If the pressure gauge on your boiler keeps dropping, or the boiler has locked out with a low-pressure fault, you are not alone. It is one of the most common problems we see in autumn and winter, when heating systems start working hard again.
What should my boiler pressure be?
Most sealed combi and system boilers run at roughly 1 to 1.5 bar when the heating is cold, rising towards 2 bar when it is hot. Your boiler manual gives the exact range for your model. Many boilers stop working and show a fault code if the pressure falls too low.
Why does a boiler lose pressure?
Bleeding radiators. Letting air out also releases a little water, so the pressure drops afterwards. This is normal: top it back up.
A leak in the system. Check visible pipework, radiator valves and the area around the boiler for drips, damp patches or corrosion. Some leaks are hidden under floors.
A faulty pressure relief valve. If it is letting water out, you may see drips from the small pipe that runs from the boiler to the outside of the house.
A failing expansion vessel. When the vessel loses its air charge, the pressure rises sharply when the system is hot and falls when it cools.
How to repressurise a combi boiler
Most combi boilers are topped up through a filling loop, either a flexible hose or a built-in key underneath the boiler. Always follow your manufacturer’s instructions, but the general steps are:
Turn the heating off and let the system cool down.
Check both ends of the filling loop are securely connected.
Open the valves slowly and watch the gauge rise to around 1 to 1.5 bar.
Close both valves fully, then reset the boiler if it is showing a fault.
If your boiler has a removable filling loop, disconnect it once you have finished.
What if the boiler pressure is too high?
A reading close to 3 bar is too high, and the pressure relief valve will usually open to protect the system. Bleeding a radiator can bring the pressure down slightly, but if it climbs again whenever the heating runs, the expansion vessel or filling loop may be at fault and the boiler needs an engineer.
When to call a heating engineer
Topping up once after bleeding radiators is normal. Needing to top up every few days or weeks is not: water is escaping somewhere, and the cause should be found before it damages the boiler or your home. Never remove the boiler casing yourself. Work inside a gas boiler must be carried out by a Gas Safe registered engineer.
If your boiler keeps losing pressure, call Heatcare Norwich on 07715 998479 or arrange a boiler repair in Norwich. Our call-out rate is £70 for the first hour.
