Home › Heating problems › Pressure keeps dropping
Heating engineer · Brussels and Wallonia
The boiler keeps losing pressure
You topped up the pressure, and a few days later the needle is below 1 bar again. Topping up now and then is normal. Having to do it often isn't: water is escaping somewhere.
The most common causes
A small leak
A fitting, a radiator, a bleed valve or a pipe that drips, sometimes without a visible trace.
The expansion vessel
It absorbs the expansion of water as it heats. Once deflated, pressure rises when hot, then water escapes.
The safety valve
It opens at around 3 bar to protect the system. If it drips, pressure then drops when the boiler is cold.
Recent bleeding
After bleeding the radiators, you need to top up once: that's normal.
What you can check yourself
- Note the pressure with the boiler cold, then once it has been heating for a while.
- Look for water under the boiler, under the radiators and around the fittings.
- Check the small discharge pipe of the safety valve under the boiler: if it's running or wet, the valve has opened.
- Top up to 1.5–2 bar with the boiler cold (guide below) and keep an eye on it for a few days.
Don't
- Topping up every week without finding the cause: fresh water brings air and limescale into the system.
- Going above 2 bar when filling a cold boiler.
- Touching the safety valve or the expansion vessel.
Call us if…
- The pressure drops below 1 bar again within a few days.
- It climbs close to 3 bar when the boiler is heating.
- The safety valve drips or runs.
- You can see a leak you can't stop.
What does a repair cost?
Call-out and diagnosis: €125 excl. VAT. The repair is extra, and you get its price before we start. We usually come within 24 hours (working days) and let you know 1 hour before we arrive.
This advice applies to most boilers. Every appliance has its quirks: if in doubt, your own manual applies, and we're happy to answer on the phone.
Frequently asked questions
My boiler is losing pressure but I can't see a leak: why?
Usually a deflated expansion vessel, or the safety valve opening when pressure rises with heat. A hidden leak, under a floor for example, is also possible. We check it on site.
The pressure climbs to 3 bar when the boiler heats up: is that serious?
Not dangerous straight away, that's what the safety valve is for. But it isn't normal: usually the expansion vessel no longer absorbs the expansion. Call us before the valve starts to leak.
What's the right pressure for a boiler?
Generally between 1 and 2 bar with the boiler cold, a little more when it's heating. Your boiler's manual gives the exact value.
Find out more
A breakdown, a question? Get in touch.
Call us or message us on WhatsApp. Or leave your number, with a photo of the boiler if you like: we'll call you back.
