Why Is My Boiler Losing Pressure?

If your boiler pressure keeps dropping, you’re probably fed up of topping it up — or worse, waking up to cold radiators. A loss of pressure can affect heating, trigger error codes, or shut your boiler down entirely. Here’s what might be causing it and what you can do next.

What’s the Normal Boiler Pressure?

For most boilers:

• Cold: Between 1.0 and 1.5 bar
• When heating’s on: Around 2.0 bar is normal

If your pressure keeps falling below 1.0 bar, the boiler may cut out as a safety feature — and you’ll lose heating or hot water.

Why Your Boiler Might Be Losing Pressure

1. A Leak Somewhere in the System

What it means: Even a small leak from a radiator, pipe joint or valve can slowly drop system pressure.

What to do: Look for signs of water under radiators, on pipework, or around the boiler. A heating engineer can pressure-test and track down hidden leaks quickly.

2. Recently Bled Radiators

What it means: Bleeding your radiators gets rid of trapped air — but also releases some system pressure.

What to do: Use the filling loop to top the pressure back up to around 1.2 bar. Always check the gauge after bleeding.

3. Faulty Pressure Relief Valve (PRV)

What it means: If the PRV is stuck or leaking, it can slowly let water escape.

What to do: Check the outside pipe for drips. If it’s leaking, the valve needs replacing by a Gas Safe engineer.

4. Expansion Vessel Issues

What it means: If your boiler’s expansion vessel is faulty or has lost charge, it can’t regulate pressure properly.

What to do: This needs diagnosis and repair by a qualified engineer. It’s a common issue in older systems.

How to Repressurise Your Boiler

1. Turn off the boiler and let it cool down.
2. Locate the filling loop (usually a silver braided hose under the boiler).
3. Slowly open both valves and watch the gauge rise to 1.2–1.5 bar.
4. Once done, close both valves and restart the boiler.

Top tip: Never overfill — if it creeps above 2.0 bar when cold, you’ve gone too far. If you’re not sure, just give us a ring and we’ll walk you through it.

Should You Keep Topping It Up?

If you’re needing to top up more than once every couple of months, there’s probably a deeper issue. Repeated refilling can cause long-term damage — like corrosion, limescale build-up or premature wear on internal components.

When to Call in a Heating Engineer

It’s time to get help if:

• Pressure drops more than once a month
• You’ve spotted a leak or water marks
• Repressurising doesn’t hold for long

In some cases, a system flush may be needed if sludge or debris is causing pressure loss or circulation issues.

Need Help With a Pressure Drop?

At Jones Heating, we’re experts in tracking down the cause of pressure loss. Whether it’s a slow leak, a faulty valve, or something deeper in the system, we’ll get it sorted — quickly and properly.

We also offer regular boiler servicing to catch small problems early and keep your system running smoothly.

Serving Dunfermline, Kirkcaldy and nearby areas — with honest advice, no-pressure service, and fast fixes you can count on.