What the pressure gauge should show
On the front or underside of most boilers is a pressure gauge, either a dial or a digital readout. With the heating off and the system cold, it should typically sit around 1 to 1.5 bar. Many dials mark this zone in green. When the heating is running, it is normal for the pressure to rise a little.
If the gauge reads below about 0.5 bar, most boilers will shut down or show a low-pressure error code, and you will have no heating or hot water until it is topped up.
How to top the pressure up
- Switch the boiler off and let the system cool for a few minutes.
- Find the filling loop. It is usually a short braided silver hose beneath the boiler with a small valve at one or both ends. Some boilers have a built-in filling key or lever instead. Your boiler manual will show the exact arrangement.
- Open the valve slowly. You will hear water flowing. Watch the gauge climb.
- Close the valve at about 1.2 bar. Do not overfill. If you overshoot well past 2 bar, the system will be over-pressurised and may discharge water from the pressure relief pipe outside.
- Switch the boiler back on and reset the error code if it does not clear itself.
That is it. If the pressure holds, you are done.
When pressure loss means a fault
Pressure does not vanish on its own. A slow drop over months can just be tiny amounts of air working out of the system, but any of these signs point to a fault that topping up will not cure:
- You are repressurising more than every few months
- The pressure drops again within days of topping up
- You can see water under the boiler, on pipework or under radiators
- The pressure relief pipe outside drips or streams
- The pressure rises far too high when the heating runs, then drops after
The usual culprits are a small leak somewhere in the system, a failed expansion vessel or a passing pressure relief valve. All are fixable, and finding the cause early is far cheaper than the water damage a hidden leak can do. This is where we come in: see our boiler and heating repairs.