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Boiler help

Boiler losing pressure?

A boiler that keeps dropping below 1 bar usually has a small leak somewhere in the system. Topping it up is safe to do yourself, and I'll show you how. But if you're topping up more than once a month or so, something is letting water out and it needs finding.

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Smell gas? Stop here. Open windows and doors, don’t flick switches, and call the National Gas Emergency line free on 0800 111 999 before anything else.

What’s going on

What the pressure gauge is telling you

Sealed heating systems run at around 1 to 1.5 bar when cold. Below about 0.5 bar most boilers lock out and stop heating, and some show a fault code for it (F22 on a Vaillant, for example). Pressure does drift down slowly over months, and bleeding radiators drops it too. That's normal. A fast or repeated drop is not.

Where the water usually goes

The common culprits are a weeping radiator valve, a pinhole in pipework under the floor, or a failing expansion vessel inside the boiler. A tired relief valve letting water out through the overflow pipe outside is another regular. Some are inexpensive fixes. All of them need a qualified engineer to locate and repair properly.

How to top the pressure back up

  1. 1Let the boiler cool down. Pressure reads higher when the system is hot, so always judge it cold.
  2. 2Find the filling loop. It's usually a braided silver hose under the boiler with one or two small valves, though some boilers have a built-in filling key instead. Your manual will show which you have.
  3. 3Open the valve(s) slowly. You'll hear water flowing in and see the gauge climb.
  4. 4Stop at around 1.2 bar, and no higher than 1.5 when cold. If you overshoot, bleed a radiator briefly to let some water back out.
  5. 5Close the valves fully, then reset the boiler if it was showing a fault.

When to stop and call me

  • You're topping up more than about once a month. That's a leak.
  • The pressure drops within hours or days of topping up.
  • You can see water: under the boiler, from the copper overflow pipe outside, or a damp patch on a ceiling or floor.
  • The gauge swings high when the heating runs, then drops right back. That points at the expansion vessel.
  • The filling loop is missing, seized, or won't shut off properly.

I’m Jamie, a Gas Safe registered engineer (No. 977838) in Burgess Hill. I handle boiler and heating repairs in Burgess Hill and across Mid Sussex, Mon to Fri with some evenings.

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FAQ

Common questions

Around 1 to 1.5 bar when the system is cold. It will rise a little when the heating is running. If it's regularly below 1 or above 2.5, get it looked at.

Topping up now and then is fine. Doing it every week isn't: fresh water brings oxygen and minerals into the system, which corrode radiators and clog the boiler over time. Cheaper to fix the leak than to replace what the leak ruins.

Bleeding lets air out of the system, and the water that replaces the air lowers the overall pressure. That's expected. Just top back up to about 1.2 bar afterwards.

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Get a free quote for the job

Send me a few details, and a photo of the boiler if you can. I'll get back to you quickly with a clear, no-obligation quote and honest advice.

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  • Gas Safe registered · 977838
  • Burgess Hill & Mid Sussex
  • Mon–Fri
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