Boiler not firing up after summer: what to check first (London)
- Checked September 2026
- 4 min read
- By PROS AI editorial team
Quick answer
The short version, before the detail.
A boiler that will not fire on the first cold morning is usually stopped by something outside the boiler: a control still set for summer, low pressure, a switch that is off, or a lockout that needs one reset. Work through the checks below in order; none of them opens the casing. If it still will not fire, book a Gas Safe registered engineer.
What is the likely cause?
Most common first. Each line says how to recognise it.
Controls still on the summer setting: a combi left on hot water only, a timer that is off, a thermostat set below room temperature or with flat batteries.
How to tell: the taps run hot but every radiator is cold.
Low system pressure.
How to tell: the gauge reads below 1 bar or sits in the red, with a low-pressure warning on the display.
No power or no gas.
How to tell: the display is dark (fused spur off, breaker tripped, prepayment electricity meter empty), or a gas hob will not light (prepayment gas meter empty).
A lockout after a failed ignition.
How to tell: a fault light or code is showing and the boiler is silent. One reset clears a one-off.
Condensate not draining.
How to tell: gurgling when the boiler tries to start, then repeated lockouts. In September the usual cause is an outside drain blocked with leaves or moss, not ice.
A stuck diverter valve or pump.
How to tell: hot water works but the heating never comes on, or the boiler fires briefly and stops. Valves can stick over time; this is engineer work.
On an older boiler, a pilot light that has gone out.
How to tell: no small blue flame where the manual says there should be one.
What can I safely check myself?
Every step here is done from the outside of the boiler. Nothing below opens the casing, removes a cover, or touches gas pipework or wiring.
- Controls. Put the boiler in heating mode (on a combi, not hot water only). Check the timer is on and shows the right time, because a power cut or the clock change can reset it. Turn the thermostat above room temperature and put fresh batteries in a wireless one.
- Power. Check the fused spur switch beside the boiler is on and its breaker on the consumer unit is up. The display should light.
- Pressure. Cool, the gauge should read 1 to 1.5 bar. Below 1 bar, open the filling loop until it reaches 1 to 1.5 bar, then close both valves fully. Below 0.5 bar, water has left the system and needs replacing before you try again.
- Reset once. Press the reset button for a few seconds (no more than ten) and wait two to three minutes. If it locks out again, do not keep pressing; a boiler that needs regular resets has a fault.
- Gas. Check another gas appliance lights, and top up a prepayment gas meter if you have one.
- Condensate pipe. Find the white plastic pipe where it leaves the wall outside and check the drain it feeds is clear of leaves. In a freeze, thaw it with warm, not boiling, water and reset once.
- Water. Run a cold tap. If nothing comes out, check the stopcock is open.
When do I stop and call an engineer?
- If you smell gas, leave the property, do not switch anything on or off, and call the National Gas emergency line on 0800 111 999 from outside.
- If water is dripping near the fused spur, sockets or the consumer unit, switch off at the main switch only if it is dry and safe to reach; otherwise keep away from it.
- Stop if the boiler locks out again after a reset, if the pressure falls again within days, if the pump is noisy or hot, or if a pilot light will not stay lit. These are internal faults.
- Anything behind the casing is gas work and must be done by a Gas Safe registered engineer. Ask for the ID card and check the seven-digit licence number on gassaferegister.co.uk, or text Gas and the number to 85080.
What does it cost in London?
A weekday call-out with diagnosis in London is £60 to £125, typically £120, and many firms waive it if the repair goes ahead. Labour is £80 to £140 an hour. Parts behind a no-fire fault run from £120 to £280 for a pressure or valve fault to £200 to £420 for a fan and £290 to £650 for a circuit board; most repairs land between £150 and £400 with the part.