Why Is My Boiler Pressure Too High?
Noticed the gauge creeping past 2.5 bar? Here is why it happens and how to fix it safely.
What is "Normal" Boiler Pressure?
Before you panic, it is important to know what normal looks like. Most modern combi boilers are happiest when the pressure gauge reads:
- Cold System: Between 1.0 and 1.5 bar
- Heating On: Around 2.0 bar (it rises as water expands)
If your gauge keeps climbing above 2.5 bar, or if you see water dripping from the pressure relief valve (PRV) outside, your boiler is under strain and needs attention.
5 Common Reasons for High Pressure
High pressure is usually caused by one of these five issues. Some you can fix yourself, others need a Gas Safe engineer.
1. Overfilled System
Cause: Adding too much water when using the filling loop.
Fix: Bleed a radiator to release pressure. Aim for 1.2 bar when cold.
2. Faulty Filling Loop
Cause: If the valves aren't fully closed, mains water constantly leaks in.
Fix: Check both valves are tight. If pressure still rises, the loop needs replacing.
3. Expansion Vessel
Cause: The vessel absorbs pressure. If it loses charge, pressure spikes rapidly.
Fix: Needs repressurising or replacing by an expert engineer.
4. Sludge Blockage
Cause: Sludge restricts water flow, causing pressure build-up.
Fix: A professional Power Flush is required to clear the system.
5. Faulty PRV (Pressure Valve)
Cause: The safety valve is stuck or damaged.
Fix: Requires a replacement part fitted by a Gas Safe engineer.
How to Safely Reduce Pressure
If you have overfilled the system, you can reduce the pressure yourself without tools.
- Turn off the boiler and let it cool down completely.
- Locate the bleed valve on a radiator (usually at the top side).
- Use a bleed key and an old towel to catch water.
- Open the valve slowly. Air will hiss, then water will trickle.
- Watch the boiler gauge. Close the valve when it hits 1.2 bar.
Is it dangerous? Boilers have safety valves, so it won't explode. However, running at high pressure long-term can damage seals, pumps, and pipework, leading to expensive leaks.
Pressure Keeps Rising?
If you have bled the radiators and the pressure shoots back up within hours or days, you have a mechanical fault.
We are your local heating experts in Fife & The Lothians. We can diagnose and fix pressure issues on the first visit.
Book a RepairOr Call 01383 660098