Boiler Losing Pressure Overnight? 6 Causes Engineers Check First

If your boiler pressure drops overnight, one of six things is almost always to blame. A Gas Safe engineer explains what to check before calling out.

Boiler Losing Pressure Overnight? 6 Causes Engineers Check First — illustrative photo of UK home emergency engineers at work

Combi boiler pressure that reads 1.2 bar at bedtime and 0.4 bar by breakfast is one of the top three winter callouts I get. Water isn't disappearing by magic — it's going somewhere. Here are the six places I check, in the order I check them.

1. The pressure relief valve is weeping outside

Go outside and look at the copper overflow pipe from the boiler (usually near an outside wall). If it's damp, dripping, or has staining or green marks on the wall below, your PRV is passing. That accounts for roughly 40% of overnight pressure loss jobs I attend. A new PRV cartridge is a 45-minute fix.

2. The expansion vessel has lost its charge

Every sealed boiler has a rubber-diaphragm vessel pre-charged with air. When it fails, the system has nowhere to expand into — pressure spikes when the heating runs, blows through the PRV, and then reads low overnight when it cools. A quick test: if the pressure gauge swings wildly between cold and hot, the vessel is finished. Re-pressurising the vessel or swapping it costs £80–£180.

3. A hidden radiator or pipe leak

Check under every radiator valve with a piece of dry kitchen roll. Lift floorboards near junctions if you've had recent work done. Most micro-leaks show as tidemarks or slight verdigris on the pipe. Even a leak that produces one drop a minute will lose 0.8 bar overnight.

4. The auto-air-vent is passing

The little brass valve on top of the pump releases air automatically — but when the seal perishes it releases water too. You'll usually spot water inside the boiler case around the pump head.

5. A cracked heat exchanger

This is the one nobody wants. A hairline crack in the primary heat exchanger dumps water down the condensate. Tell-tale signs: white residue around the flue, and pressure loss only when the boiler is firing. On boilers older than 12 years this is often uneconomic to repair — time to look at a new boiler.

6. You've recently bled radiators

If you bled radiators yesterday and re-pressurised, the trapped air is still working its way out. Give it 48 hours and top up once more before assuming there's a fault.

The 30-second re-pressurise (safely)

  1. Turn the boiler off at the front panel.
  2. Find the filling loop — a silver braided hose or two chrome levers under the boiler.
  3. Open both valves slowly until the gauge reads 1.2–1.5 bar cold.
  4. Close both valves fully and turn the boiler back on.

If you're topping up more than once a fortnight, stop patching it and get it diagnosed. Every top-up dilutes your inhibitor and speeds up sludge build-up.

Is this covered under a boiler policy?

PRVs, expansion vessels, pumps and diverter valves are covered as standard on our boiler cover plans. Heat exchanger cracks are usually only covered if the boiler is under 10 years old — check the policy wording. Compare features side by side on our boiler cover comparison.

FAQ

What's the correct boiler pressure?

1.0–1.5 bar cold, up to 2.0 bar when the heating is running. Above 2.5 bar and your PRV should be dumping.

Why does it drop faster in winter?

Colder pipework contracts and highlights existing leaks that were sealed by summer expansion.

Can I keep topping up indefinitely?

No. Every top-up brings in fresh oxygenated water which accelerates internal corrosion. Get the underlying leak sorted.

Summary