Check the gauge first
Combi and system boilers run on a sealed system, with a pressure gauge on the front of the boiler or underneath it. With the heating off and the system cold, most want somewhere around 1 to 1.5 bar. Your boiler manual gives the exact figure, and it is worth looking up, because some models ask for a little more or less.
Pressure rises as the water warms up, so a higher reading with the radiators hot is normal. If the cold reading has fallen well below the figure in the manual, the boiler may refuse to fire or show a low-pressure fault.
Regular boilers with tanks in the loft have no gauge and never need topping up. The tank keeps the system full.
Find the filling loop
The filling loop connects the heating system to the mains cold water so you can let more in. It is usually under the boiler and comes in two common forms. If yours looks like neither, check the manual before you touch any other valve.
- A braided silver hose between two pipes, with a valve at one or both ends
- A built-in loop with a lever, tap or plastic key under the boiler, common on newer models
Top it up
- Turn the heating off and let the system cool, so the gauge gives a true reading
- Open the filling loop slowly. With two valves, open one fully and use the other to control the flow. You will hear water running
- Watch the gauge, and close the valves as soon as it reaches the pressure your manual gives
- Make sure every valve is fully closed and nothing is dripping from the connections
- If the boiler has locked out, reset it the way the manual describes
- Turn the heating back on and check the gauge again once the system is warm
- If you overshoot, bleed a radiator to let a little water out and bring the pressure down
- If the manual says to disconnect the hose afterwards, do, and keep it somewhere you will find it next time
Why it keeps dropping
Topping up after bleeding radiators is normal, because bleeding lets water out along with the air. Topping up every week or two is not. A sealed system does not use water, so if the pressure keeps falling, the water is going somewhere. The usual suspects:
- A leak: look for drips or green crusting at radiator valves, damp patches on ceilings or floors, and wet pipework under the boiler
- The pressure relief valve: it vents through a small copper pipe that runs from the boiler to outside. If the end of that pipe drips or has left a stain on the wall, the valve is letting water out
- The expansion vessel: if the pressure climbs sharply when the heating comes on and falls again afterwards, the vessel has probably lost its charge, and the relief valve is opening to protect the system
When to stop and call
- You are topping up regularly and cannot find an obvious leak
- The gauge climbs close to 3 bar, or into the red, when the heating is on
- Water is coming from the relief pipe outside
- You suspect the relief valve or the expansion vessel. Both sit inside or behind the boiler, and only an engineer should open the casing
- You cannot find the filling loop, or a valve will not close properly
- There is water under the boiler, or anywhere near electrics
- The pressure is right and the boiler still will not fire after a reset
If you smell gas
Stop what you are doing. Do not use light switches, sockets or anything electrical, and do not smoke or light anything. Open doors and windows, turn the gas off at the meter if it is safe to, leave the property and call the National Gas Emergency Service on 0800 111 999.