Why your boiler keeps losing pressure

What boiler pressure should be

Modern combi and system boilers run as a sealed, pressurised system. When the heating is cold, the gauge should normally sit between 1 and 1.5 bar. It is quite normal for the needle to rise a little when the heating is hot and the water expands, then settle back as it cools.

You have a genuine problem when the cold pressure keeps falling below about 1 bar, or when you find yourself topping it up again and again. At very low pressure many boilers simply shut down on a safety lockout and refuse to fire, leaving you with no heating or hot water.

Common causes of a boiler losing pressure

• A leak somewhere in the system. Even a pinhole weep on a radiator valve, a towel-rail joint or a hidden pipe under the floor will drop the pressure over days or weeks. Look for damp patches, tide marks under valves and rusty or green staining.
• A failed expansion vessel. This sealed tank holds a cushion of air that absorbs the extra volume as water heats up. When it loses its air charge, the expanding water has nowhere to go, the pressure spikes when hot and is forced out through the safety valve, so the gauge reads low once everything cools.
• A passing pressure relief valve. If this safety valve is weeping, water escapes down the overflow pipe that ends outside, usually low on an external wall. A dripping outside pipe is a classic sign.
• Recently bled radiators. Letting air out lowers the pressure, so a small top-up afterwards is normal.

How to top up your boiler pressure

Topping up is a job most homeowners can do with the filling loop, the braided silver hose beneath the boiler with a valve at each end. With the boiler switched off and cold:

• Find the loop and its two taps or levers.
• Open both slowly. You will hear water flowing and see the gauge begin to rise.
• Close the valves as soon as the needle reaches about 1.2 to 1.5 bar. Do not overfill, as too much pressure will simply push water back out of the safety valve.
• Switch the boiler on and reset it if needed.

If your model has a built-in keyless or digital filling system, follow the steps in the manufacturer's manual instead.

When to stop and call an engineer

Topping up once in a while is nothing to worry about. Treat the following as a sign that the underlying fault needs fixing rather than repeating:

• You are re-pressurising more than about once a month.
• Water is dripping from the overflow pipe outside.
• The pressure climbs far too high when hot, or drops to zero quickly.
• You can see a leak but cannot safely reach or isolate it.

A tired expansion vessel can usually be re-charged or replaced, and a passing safety valve swapped. Both are routine repairs, but they involve the sealed gas appliance and should be left to a Gas Safe registered engineer. If you ever smell gas, do not touch electrics or the boiler, leave the property and ring the National Gas Emergency Service on 0800 111 999.

Most pressure loss comes down to a small leak, a tired expansion vessel or a weeping safety valve, and all three are fixable. If you are topping up again and again, a proper diagnosis will find the real cause and stop the cycle for good. Omega Breakdown confirms a fixed price before any work begins, so you know exactly where you stand before an engineer lifts a spanner.

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