# Why is my boiler losing pressure?

*David · Founder, Datum · Datum · updated 2026-10-19 · Boilers*

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.
>
> — David, Founder, Datum

## Related questions

### What pressure should my boiler be?

When the system is cold the gauge should sit in its green zone. Baxi gives a normal range of 1.0 to 2.0 bar and says to top up if it falls below 1.0 bar, but models differ, so check your own manual for the exact figure.

### Is a boiler losing pressure dangerous?

Low pressure itself usually just stops the boiler working. The underlying leak is the real cost — water finding its way into floors and ceilings. It's a 'book it soon' problem, not a 'panic tonight' one.

### How much does it cost to fix?

Depends entirely on the cause — a radiator valve is a small job; an expansion vessel is mid-range. We diagnose first and give you the fixed price before any work starts.

### Why does my boiler lose pressure overnight but hold during the day?

A pressure drop is a sign of a leak or a fault wherever it shows up. Vaillant lists a faulty expansion vessel or passing filling loop, worn seals and corrosion among the causes, and a pressure relief valve lets water out if pressure gets too high. It's worth tracing, not just topping up.

### Does living in an older home or a new-build change the likely cause?

It shifts the odds. Victorian and older conversions tend to have microbore pipework and older radiator valves — more joints for water to weep from. A newer home losing pressure in its first year or two is more often a filling loop left slightly open or the system still settling. We trace which it is rather than guess.

## Sources & verification

- [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)](https://www.baxi.co.uk/help-and-advice/faqs/what-should-my-boiler-pressure-be-set-at)
- [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)](https://www.vaillant.co.uk/advice/understanding-heating-technology/boilers/why-is-my-boiler-leaking-water/)

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