The heating circuit on a combi boiler is a pressurised sealed system. The system is pressurised using the mains water pipe at the boiler using the filling loop.
Over time there will be pressure drops in the system due to small leaks in the pipe work or radiators. These leaks of water are so small that they are invisible to the eye and totally harmless. These small leaks may cause a small ingress of air into the system and if your radiators become cool at the top and you have to bleed them this will further reduce the pressure.
Depending on your system you may need to top up pressure 3 to 4 times per year or it could be as low as once per year.
Check the pressure gauge on the boiler every couple of months and if its below 1 Bar top up pressure to 1.5 Bar.
Check and top up pressure whenever radiators have been bled.
There are some images to the left of typical boiler pressure gauge and filling loop set ups.
Please note – Water regulations state that a boiler filling loop should not be permanently connected to a boiler. This rule is often ignored. You should always leave at least one end of the filling loop disconnected. Although there is a low risk it is possible that if you leave both ends connected, the water circulating through the central heating system (complete with sludge, rust and treatment chemicals) can contaminate your drinking water.
Procedure for topping up boiler pressure is as follows:
- Switch Off Boiler.
- Connect both ends of filling loop make sure it is tightened properly.
- Open valves on filling loop (some loops have 2 valves and others only have one) If there are 2 it is necessary to open both. Open valves slowly to avoid over pressurising the boiler.
- As the valve is opened you will hear water rushing into the boiler. Keep an eye on the pressure gauge. When the pressure gauge reaches 1.5 Bar close the valves. Ensure valves are fully closed.
- Disconnect filling loop.
- Switch on boiler.
If you accidently over pressurise the boiler don’t worry, the pressure can be reduced by releasing some water from a radiator bleed point. If you keep on pressurising the boiler and don’t close the valve the boiler will release the pressure through its safety valve. This is quite noisy and the excess water will be expelled through the safety outlet pipe through an outside wall. If this happens the safety valve will close when the pressure has dropped and you will have to go through the procedure again.