Boiler 25 September 2026

Boiler Not Firing Up? Why It Won't Light and What You Can Safely Check

If your boiler is not firing up, the cause is usually one of four things: it is not being asked to fire (a timer, thermostat or smart control issue), it is not getting gas, it has locked out to protect itself, or a part in the ignition sequence has failed. You can often rule out the first three yourself in ten minutes without taking a cover off; the fourth is a job for a Gas Safe registered engineer.

This is the order our engineers work through when we are called out across west Kent on the first cold mornings of the year, when boilers that have sat idle all summer are suddenly asked to work hard.

Why is my boiler not firing up?

A modern gas boiler goes through a set sequence every time it lights. The controls call for heat, the fan starts and proves the flue is clear, the gas valve opens, the ignition electrode sparks and a flame sensor confirms there is a flame. If any step fails, the boiler stops and tries again, and after a few failed attempts it locks out.

So listen for a minute before doing anything else. A boiler that makes no sound at all has a different problem from one where the fan runs and the electrode clicks but the flame never catches.

Boiler not firing up but no error code

This is the most common version we see, and in most cases the boiler simply is not being asked to run. Check these first:

• The timer or programmer. After a power cut, many older mechanical and digital timers lose their time or revert to “off”. Check the clock is right and the heating channel is set to on or to a timed period that covers now.
• The room thermostat. Turn it up well above the current room temperature. If it is set to 20°C and the hall is already 21°C, the boiler has no reason to fire.
• Smart controls such as Hive or Nest. If the boiler is not firing up with Hive, check the app shows the receiver as online. A thermostat that has lost its connection to the receiver will happily show a target temperature on screen while the boiler never hears about it. Our guide to Hive thermostat problems covers the reconnection steps.
• Batteries in a wireless thermostat. Flat batteries are a regular cause of an early winter call-out.
• The fused spur. Check the switch next to the boiler is on and, if the display is blank, whether the fuse has blown or a breaker has tripped at the consumer unit.

If every control is calling for heat and the display shows nothing wrong, the problem is further inside the boiler.

Boiler not firing up for heating but does for hot water

On a combi this split is useful, because it proves the burner, gas valve and ignition all work. The fault is in how the heating demand reaches the boiler or in the heating side of the system.

The usual suspects are the controls above, a diverter valve stuck in the hot water position, or a pump that is not moving water round the radiators. We have written separately about hot water but no heating and about the signs of a central heating pump not working, so if this matches what you are seeing, start there.

On a system or regular boiler with a cylinder, the equivalent culprit is often a motorised zone valve or a three-port valve that has stuck after a summer off.

Boiler not firing up when hot water is turned on

The reverse, where the heating works but the boiler will not light when you open a hot tap, points at the flow sensor on a combi. The boiler only fires for hot water when it detects water moving through it. A sensor clogged with debris or scale will not register the flow, particularly on low-flow taps and showers. In our area the water is hard, so scaled-up hot water components are something we find often.

Try a full-bore hot tap such as the kitchen or the bath before assuming the boiler has failed. If it fires on the bath tap but not the basin, the flow sensor is the likely cause.

Boiler not firing up after summer

Boilers that have not run for months are the ones that fail in the first cold week. The things we most often find are:

• Low pressure. A sealed system slowly loses a little water over the summer. Most boilers will not fire below about 0.5 to 1 bar on the gauge. Our post on what pressure your boiler should be explains how to read it and top it up.
• A seized pump. Sludge settles while the system is idle and can lock the pump rotor.
• A stuck diverter or zone valve. Parts that have not moved since spring can stick in one position.
• Ignition electrode wear. A tired or poorly set electrode may have managed last winter and then give up on the first attempt this autumn.

This is exactly why we recommend getting the boiler serviced before the heating goes on for the season, not after it has broken. You can book a boiler service with us at a flat £95+VAT.

Boiler not firing up after a power cut or reset

After a power cut, check the timer first, as above. If the display shows a lockout, pressing reset once is reasonable and is what the manufacturer’s manual will tell you to do.

Please do not press reset repeatedly. A boiler that locks out every time has a fault in the ignition, flame detection, gas supply or flue, and on some models repeated resets cause a permanent lockout that needs an engineer to clear.

Boiler not firing up when the temperature drops

If the boiler works on mild days but not on frosty mornings, the outside condensate pipe may have frozen, and the boiler shuts itself down until it clears. If that happens every cold snap, the pipe run needs looking at rather than the boiler.

Check the gas supply

If the boiler clicks and the fan runs but it never lights, check whether your other gas appliances work. If the hob will not light either, or the meter display says credit has run out on a prepayment meter, the boiler is not the problem.

If you smell gas at any point, stop. Do not operate electrical switches, open the doors and windows, turn off the supply at the meter if you can do so safely, and call the National Gas Emergency Service on 0800 111 999 straight away.

When to call a Gas Safe engineer

Once the controls are calling for heat, the pressure is right and there is gas at the hob, anything left is inside the sealed part of the boiler: the ignition electrode, flame sensor, gas valve, fan, printed circuit board or flue. None of that is DIY. By law, any work on a gas appliance must be carried out by a Gas Safe registered engineer.

Our engineers test each stage of the sequence and tell you plainly what has failed before anything is fitted. Our boiler repair service is charged at £95+VAT for the first hour, then £47.50+VAT for each 30 minutes after, and any If the boiler is old and the fault is a major part, we will tell you honestly whether a repair is worth it or whether a new boiler installation makes more sense, with a free fixed written quote.

If your boiler is not firing up and you have worked through the checks above, call our team on 01732 792 454. We are Gas Safe registered (number 951167) and cover Tonbridge, Tunbridge Wells, Sevenoaks, Maidstone and the villages in between, so we can get someone out to you and get the heating back on.