Why Is My Boiler Losing Pressure?

The common causes of boiler pressure loss, what you can safely check, how to top up, and when to call an engineer.

Last updated: 2026-09-01

Pressure loss is one of the most common boiler complaints. Most systems sit at 1–1.5 bar when cold. If yours repeatedly drops below 1 bar or the boiler locks out, something needs attention.

Common causes

• Small system leaks — weeping radiator valves, unions or pipe joints. Look for damp patches, staining or drips under radiators.
• Bleeding radiators — every bleed drops system pressure slightly. Repeated bleeding without topping up adds up.
• Expansion vessel faults — pressure that swings high when hot and then drops to zero is a classic sign.
• Pressure relief valve discharge — check the copper pipe outside. Dripping there means the valve has been lifting.
• Boiler-internal leaks — visible underneath the casing or via the condensate. This needs an engineer.

Safe checks you can make

With the system cold, note the gauge reading, inspect visible pipework and radiator valves for moisture, and check the external relief pipe for dripping. If pressure is simply low with no visible fault, topping up via the filling loop to around 1.2 bar, following the boiler manual, is usually fine. Then watch whether it holds.

When to call an engineer

Call if pressure drops again within days, the relief pipe drips, you find any leak you cannot isolate, or the boiler locks out repeatedly. Never keep resetting a locking-out boiler, and never open the casing. Contact SureFlow with your boiler make, model, and what the gauge does hot versus cold. Our boiler repair service covers diagnosis and repair.