Most combi and system boilers run at 1–1.5 bar when cold, rising to around 2 bar when the heating is on. Below 0.5 bar, most boilers lock out.
Topping up
Find the filling loop, usually a braided hose or a lever under the boiler. Open it slowly until the gauge reads about 1.2 bar, then close it fully. If your boiler has a built-in key, follow the manual.
If it keeps dropping
- A small leak on a radiator valve or pipe joint, often hidden under floors.
- A pressure relief valve that is letting by. Check for drips from the small copper pipe outside.
- A failed expansion vessel, which makes the pressure jump up when hot then drop.
- Air being bled from radiators without topping back up.
Repeatedly topping up adds fresh oxygenated water, which speeds up corrosion. If you’re topping up more than once a month, book a repair.