How to Fix Low Boiler Pressure
Last updated: 7 September 2026
Check the pressure gauge, repressurise using the filling loop, and check for leaks.
Overview
Low boiler pressure is a common issue that can affect your heating and hot water. This guide covers diagnosing and fixing low boiler pressure.
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, locate the pressure gauge on the front of your boiler. This is usually a dial or digital display that shows the current system pressure. The needle should be between 1 and 1.5 bar when the system is cold.
- If the pressure is below 1 bar, locate the filling loop. This is typically a silver flexible hose with two valves, often found underneath the boiler. It connects the mains water supply to the heating system.
- Open both valves on the filling loop slowly. Turn one valve fully and then gradually open the second valve while watching the pressure gauge. Allow the pressure to rise to 1.5 bar.
- Close both valves firmly once the pressure reaches 1.5 bar. Ensure they are fully closed to prevent over-pressurisation of the system.
- Check the pressure gauge again after 10-15 minutes. If the pressure has dropped, there may be a leak in the system that needs to be addressed.
- If the pressure drops again quickly, inspect all visible pipework and radiators for signs of leaks. Check under sinks, around the boiler, and at radiator valves for any drips or damp patches.
- If you cannot find any visible leaks and the pressure continues to drop, there may be a leak in the pipework under the floor or behind walls. This requires professional investigation by a heating engineer.
- After repressurising, check that the heating system is working correctly. Turn on the heating and listen for any unusual noises or check that radiators are heating up evenly.
- If the boiler has an error code related to low pressure, it may need to be reset after repressurising. Consult your boiler manual for the specific reset procedure.
- If you are unsure about any step, or if the pressure drops repeatedly, contact a Gas Safe registered engineer. Low pressure can indicate a more serious issue that needs professional attention.
Common Error Codes
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 |
|---|---|---|
| E119 | Low system pressure | Repressurise using the filling loop. If the pressure drops again, check for leaks in the system. |
| E9 | Low system pressure (Worcester Bosch) | Repressurise the boiler using the filling loop. If the pressure drops again, there may be a leak. |
| E133 | Ignition fault | Check the gas supply and the condensate pipe. If the error persists, call a Gas Safe registered engineer. |
| EA | Ignition fault | Check the gas supply and the condensate pipe. If the error persists, call a Gas Safe registered engineer. |
Frequently Asked Questions
Why does my boiler keep losing pressure?
A boiler that keeps losing pressure usually has a leak in the system. This could be a small leak on a radiator valve, a pipe joint, or a more serious leak in the hidden pipework. The expansion vessel may also be faulty.
Can I repressurise my boiler myself?
Yes, you can repressurise your boiler yourself using the filling loop. Ensure you follow the correct procedure and do not over-pressurise the system. If you are unsure, consult your boiler manual or call a professional.
What pressure should my boiler be at?
The ideal pressure for a cold boiler is between 1 and 1.5 bar. When the heating is on, the pressure may rise slightly as the water expands. If the pressure exceeds 2.5 bar, there may be an issue with the expansion vessel.
How often should I check my boiler pressure?
You should check your boiler pressure monthly, or whenever you notice that the heating is not working as efficiently as usual. Regular checks can help identify issues before they become serious.