Unvented hot water cylinders, and what G3 actually means
Updated 1 October 2026 · 2 minute read
If you have a hot water cylinder and no cold water tank in the loft, you probably have an unvented cylinder. It stores hot water at mains pressure, which is why the shower feels as strong as the cold tap.
How it works
- Fed from the mains. No loft tank. Hot water arrives at the same pressure as the cold.
- Heated by your boiler, through a coil inside the cylinder. Most also carry an immersion heater as a backup.
- An expansion vessel takes up the extra volume as the water heats, because in a sealed system that expansion has nowhere else to go.
- A pressure relief valve, a tundish and a discharge pipe carry water safely outside if the pressure or temperature ever gets too high.
Why G3 matters
That safety chain is the reason unvented cylinders have their own qualification, G3. Fitting one is notifiable building work, and it must be done by a G3 qualified installer. Our cylinder installs are done by a G3 qualified engineer, and we notify Building Control as part of the job.
The fault we see most
When the expansion vessel loses its charge, the extra volume has nowhere to go, so the pressure relief valve starts to weep. You might see water dripping from the tundish, or from a pipe outside. It is the most common unvented fault we get called to, and it should not be ignored.
What size do you need?
Sizing is about how much hot water gets used at once and how fast it reheats, not about copying whatever is there now. Our starting point:
| Cylinder | Household | Bathrooms |
|---|---|---|
| 120 litres | 1 to 2 people | One, light use |
| 150 litres | 2 to 3 people | One |
| 180 litres | 3 to 4 people | One plus an en suite |
| 210 litres | 4 to 5 people | Two |
| 250 litres | 5 to 6 people | Two or three |
| 300 litres | Larger household | Three or more |
Too big and you pay to heat water nobody uses. Too small and it runs cold halfway through the second shower. We also check your incoming mains flow first, because an unvented cylinder on weak mains will disappoint whatever its size.
Getting a cylinder fitted in Bridgend
We fit vented and unvented cylinders across Bridgend county. Tell us how many people and bathrooms, send a photo of what you have now, and we will price it before we start. Price my cylinder.
Keep reading
- How a plumber in Bridgend prices your job Why you will not find a plumbing price list here, what we need from you to price a job, and why the price is agreed before we start.
- Dripping tap? Washer, cartridge or a whole new tap How to tell what kind of tap you have, why it drips, and when a repair is worth it versus a new tap. A plain guide from a Bridgend plumber.
- Toilet keeps running? What is going on in the cistern The three usual reasons a toilet keeps filling or trickling into the pan, what you can check safely, and when to call a plumber in Bridgend.