Boiler losing pressure? Causes and what to do

Low boiler pressure is one of the most common reasons a boiler stops working, especially at the start of winter. Here is how to check it and what a falling pressure usually means.

What pressure should your boiler be at?

Most sealed central heating systems sit at around 1 to 1.5 bar when the heating is cold, rising a little when it is hot. Check your boiler manual, as the recommended figure can vary. If the gauge reads below 1 bar, or the display shows a low-pressure fault, the boiler may stop firing.

Topping up the pressure

Most combi and system boilers have a filling loop, either an external braided hose with two valves or a built-in key or lever under the boiler.

• Make sure the heating is off and the system has cooled.

• Follow the manufacturer's instructions for your model.

• Open the valves slowly and watch the gauge until it reaches the recommended pressure.

• Close the valves fully and reset the boiler if needed.

Why does the pressure keep dropping?

If you have to top up more than once or twice a year, something is wrong. Common causes include:

• A small leak on a radiator valve, pipe joint or under the floor.

• A faulty pressure relief valve discharging water outside through a small copper pipe.

• A failed expansion vessel inside the boiler that needs re-charging or replacing.

• Recently bled radiators, which release air and lower the pressure.

When to call an engineer

• The pressure drops again within days or weeks.

• You can see water dripping from the pressure relief pipe outside.

• The pressure climbs very high when the heating is on.

• You find damp patches near pipework or on ceilings.

Need help?

The Essential Connection repairs and services boilers across Walton-on-Thames, Weybridge, Cobham, Esher, Kingston and South West London. All gas work is carried out by Gas Safe registered engineers. Get in touch or call 07719 664615.