How to Reset a Boiler Safely

Updated July 2026 · Emergency Plumber Chelsea

A boiler lockout is a safety feature, not a breakdown in itself. When the boiler detects something wrong, such as no flame on ignition, low water pressure, overheating or a blocked condensate pipe, it shuts off the gas and stops. Some faults clear on their own once the condition goes away. Others, which manufacturers call lockouts, keep the boiler off until someone presses reset. The reset tells the boiler to try again, so it only helps if whatever caused the lockout has gone. That is why a few quick checks before you press the button make all the difference. If the boiler shows no fault at all and simply will not come on, our guide to a boiler not working covers the power, controls and gas checks.

Start by writing down the fault code on the display, and take a photo if you can. Once you reset, the code usually disappears, and it is the single most useful piece of information an engineer can have. Codes differ by manufacturer, so an F1 on one make means something completely different on another. Our guides to Worcester fault codes, Ideal fault codes and Baxi fault codes explain the most common ones.

Next, check the gas. Try the hob or another gas appliance. If nothing lights anywhere, the problem is the supply, not the boiler, so check the credit on a prepayment meter and make sure the valve at the meter is on. If you smell gas at any point, do not reset anything. Leave the property, avoid using switches, and call the National Gas Emergency Service on 0800 111 999 from outside. If the boiler shows an ignition fault and will not stay lit, our guide to a boiler ignition lockout goes through the causes.

Then check the pressure gauge. On most combi and system boilers it should read around 1 to 1.5 bar with the heating cold. If it has dropped towards zero, the boiler will refuse to run, and a reset will not help until you top it up through the filling loop as your manual describes. If the pressure is much higher than normal, see our guide on boiler pressure that is too high before resetting.

In cold weather, check the condensate pipe. This is the plastic pipe, usually white or grey and about the width of a thumb, that runs from the bottom of the boiler to a drain, often outside. When it freezes, the boiler cannot drain its condensate and locks out, often with a gurgling noise. A reset will fail until it is thawed. Pour warm, not boiling, water along the outside section, or hold a hot water bottle against it, then reset. Our frozen condensate pipe guide explains the full process.

Now find the reset control. On many Worcester, Vaillant and Baxi boilers it is a button marked reset, or with a flame symbol and a cross or arrow. On many Ideal boilers you turn the mode knob to the reset position and hold it. Newer boilers with touch screens may have a reset option in a menu. Press and hold for the time your manual gives, usually a few seconds, then release. The boiler will run through its start-up sequence, and you may hear the fan and the ignition clicking. Give it a minute or two before deciding whether it has worked. If you have a Viessmann, our guide to Viessmann boiler fault codes explains what F2, F4 and F9 mean before you reset.

If the boiler has no display at all, it may have no power rather than being locked out. Check the fused spur switch next to the boiler, check whether any other circuits have tripped at the consumer unit, and check whether there has been a power cut. After a power cut, also check the timer or programmer, as some lose their settings and the boiler looks dead when it is simply switched off by the clock. Resetting a programmer or thermostat is not the same as resetting the boiler, and the boiler's own reset button will not fix a programmer that has lost its time.

Know when to stop. Most manufacturers limit how many resets a boiler will accept in a short period before it enters a longer lockout, and every failed ignition means gas has been released without burning. A sensible rule is one reset, and a second only if you have fixed something in between, such as topping up the pressure or thawing the condensate pipe. If the boiler locks out again, leave it off and call an engineer. Repeated lockouts point to faults such as a worn ignition electrode, a failing gas valve, a blocked flue, a stuck pump or a sludged heat exchanger, none of which can be fixed from the front panel. Do not take the casing off or try to clean parts yourself; opening the combustion side of a gas boiler is legally restricted to Gas Safe registered engineers.

Some lockouts are worth recognising on sight. A boiler that locks out every morning in winter but works later in the day often has a condensate pipe that freezes overnight. A boiler that locks out every time the heating reaches full temperature may be overheating because of poor circulation. A boiler that locks out after long hot showers may have a hot water side fault. Telling the engineer the pattern, as well as the code, often speeds up the repair. If you have heating but no hot water, or the other way round, our guides to no hot water and hot water but no heating cover the specific checks.

If your boiler will not reset, or keeps locking out, call Emergency Plumber Chelsea on 020 3002 6667. Tell us the make, model, fault code and what you have already checked. Our Gas Safe registered engineers carry out boiler repairs across Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham, and a regular boiler service is the best way to stop lockouts happening in the first place. There is no call-out fee on jobs over 3 hours.

Quick tips

• Photograph the fault code before you reset; it disappears once you press the button.
• Check the hob and the pressure gauge first; a reset will not fix a missing gas supply or low pressure.
• In a frost, thaw the condensate pipe with warm water before resetting.
• One reset, and a second only after fixing something; repeated lockouts need an engineer.
• Never reset if you smell gas: get out and call 0800 111 999.

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