Boiler Losing
Pressure?

Why a Boiler Loses Pressure

Most modern combi and system boilers run on a sealed heating system. Instead of a feed and expansion tank in the loft, the system is filled to a set pressure and then closed off. When the pressure on the boiler's gauge drops below its minimum, usually around 0.5 bar, the boiler locks out and shows a low pressure fault code rather than run dry.

A sealed system should hold its pressure. A slow drop over weeks usually means water is escaping somewhere. A sudden drop to zero usually means a larger leak, or a pressure relief valve that has opened. Pressure that climbs when the heating runs and falls again when it cools points to the expansion vessel.

Safe Checks You Can Make Yourself

• Read the gauge with the heating off and the system cold. On most domestic boilers the recommended cold pressure is between 1.0 and 1.5 bar. Your user instructions give the exact figure, often marked as a green band on the gauge.
• Look for water under the boiler, around radiator valves, at visible pipe joints, and at the pressure relief discharge pipe, which is a short copper pipe through the outside wall near the boiler. Drips from that pipe mean the relief valve is passing.
• Check the radiators. A radiator that is cold at the top and warm at the bottom has air in it. Bleeding it releases the air but also lowers the system pressure slightly, so you will need to top up afterwards.

Topping Up at the Filling Loop

Topping up is a normal user task and your boiler instructions describe it. The filling loop is either a short braided silver hose with a valve at each end underneath the boiler, or a built-in key or lever on the boiler itself. The general method is:

• Switch the boiler off and let it cool.
• Open both filling loop valves slowly, or turn the key or lever as the instructions describe. You should hear water flowing.
• Watch the gauge and close the valves when the needle reaches the cold pressure your instructions give, normally between 1.0 and 1.5 bar. Do not overfill: at around 3 bar the pressure relief valve opens and discharges water outside.
• Close both valves fully. If the loop is a removable braided hose, disconnect it afterwards. The regulations do not allow it to be left permanently connected.
• Switch the boiler back on and reset any fault code.

If the loop is stiff, the valves leak, or you are not sure which control is which, stop and call us.

When It Is a Leak

If you are topping up more than once or twice a year, the water is going somewhere. Common sources are radiator valve glands, compression joints, a corroded radiator, pipework under floors, and the boiler's own pump seal, automatic air vent or heat exchanger. Water that keeps disappearing with no visible sign is often a leak under the floor. Where the pipework is buried, our leak detection service uses thermal imaging and tracer gas to find it without lifting floors on guesswork.

When It Is the Expansion Vessel or Relief Valve

The expansion vessel is a small steel tank, often inside the boiler, with a rubber diaphragm and a charge of air on one side. It absorbs the expansion of the water as it heats. When the air charge is lost, the water has nowhere to go, the pressure climbs sharply as the heating runs, the pressure relief valve opens at 3 bar and discharges water outside, and the pressure is then low once the system cools. Recharging or replacing the vessel is a routine job, but the relief valve often needs replacing at the same time, because once it has opened it rarely seals perfectly again.

When to Call an Engineer

• The pressure drops again within days of topping up
• The pressure rises above 2.5 bar when the heating is on
• There is visible water at the boiler or a pool underneath it
• The boiler locks out repeatedly, or the fault code is not a simple low pressure code
• The filling loop will not close and the pressure keeps climbing
• The hot water flow from a combi has changed at the same time

A boiler that keeps losing pressure is not dangerous in itself, but every top-up adds fresh oxygenated water, which corrodes radiators and produces sludge. Fixing the cause is cheaper than the damage it leads to. We cover Scunthorpe and North Lincolnshire and carry common parts on the van. See our boiler servicing and repair page, call 01724 304 304, or use the contact form.

Frequently Asked Questions