Why is my boiler losing pressure and how do I fix it?

Boiler pressure is one of the most common problems London homeowners and landlords contact us about. When the needle on the pressure gauge drops, the heating system can stop working properly, leaving radiators cold and no hot water.

The good news is that a small drop in pressure is often easy to resolve. The key is knowing what is safe to do yourself and what needs a Gas Safe engineer.

What boiler pressure should be

Most combi and system boilers should sit at around 1 to 1.5 bar when the system is cold. If the gauge reads below 1, or shows zero, the boiler may refuse to start as a safety measure.

Check the reading when the system is cold, not after the heating has been running, because warm water increases the pressure reading.

Common causes of pressure loss

Pressure drops happen for a few reasons:

• Bleeding radiators releases air and can lower system pressure.
• A small leak in pipework, a radiator valve or the boiler itself.
• A faulty pressure relief valve letting water out.
• An expansion vessel problem, which is common on older systems.
• A recent service or repair where the system was drained and not fully re-pressurised.

If the pressure drops only once after bleeding radiators, topping it up may be all that is needed. If it keeps dropping, there is almost certainly a fault worth investigating.

How to top up boiler pressure safely

Many boilers have a filling loop, a flexible silver hose or two small valves under the boiler. Open the valves slowly until the gauge reaches about 1.5 bar, then close them again.

Only do this if you are confident. If you are unsure, or the pressure drops again quickly, do not keep topping it up — that can mask a real leak and waste water.

When to call a heating engineer

Call a Gas Safe engineer if:

• The pressure drops repeatedly after topping up.
• You can see a visible leak or damp patch near the boiler or pipework.
• The pressure is very high or the relief valve keeps discharging.
• You smell gas or suspect a gas-related fault — turn off the gas and call a professional immediately.

If your boiler keeps losing pressure, our heating service can trace the cause and get the system back to a safe operating level. For winter prep and regular maintenance, see our guide to boiler maintenance mistakes.

All gas work should be carried out by a registered engineer. You can verify any engineer on the Gas Safe Register, and the HSE gas safety pages explain the rules around gas appliances.