How to Fix Boiler Pressure Dropping
Last updated: 30 September 2026
Check for leaks, repressurise the system, check the expansion vessel, and check the pressure relief valve.
Overview
A boiler that keeps losing pressure indicates a leak or a faulty component. This guide covers diagnosing and fixing the issue.
Step-by-Step Fix
Before beginning any repair, ensure you have isolated power or water supplies. Clear your workspace, gather standard hand tools, and wear appropriate personal protective equipment (PPE).
- First check the boiler pressure gauge. It should read between one and one point five bar when the system is cold. If the pressure is below one bar repressurise the system using the filling loop. Note how often the pressure drops. If it drops within days there is a leak.
- Check all visible pipework and radiators for leaks. Look under sinks around the boiler and at radiator valves. Even a small leak can cause the pressure to drop over time. Check for damp patches on walls and ceilings.
- Check the pressure relief valve. This valve releases water if the system pressure gets too high. If the valve is faulty it may leak continuously causing the pressure to drop. Look for a discharge pipe outside the property and check if water is dripping from it.
- Check the expansion vessel. The expansion vessel absorbs the increase in water volume as the system heats up. If the vessel has lost its air charge the pressure will rise excessively when hot and then drop when cold. Check the vessel pressure using a tyre pressure gauge on the Schrader valve.
- If the expansion vessel has lost pressure recharge it using a bicycle pump to one bar. If the vessel does not hold pressure the internal diaphragm may have failed and the vessel needs replacing. This is a job for a Gas Safe engineer.
- Check for air in the system. Air can cause the pressure to fluctuate. Bleed all radiators starting with the lowest and working upwards. Check the boiler pressure after bleeding and repressurise if needed.
- If the pressure drops quickly and you cannot find any leaks the leak may be under the floor or in the boiler itself. A Gas Safe engineer can use a thermal camera or pressure test to locate hidden leaks.
- If the pressure relief valve is faulty it must be replaced. This is a job for a Gas Safe engineer. Replacing the valve involves draining the boiler and working on the sealed system.
- Monitor the boiler pressure over the next few weeks after any repair. If the pressure remains stable the repair is successful. If it drops again there may be a leak elsewhere that requires further investigation.
- To prevent future pressure problems have the boiler serviced annually. Check the pressure monthly and address any drops promptly. Keep the system topped up to the correct pressure.
Common Symptoms & Diagnostic Resolutions
Review the common error codes and operating symptoms below to quickly identify the root cause of the fault before ordering replacement parts.
| Symptom / Code | Likely Root Cause | Recommended Resolution |
|---|---|---|
| Pressure drops quickly | Leak in the system or faulty pressure relief valve | Check for visible leaks. If no leaks are found the pressure relief valve may be faulty and need replacing. |
| Pressure rises when heating is on | Expansion vessel has lost its air charge | Recharge the expansion vessel to one bar. If it does not hold pressure replace the vessel. |
| Pressure drops after bleeding radiators | Water lost during bleeding | Repressurise the system using the filling loop to between one and one point five bar. |
| Water dripping from pressure relief pipe | Faulty pressure relief valve or overpressure | Check the system pressure. If the valve continues to leak it needs replacing by a Gas Safe engineer. |
Frequently Asked Questions
Why does my boiler keep losing pressure?
A leak in the system a faulty pressure relief valve or a failed expansion vessel are the most common causes. Check for visible leaks first then investigate internal components.
Can I fix a boiler losing pressure myself?
You can repressurise the system and check for visible leaks. If the leak is internal or the pressure relief valve is faulty you need a Gas Safe registered engineer.
How often should I check boiler pressure?
Check the boiler pressure monthly. The pressure should be between one and one point five bar when cold. If it drops frequently there is an underlying issue that needs investigating.
What is an expansion vessel?
An expansion vessel is a component that absorbs the increase in water volume as the heating system heats up. It contains a diaphragm and an air charge. If the air charge is lost the vessel cannot absorb expansion causing pressure fluctuations.