# Why is my boiler losing pressure?

*David · Founder, Datum · Datum · Boilers*

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, Founder, Datum

## Related questions

### What pressure should my boiler be?

Typically 1–1.5 bar when the system is cold — the gauge usually shows a green zone. Check your model's manual for its 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 drop that shows mostly overnight usually means a small leak that opens up as the system cools and the metal contracts — or a pressure-relief valve weeping while the boiler's off. Daytime heating expands everything and masks it. It's still a leak 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.

---
Canonical page: https://www.datumclimate.co.uk/guides/boiler-losing-pressure