Boilers
Why is my boiler losing pressure?
3 min read · Last updated · Reviewed for accuracy before publishing
A cold boiler that sits in the green zone on its pressure gauge is normal; your manual gives the exact range for your model. An occasional top-up is 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 is a drop in boiler pressure normal?
A slow drift over many months is normal, and so is a single top-up after bleeding radiators, because bleeding removes air. An occasional top-up is ordinary. Repeated top-ups every few weeks are not: that points to a leak somewhere on the system.
Vaillant lists a faulty expansion vessel or passing filling loop, corrosion and worn seals among the causes.
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.”
Sources
- Baxi — What should my boiler pressure be set at? (normal range 1.0 to 2.0 bar; repressurise below 1.0 bar; check your manual; accessed 19 Oct 2026)
- Vaillant — Is my boiler leaking water? (excessive pressure is among the most common causes; the pressure relief valve lets water out above 3 bar; a faulty expansion vessel or passing filling loop, corrosion, worn seals and a cracked heat exchanger are other causes; turn off the water supply and heating and call a Gas Safe registered engineer; accessed 19 Oct 2026)
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.