by. Servicing an unvented cylinder means working directly on the safety devices, so it needs G3 just as installation does.
• Installation and commissioning of a new cylinder
• Replacing a failed cylinder, like for like or resizing
• Expansion vessel recharge or replacement
• Pressure reducing valve, expansion relief and temperature relief valve replacement
• Immersion heater replacement
• Fault finding on poor hot water pressure, fluctuating temperature or water discharging outside

Signs yours needs attention

• Water dripping or running from the discharge pipe outside. This is the safety system doing its job, and it means something has failed — usually the expansion vessel or a relief valve. Don’t ignore it and don’t let anyone cap it off.
• Hot water pressure dropping, or surging and dropping while running
• Banging or knocking in the pipework when a tap is turned off
• Hot water running out much faster than it used to
• No hot water at all, but the heating works
• It has never been serviced, or you don’t know when it last was

Servicing and your warranty

Unvented cylinders carry manufacturer warranties, typically of several years, and nearly all of them require a documented annual service by a G3 qualified engineer. Miss it and a failed cylinder becomes your bill rather than theirs — and a replacement cylinder plus fitting is not a small number. Keep every service record.

Where we see these most

Unvented cylinders are standard in larger houses across Chiswick and Bedford Park, Notting Hill and Holland Park, Kensington, Chelsea and Maida Vale — anywhere with several bathrooms, where a combi could never keep up.

Because we hold both Gas Safe registration and G3, the boiler and the cylinder can be dealt with on the same visit instead of you booking two separate trades and coordinating them.

Common questions

How do I know if I have an unvented cylinder?

It is a large insulated metal cylinder, usually white, often in an airing cupboard, loft or utility room, with a small red or blue expansion vessel next to it and a pipe running outside. If there is no cold water tank in your loft feeding it, it is almost certainly unvented. Send us a photo when you ring and we will tell you.

How often should it be serviced?

Once a year, and the manufacturer’s warranty will normally require it. The service checks the expansion vessel charge, the relief valves, the thermostat and the thermal cut-out — the parts that keep it safe.

Water is running out of the pipe outside. Is it urgent?

It needs looking at, yes. The discharge pipe exists so that pressure can escape safely, so water coming out means a control has failed and the system is protecting itself. It is not an emergency in the sense of evacuating the house, but do not leave it for weeks and never let anyone block or cap that pipe.

Can you service the boiler and the cylinder at the same time?

Yes, and it is cheaper than two separate visits. Tell us what you have when you ring and we will price it as one job.

Is G3 the same as Gas Safe?

No. Gas Safe covers gas appliances. G3 is a separate qualification for unvented hot water storage, named after Approved Document G3 of the Building Regulations. An engineer can be Gas Safe registered and still not be allowed to touch your cylinder. We hold both.