You top up the boiler pressure, and a few days later it's back down near zero. Pressure doesn't just disappear. If it keeps dropping, water is escaping from your heating system somewhere, and it's worth finding out where before it causes damage.
What's normal
Most combi boilers should sit at about 1 to 1.5 bar when the heating is off. It rises a little when the heating is on. A small drop after you've bled radiators is normal. A steady drop every few days is not.
The usual causes
- A small leak on the heating pipes or radiators. Look for drips or damp under radiator valves, along visible pipes, and around towel rails. Check for brown or green crusting on joints.
- The pressure relief valve. This is a safety valve on the boiler. If it's letting water out, you'll often see a pipe dripping outside, usually a copper pipe from the bottom of the boiler going through the wall.
- The expansion vessel inside the boiler. If it has lost its air charge, the pressure jumps up when the heating is on and the relief valve releases water, so it drops again later.
- Recently bled radiators. Bleeding lets air and a little water out, so you may need one top-up afterwards.
What you can check yourself
- Write down the pressure with the heating off and cold. Check again each day.
- Look under and around every radiator, especially the valves, and along any pipes you can see. Put dry kitchen paper under suspect joints.
- Check outside for a dripping pipe from the boiler.
- Look for damp patches on ceilings or floors, which can mean a leak on a pipe you can't see.
Don't keep topping it up
Topping up now and then is fine, but if you're doing it every few days, fresh water keeps entering the system. That brings in oxygen, which speeds up rust and sludge inside the radiators and boiler. It's better to find and fix the cause.
Who to call
- If you can see a leak on a radiator, valve or pipe, a plumber can fix it. Call us.
- If the problem is inside the boiler, such as the relief valve or expansion vessel, it must be checked by a Gas Safe registered engineer.
- If water is dripping through a ceiling, turn off the stopcock and see water leaking through the ceiling.
Common questions
Why does my boiler pressure keep dropping?
Water is escaping from the heating system, usually through a small leak on a radiator or pipe, or through the boiler's pressure relief valve.
Is it dangerous if my boiler pressure keeps dropping?
It isn't usually dangerous, but low pressure can stop the heating working, and the leak causing it can damage floors and ceilings.
How often should I have to top up my boiler?
Rarely, perhaps once or twice a year. If you're topping up every few days or weeks, there's a leak that needs finding.