boilers
Boiler Pressure Too Low: How to Fix It in Your London Home
What Is the Correct Boiler Pressure?
Most combi and system boilers in London homes should operate at 1.0–1.5 bar when cold, rising to around 2.0 bar when hot. If your pressure gauge reads below 0.5 bar, the boiler will lock out (often showing F22 on Vaillant or EA on Worcester Bosch).
How to Repressurise a Combi Boiler
Most combi boilers have a filling loop — a braided flexible hose connecting the cold mains supply to the central heating circuit. The process:
- Turn off the boiler and let it cool for 30 minutes
- Locate the filling loop (usually under the boiler, silver braided hose with two valves)
- Open both valves slowly — you will hear water entering the system
- Watch the pressure gauge — stop when it reaches 1.0–1.2 bar
- Close both valves and restart the boiler
Why Does Boiler Pressure Keep Dropping?
If you are repressurising your boiler more than once a month, there is a system leak. Common locations in London homes:
- Radiator valves — especially old brass TRVs and lockshields
- Boiler heat exchanger micro-leak (common in boilers over 8 years)
- Expansion vessel failure — common cause of recurring pressure loss
- Pressure relief valve dripping — look for a pipe dripping water outside
Frequently asked questions
Is it safe to repressurise my boiler myself?
Yes — repressurising a combi boiler using the filling loop is a straightforward task. You are simply adding cold water to the closed heating circuit. Follow your boiler manual instructions and do not overpressurise above 1.5 bar.
Why does my London boiler keep losing pressure?
Recurring pressure loss almost always indicates a system leak. London properties with aging pipework and compression fittings are particularly susceptible. A Gas Safe engineer can carry out a pressure test to identify the leak location.
What is an expansion vessel and why does it cause pressure loss?
The expansion vessel absorbs the increased volume of water as it heats up, preventing excess pressure. When the vessel's internal diaphragm fails (common after 8–10 years), the vessel cannot absorb expansion, causing the pressure relief valve to release water and the system pressure to drop repeatedly.