If your boiler pressure drops when the heating is running but stays stable when it’s off, that’s not normal system behaviour.
Pressure loss under heat usually means the system is being stressed and revealing a weakness.
This is exactly what our Central Heating Leak Detection service is designed to diagnose properly.
This page explains what’s happening and what to do next.
What This Usually Means
If pressure drops only when the heating is on, it usually means the system is losing water when it expands under heat.
This points to a leak or weakness that only becomes active during operation.
The system is holding when idle but failing under load.
Why This Happens
When your heating system warms up, water expands and pressure increases. This can expose weak points that remain sealed when the system is cold.
This is why the problem appears only when the heating is running.
To understand this better, see central heating leaks explained.
The change in temperature is revealing the issue — not causing it.
How to Tell If This Is the Problem
- Pressure drops when heating is on
- Pressure stable when heating is off
- No visible leaks
- Requires topping up after use
What Most People Get Wrong
Many assume this is a boiler fault, but in most cases it’s a system issue rather than the boiler itself.
The behaviour is coming from the system, not just the appliance.
How the Problem Is Actually Solved
We test the system under working conditions and trace where pressure is being lost.
This allows us to pinpoint the exact source without guesswork.
The focus is on identifying the weak point, not replacing parts unnecessarily.
What to Do Next
If your pressure drops only when the heating is on, it’s best to investigate early.
Our Central Heating Leak Detection service will identify the cause clearly.
This type of issue rarely resolves on its own.