Boiler Not Working? Common Causes & Fixes UK 2026
If your boiler won’t fire up, the cause is often something simple like low system pressure, a tripped fuse, a thermostat/programmer setting, or a lockout that needs a reset. This UK guide covers the most common reasons a boiler stops working, safe checks you can do yourself, and when it’s time to call a Gas Safe engineer.
First: Check if It’s Just Heating, Hot Water, or Both
The fastest way to narrow it down is to see what’s affected. If you have no heating but hot water works, the issue may be controls, thermostat, motorised valve, or circulation. If you have neither, power, gas supply, pressure, or lockout becomes more likely.
Common Cause 1: Boiler Pressure Too Low (Sealed Systems)
Many UK combi and system boilers are sealed systems and need adequate pressure to run. If pressure is below the safe cold range (often around 1–1.5 bar), the boiler may not fire. You may need to top up using the filling loop (follow your manual).
Common Cause 2: Power, Fused Spur, or Tripped Breaker
Check the boiler’s fused spur (usually near the boiler) and your consumer unit for tripped breakers. If the boiler display is off, power is the first thing to confirm. If you suspect an electrical fault, stop and call an electrician or engineer.
Common Cause 3: Thermostat or Programmer Settings
Schedules, incorrect time settings after a power cut, or a low thermostat setpoint can make it seem like the boiler isn’t working. Try setting the thermostat higher and switching the programmer to “on” or “override” to test.
Common Cause 4: Boiler Lockout or Error Code
Boilers lock out to protect themselves after detecting faults (like ignition failure, flame detection problems, low pressure, or overheating). If it’s safe, you can try a single reset. If it locks out again, stop resetting and book an engineer.
When to Call an Engineer
Call a Gas Safe engineer if you smell gas, see repeated lockouts, suspect a leak, hear unusual noises, or if basic checks (pressure/power/controls) don’t restore operation. Don’t remove the boiler case or attempt internal repairs.
If You’re Replacing the Boiler, Size It Right
If your boiler is repeatedly failing, replacement might be more cost-effective. Use our free calculator to estimate the right kW output range for your home.
Use the Boiler Size CalculatorFrequently Asked Questions
Why won’t my boiler fire up?
Common causes include low pressure, a power issue, thermostat/programmer settings, or a lockout condition. Start with safe checks before calling an engineer.
Is low boiler pressure dangerous?
Low pressure usually isn’t dangerous; the boiler may shut down for protection. The underlying cause (like a leak) may need attention if pressure drops repeatedly.
Can I reset my boiler?
You can try one reset if the boiler indicates lockout and there’s no sign of gas smell or leaks. If it locks out again, stop resetting and contact a Gas Safe engineer.
Why does my boiler keep switching off?
Repeated switching off can be caused by pressure issues, poor circulation, overheating, faulty sensors, or control problems. An engineer can diagnose if basic checks don’t help.
When should I replace my boiler instead of repairing?
If it’s old, repairs are frequent, or parts are costly, replacement can make sense—especially if efficiency and reliability are poor.