If you’ve noticed your boiler pressure shooting up every time the heating comes on, you’re not alone. Boiler pressure too high when heating on is one of the most common issues homeowners experience and it’s something that shouldn’t be ignored.
A boiler that’s constantly over-pressurising can lead to leaks, system breakdowns, or even a complete boiler shutdown. The good news? In many cases, the cause is easy to understand and sometimes easy to fix.
Let’s break it down in plain English.
What Boiler Pressure Should Be
Most modern boilers are designed to run at around 1 -1.5 bar when cold.
When the heating is on and the system warms up, the pressure will naturally rise slightly usually to around 2 bar.
That’s normal.
However, if the pressure climbs to 2.5 bar, 3 bar, or higher, you’ve got a problem.
If your boiler pressure keeps rising only when the heating is on, it’s a strong sign something in the system isn’t doing its job.
Why Is Boiler Pressure Too High When Heating Is On?
Here are the most common reasons this happens.
1. The Expansion Vessel Has Failed (Very Common)
The expansion vessel absorbs the extra pressure created when water heats up and expands. Inside it is a rubber diaphragm and an air pocket.
If:
The diaphragm splits
The air pressure is lost
Or the vessel is waterlogged
…the system has nowhere for that expanding water to go. The result?
Pressure skyrockets when the heating comes on.
This is one of the most frequent causes engineers see.
2. The Filling Loop Has Been Left Open
The filling loop is used to top up pressure but if it’s left slightly open, mains water can keep feeding into the system.
You might notice:
Pressure slowly rising on its own
Pressure jumping higher when heating is on
Always check the filling loop valves are fully closed.
3. Faulty Pressure Relief Valve (PRV)
When pressure gets too high, the pressure relief valve is supposed to release excess water safely.
If the PRV is faulty or blocked:
Pressure may keep climbing
Water may leak outside through the discharge pipe
The boiler may lock out
Sometimes the PRV itself fails because of ongoing high pressure often linked back to the expansion vessel.
4. Recently Re-Pressurised System
If you’ve recently topped up the boiler and added too much water, the pressure might seem fine when cold but spike dangerously once the heating is on.
This is why boiler pressure should always be set when the system is cold.
Is High Boiler Pressure Dangerous?
Short answer: yes, it can be.
High pressure can:
Cause leaks from radiators or pipework
Damage internal boiler components
Trigger constant shutdowns
Shorten the life of the boiler
Repeated pressure relief valve discharges can also lead to corrosion and further faults.
If your boiler pressure keeps rising every time the heating runs, it’s not something to “keep an eye on” it needs fixing.
What You Can Safely Check Yourself
Before calling an engineer, there are a couple of safe checks you can do:
- Check the pressure when the boiler is cold
- Make sure the filling loop is fully closed
- Bleed one radiator (only if pressure is already high)
- Check if water is dripping from the external PRV pipe
If the pressure still climbs once the heating is on, the issue is almost certainly internal.
When to Call a Gas Engineer
If you’re dealing with boiler pressure too high when heating on, you should call a Gas Safe registered engineer if:
Pressure rises above 2.5–3 bar
The boiler keeps losing or gaining pressure
Water is leaking from the boiler or outside pipe
The boiler repeatedly shuts down
Expansion vessel faults, PRV replacements, and internal pressure issues are not DIY jobs and should always be handled by a professional.
Don’t Ignore High Boiler Pressure
Boiler pressure problems rarely fix themselves and ignoring them usually leads to bigger, more expensive repairs.
If your boiler pressure shoots up every time the heating comes on, getting it checked early can save you:
Emergency breakdowns
Water damage
Boiler replacement costs
A qualified gas engineer can diagnose the issue quickly and get your heating running safely again.