What the pressure should be
On most sealed-system and combi boilers, the pressure gauge should read around 1 to 1.5 bar when the heating is cold, and rise a little when it is running. If it sits below 1 bar the boiler may lock out. If it climbs towards 3 bar, switch the boiler off and call an engineer.
Common causes of pressure loss
- A leak somewhere in the system. Radiator valves, pipe joints and the boiler itself are the usual places. Look for damp patches, staining and drips.
- Recently bled radiators. Letting air out also lets pressure drop. This one is normal and just needs topping up.
- A faulty pressure relief valve. If the small copper discharge pipe outside is dripping, the valve may be releasing water.
- A failing expansion vessel. This absorbs pressure changes when water heats. When the air charge is lost, pressure spikes, the relief valve opens and pressure falls over time.
What you can safely check
Look at your boiler manual first. With the boiler off and cool, you can usually top the pressure up using the filling loop, a flexible braided hose with a valve at each end under the boiler. Open the valves slowly until the gauge reaches 1 to 1.5 bar, then close both firmly.
Check radiators and visible pipework for signs of leaks. That is as far as home checks should go.
When to call an engineer
- You are topping up more than once a month
- The pressure rises sharply when the heating comes on
- You can see water coming from the boiler or its pipework
- You are unsure about any step
Do not remove the boiler casing or tinker with gas components. That is work for a Gas Safe registered engineer by law.
Repeated pressure loss usually points to a small fault that gets worse. We carry out emergency and same-day heating repairs across Hull and East Yorkshire.