Your boiler's pressure gauge is sitting below 1 bar. The heating isn't working properly. You're not sure what's caused it or whether you need to call someone out.
Here's a clear, honest breakdown.
What is boiler pressure, and what should it be?
Most combi boilers operate correctly between 1 and 1.5 bar when the heating is off (cold system). When the heating is running, it may rise to around 2 bar — that's normal.
If the gauge is showing below 0.5 bar, your boiler will likely lock out and stop working entirely until pressure is restored.
Common reasons your boiler is losing pressure
1. A small leak somewhere in the system
The most common cause. It doesn't have to be dramatic — a tiny weep from a radiator valve, a pipe joint, or a fitting is enough to cause a gradual pressure drop over days or weeks.
Check around all radiators (particularly the valves at the bottom), under the boiler itself, and any visible pipework for damp patches, watermarks, or corrosion staining.
2. A recently bled radiator
If you've bled a radiator recently, you'll have released water along with the air. That drop in water volume reduces system pressure. This is normal and fixable by repressurisng the system.
3. The pressure relief valve has discharged
The pressure relief valve (PRV) is a safety device that releases water if pressure gets too high. If it's discharged, you'll often find a small amount of water outside, usually near a pipe exiting the building. This is a sign the system reached unsafe pressure at some point — worth having an engineer look at the cause.
4. The expansion vessel needs recharging
The expansion vessel absorbs changes in water volume as the system heats and cools. If its internal pressure drops (this happens naturally over years), the system pressure becomes unstable — dropping when cold and rising excessively when hot.
This is a common issue in systems over five years old and is straightforward for an engineer to check and correct.
5. A faulty filling loop or pressure gauge
Sometimes the gauge itself is inaccurate, or the filling loop (the device used to repressurise the system) has a slow leak. If pressure keeps dropping even after topping up, this is worth investigating.
Can I repressurise the boiler myself?
Yes — most homeowners can safely repressurise a combi boiler using the filling loop (usually a braided hose with a valve, located beneath the boiler). The process is:
- Ensure the heating is off and the system is cold
- Connect the filling loop if it's not permanently attached
- Open the valve slowly until the gauge reads around 1.2–1.5 bar
- Close the valve and disconnect the filling loop
- Restart the boiler
Your boiler's manual will have model-specific instructions.
Important: repressurisng is a temporary fix if there's an underlying leak. If you're topping up more than once every few months, there's a cause that needs finding.
When should I call an engineer?
Call a Gas Safe registered engineer if:
- You can't identify where the pressure is going
- The boiler pressure drops again within days or weeks of topping up
- You've found damp, staining or corrosion around pipework or radiators
- The pressure rises excessively when heating is running (above 2.5–3 bar)
- The boiler is showing an error code related to pressure
Don't ignore persistent pressure drops. What starts as a minor issue can develop into a failed component, water damage, or — in worst cases — a safety risk.
If your boiler keeps losing pressure and you're based in Hemel Hempstead, Ealing, or NW London — get in touch. We'll diagnose the cause properly, tell you exactly what it is, and fix it.