Why Is My Boiler Losing Pressure?
If your boiler pressure keeps dropping, it’s more than just an inconvenience — it could be a sign of an underlying fault. A loss of pressure can reduce heating efficiency or cause your boiler to shut down entirely. Here's how to identify the issue and get it sorted.
What Is Normal Boiler Pressure?
Most boilers operate best with cold pressure between 1.0 and 1.5 bar. When the heating is on, it may rise slightly to 2.0 bar. If the pressure falls consistently below 1.0 bar, your boiler may cut out as a safety measure.
Common Causes of Low Boiler Pressure
1. Leaks in the Heating System
Cause: Leaks from radiators, pipework or connections cause gradual loss of water — and therefore pressure.
Fix: Visually check for damp spots, greenish residue or dripping joints. A qualified heating engineer can pressure-test and trace hidden leaks.
2. Recently Bled Radiators
Cause: Bleeding radiators removes trapped air but also reduces system pressure.
Fix: Use the filling loop to top up the boiler back to around 1.2 bar. Always check pressure after bleeding radiators.
3. Faulty Pressure Relief Valve (PRV)
Cause: A stuck or leaking PRV may slowly release water, dropping pressure.
Fix: Check for discharge pipe drips. A faulty PRV must be replaced by a Gas Safe engineer.
4. Expansion Vessel Problems
Cause: The internal expansion vessel may lose air charge or split, preventing it from absorbing pressure changes.
Fix: Needs diagnosis and repressurising or replacement by a professional.
How to Repressurise Your Boiler
- Turn off the boiler and let it cool.
- Locate the filling loop (usually a silver braided hose underneath).
- Open the valves slowly until pressure reaches 1.2–1.5 bar.
- Close the valves fully and restart the boiler.
Important: Never overfill above 2.0 bar, and only top up slowly. If unsure, call an engineer.
Is It Safe to Keep Topping Up the Boiler?
If you're topping up the pressure more than once every couple of months, there's likely a deeper issue — such as a small leak or a failing expansion vessel. Constant refilling can lead to corrosion, limescale build-up, and damage to internal components.
When to Call a Heating Engineer
You should contact a professional if:
- The boiler loses pressure more than once a month
- You notice visible leaks or water stains
- The pressure drops even after bleeding and topping up
Attempting to fix pressure-related faults yourself (beyond basic top-ups) can void your warranty or damage your system further.
Need Help With a Pressure Drop?
Jones Heating specialises in boiler diagnostics and pressure-related repairs. Whether it's a leaking valve or a hidden system fault, we’ll find the root cause — fast.
📞 Call us: 01383 660098
🌐 Visit: jones-heating.co.uk/boiler-repairs
We cover Dunfermline, Kirkcaldy, and surrounding areas — with honest advice and fast service. Don’t ignore a pressure drop. We’ll sort it properly.
