What normal pressure looks like
Most combi and system boilers should read between 1 and 1.5 bar when the heating is cold. It rises a little when the heating is on. If the gauge keeps dropping below 1, water is escaping somewhere, and many boilers lock out and stop heating once it falls too low.
Common causes
A slow drop in pressure is almost always one of these.
- A leaking radiator, radiator valve or towel rail
- A weeping pipe joint under the floor or behind a wall
- A faulty pressure relief valve letting water out through the pipe outside
- A failed expansion vessel inside the boiler
- Bleeding the radiators, which lets water out as well as air
What you can check
Look for drips, stains or green corrosion under each radiator and valve, and on any pipes you can see. Then check the small copper pipe on the outside wall near the boiler after the heating has run. Drips or a wet patch below it point to the pressure relief valve.
How to top up the pressure
Most combi boilers have a filling loop underneath: two small valves joined by a braided hose. Check your boiler manual first, as some models use a built-in key or lever instead.
Topping up, step by step
Do this with the heating off and the system cold.
- Find the filling loop under the boiler and check both ends are connected
- Open the first valve, then open the second one slowly. You'll hear water flowing
- Watch the gauge and close both valves when it reads between 1 and 1.5 bar
- Turn the heating back on and reset the boiler if it shows a fault
- Note the date. If you need to do it again within a week, there's a leak
If the pressure is too high
A reading above 2.5 bar when cold usually means the filling loop has been left open or is letting water through. Check both valves are fully closed. If the pressure stays high, or water comes out of the relief pipe outside, turn the boiler off and call us.
Is it safe to keep using the boiler?
A boiler that is losing pressure slowly is not dangerous in itself, and topping it up will keep the heating going for now. But a hidden leak can damage floors and ceilings over time, and a failing relief valve or expansion vessel will get worse.
When to call us
If you're topping up more than once a week, there's a leak or a failing part. Our RGI-registered engineers will find it and fix it, often on the first visit.