Boilers
Why is my boiler losing pressure?
3 min read · Reviewed for accuracy before publishing
Boiler pressure between 1 and 1.5 bar (cold) is normal. If you've topped it up once this year, that's ordinary. If you're topping up every few weeks, something is letting water out — most often a tiny leak on a radiator valve or joint, a weeping pressure-relief valve, or an expansion vessel on its way out. None of those fix themselves, and repeated topping up masks the symptom while the cause quietly gets worse.
When it's fine
A slow drift over many months, or a single top-up after bleeding radiators — normal. Bleeding removes air, and the pressure drop afterwards is expected.
When to get it looked at
Topping up more than every couple of months, visible drips anywhere on the system, damp patches under radiators, or a boiler that locks out on low pressure. A pressure test traces it in one visit — and it's far cheaper than the water damage a slow leak leaves behind.
A note on topping up
Topping up via the filling loop is a manufacturer-sanctioned homeowner action on most modern boilers — your boiler's manual shows the exact steps for your model. If you're unsure, don't guess: send us the model and we'll point you at the right page, or just book us in.
“Topping up is not a fix — it is a symptom you are feeding. Once a year is normal life; every few weeks is a leak with your name on it.”
David founded Datum after years on the tools across heating and plumbing. He writes the boiler and cost guides from what actually happens on real installs.