Low boiler pressure is the single most common reason I get called out for "my heating isn't working". Nine times out of ten, you can fix it yourself in five minutes. Here's how, and when to stop and call me instead.
What pressure should your boiler be at?
Look at the pressure gauge on the front of the boiler. When the heating is OFF and cold, pressure should sit between 1.0 and 1.5 bar (the green zone on most gauges). When the heating is on and hot, pressure rises naturally to 1.5 to 2.0 bar. Below 1 bar and most modern boilers will lock out with an error code.
How to top up the pressure
Find the filling loop. It's a short silver braided hose underneath the boiler, with a valve at each end. Both valves are usually closed. Here's the sequence:
- 1. Turn off the boiler at the switch and let it cool for 10 minutes
- 2. Open both filling loop valves SLOWLY
- 3. Watch the pressure gauge: stop as soon as it reaches 1.2 bar
- 4. Close both valves tightly
- 5. Switch the boiler back on, reset any error code, and run the heating
Open the valves slowly. Opening them fully can over-pressurise the system, blow the pressure relief valve, and leave you with water dripping outside the property. If you go above 2 bar, you may need to bleed a radiator to drop it back down.
When pressure keeps dropping
If you top up and pressure drops again within a few days, you have a leak somewhere in the system. Look for:
- Damp patches on ceilings or walls below the heating system
- Wet pipes under the boiler
- Water dripping from the pressure relief valve outside (small white pipe sticking out of the wall)
- A faulty expansion vessel inside the boiler (a job for an engineer)
Topping up daily is a sticking-plaster. If you're doing it more than once a fortnight, call me and I'll find the leak.
If you can't find a filling loop on your boiler, you may have a combi with a built-in "easy-fill" system. Check your boiler manual or send me a photo and I'll talk you through it.