Most boiler breakdowns come from a short list of causes. Some you can spot and sort yourself in a few minutes. Others need a Gas Safe registered engineer. Knowing which is which saves you time, and sometimes a call-out.
Low pressure
A sealed heating system needs water at the right pressure to run. Most boilers want about 1 to 1.5 bar when the system is cold, and the gauge on the front shows it. If it has dropped close to zero, the boiler will usually lock out and show a fault.
A slow drop usually points to a small leak somewhere in the system, or to radiators that were recently bled. If you top it up and it keeps falling, call us. Repeatedly topping up a leaking system adds fresh oxygen to the water and speeds up corrosion.
A frozen condensate pipe
Condensing boilers produce acidic water as a by-product, and it leaves the house through a plastic pipe, usually to a drain outside. In a hard frost, that water can freeze in the pipe. The boiler then shuts down, often with a fault code and a gurgling noise.
If the pipe is outside and you can reach it safely, pouring warm (not boiling) water along it often clears the ice. Boiling water can crack the plastic. If it keeps happening, the pipe may need re-routing or insulating, which is a quick job.
Trapped air
Air in the radiators leaves cold patches at the top and a noisy system. It is also a sign something is allowing air in. Bleeding the radiators with a radiator key takes a few minutes, and afterwards you will probably need to top the pressure back up.
Sludge and debris
Over the years, rust particles and sludge build up in the water. They settle in the radiators, cause cold spots at the bottom, and can block the boiler's heat exchanger. A magnetic filter catches much of it, and a power flush clears an older system that has already built up.
Faulty parts
- Thermostat. If the heating does not respond, a failed room thermostat is often the culprit.
- Pump. A worn pump gives you a boiler that fires but radiators that stay cold.
- Diverter valve. On a combi, it switches between heating and hot water. When it sticks, you may get one without the other.
- Ignition or sensors. A boiler that tries to light and gives up can have a dirty or worn electrode or sensor.
Age and lack of servicing
A boiler that has never been serviced has had no one looking for small problems before they turn into breakdowns. An annual service is the cheapest way to avoid the call at midnight in January. We cover how often a boiler should be serviced in a separate guide.
What you should not do
Do not take the casing off a gas boiler, and do not try to repair anything gas-related. That is unsafe and illegal for anyone not registered. If you smell gas, follow our gas leak advice straight away.
When to call
If the boiler has no heating or hot water, shows a fault code that will not clear after a reset, or leaks, call us. We cover emergency callouts 24/7 across North Norfolk, Hertfordshire and Bedfordshire. If the boiler is more than about 15 years old and keeps breaking down, our guide to when to replace your boiler will help you decide whether repairing it still makes sense.
