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Heating pressure keeps dropping

By Declan Barry, plumber, Stockport. Updated 2026-09-10.

If the pressure gauge on your heating drops from around 1.5 bar to zero over days or weeks, water is leaving the system. The usual causes are a weeping radiator valve, a failed expansion vessel, a pressure relief valve that has let go, or a small leak under a floor. Topping up keeps the heating running but it does not fix any of those.

What to do

  1. Note how fast it drops

    Dropping overnight means a real leak. Dropping over a month might be a tiny weep or a failing expansion vessel. Write down the gauge reading each morning for a week.

  2. Check the radiator valves

    Look at the valves at both ends of every radiator for damp, green staining or a drip mark on the floor. A weeping valve is the commonest cause and the cheapest fix.

  3. Check the pressure relief pipe outside

    A copper pipe through the outside wall near the boiler. If it drips or has a limescale trail under it, the pressure relief valve is passing water and the expansion vessel probably needs recharging or replacing.

  4. Top up if you need heating

    Use the filling loop under the boiler to bring the gauge back to about 1.5 bar cold, then close both valves on the loop. Do not leave it open. If you are topping up more than once a month, it needs looking at.

When to ring a plumber

  • You are topping up weekly or more. That is a leak, and it is getting worse.
  • The pressure relief pipe outside is dripping. That is a valve or vessel fault and it will not fix itself.
  • You can hear water running under a floor, or there is a damp patch you cannot explain.

Questions

What should the pressure be?

Between 1 and 1.5 bar when the system is cold. It rises a little when hot. Above 2.5 bar is too high and the relief valve will let water out.

Is it dangerous?

Low pressure is not dangerous, the boiler just shuts down. The leak causing it can be, if it is under a floor and soaking a joist for months.

Related

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