Combi Boiler Not Working? Common Causes & Fixes for London Homes
Combi boiler not working in your London flat or house? This guide walks through the most common reasons a combi boiler stops working and which ones you can fix yourself before calling an engineer.
Is Your Combi Boiler Not Working?
A combi boiler that stops working is one of the most disruptive household emergencies, particularly in London's cold winter months. Before calling an engineer, there are several quick checks you can do that resolve the problem for free in around 30% of cases.
1. Check the Pressure
Low water pressure is the most common reason a combi boiler stops working — it accounts for over 30% of boiler callouts we receive in London. Look at the pressure gauge on the front of the boiler (usually a round dial or a display reading in bar).
Normal range: 1-1.5 bar when cold, up to 2 bar when hot.
If the gauge reads below 0.8 bar, the boiler has shut down to protect the system. You can fix this by repressurising using the filling loop — a small valve (or pair of valves) usually found underneath the boiler. Turn the valve(s) until the gauge reaches 1.5 bar, then close it. The boiler should restart.
2. Check the Condensate Pipe (In Winter)
Combi boilers installed since 2005 have a condensate pipe that drains acidic water vapour condensed from the flue gases. This pipe runs outside the property to a drain, and it can freeze in cold weather — blocking the boiler and triggering a lockout.
Signs of a frozen condensate: the fault code on display (Vaillant F.28/F.29, Worcester Bosch EA 227, Baxi E168 can all be triggered by a frozen condensate), and you can often see ice or frost around where the pipe exits the building.
Fix: Pour warm (not boiling) water over the external pipe until it thaws. Then reset the boiler.
3. Check the Electricity Supply
Combi boilers require electricity to operate — they cannot function during a power cut. Check the consumer unit (fuse board) to confirm the boiler circuit breaker has not tripped. A tripped breaker on the boiler circuit (often labelled "Boiler" or "CH") should be reset by switching it fully off then on again.
4. Check the Thermostat and Timer
A boiler that appears dead but shows no fault code may simply have no demand from the thermostat. Check:
- Room thermostat temperature is set higher than the current room temperature
- The programmer/timer is set to the correct time (particularly after a power cut)
- The hot water cylinder thermostat (if applicable) is set above 60°C
- Smart thermostat apps are connected and have the correct schedule
5. Check for a Fault Code
If none of the above apply, check the boiler display for a fault code. Most combi boilers will show an error code when they lockout for a fault. Look up the code for your boiler brand in our fault code guides for Baxi, Ideal, Glow-worm, Vaillant or Worcester Bosch.
When to Call a Gas Safe Engineer
Call an engineer without attempting further resets or investigation if:
- You smell gas near the boiler or anywhere in the property
- The carbon monoxide alarm has sounded
- The boiler is making unusual noises (banging, hissing)
- Low pressure returns within days of repressurising (indicating a leak)
- The fault code is not resolving after one reset attempt
Frequently asked questions
Why has my combi boiler stopped working with no fault code?
A combi boiler that shows no fault code but will not fire is usually caused by: no heating demand from the thermostat/timer (check settings), a power supply issue (check consumer unit), or a gas supply interruption (check other gas appliances). Reset the boiler once and if it does not start, call a Gas Safe engineer.
Why does my combi boiler work for hot water but not heating?
This usually indicates a problem with the 3-port mid-position valve (in older systems), a thermostat set incorrectly, or the programmer set to hot water only mode. Check the programmer settings first. If heating is selected but not working, the diverter valve or pump may need attention — call a Gas Safe engineer.
My combi boiler pressure keeps dropping — what is wrong?
Repeated pressure loss indicates a system leak. This could be a weeping radiator valve, a pinhole in a pipe, or a pressure relief valve discharging. A Gas Safe engineer can pressure test the system to find and repair the leak.
How do I repressurise my combi boiler?
Find the filling loop — usually a silver braided hose with one or two small valves underneath the boiler. Turn the valve(s) slowly until the pressure gauge reads 1.5 bar, then close the valve(s) firmly. Reset the boiler. If you cannot find the filling loop, call a plumber — different boiler models locate it differently.