If your boiler is not turning on, you can be left without heating and hot water. Often the cause is simple – a thermostat setting, the electricity supply or the gas supply – and you can check it yourself. This guide explains what you can safely check before calling a technician and which problems are strictly a job for a professional.

Why a boiler won’t turn on

According to an official guide from the boiler manufacturer Vaillant, the most common causes are:

  • The room is already at the set temperature – if the thermostat is set to 18°C and the room is at 19°C, the boiler won’t come on because the home is already warm enough;
  • Electrical problems – a boiler needs electricity to ignite. Vaillant says a boiler not coming on is often an electrical issue rather than a fault in the appliance itself;
  • Gas supply problems – the boiler needs a certain gas pressure to run; with insufficient or no gas pressure it won’t ignite and will show a fault code;
  • Internal boiler faults – for example a faulty fan or gas valve, a blocked burner or frozen pipes.

Source: Vaillant – Boiler not igniting? Common issues and how to fix them (official page)

Boiler not turning on – wall-mounted gas boiler

What you can safely check yourself

The steps below do not require opening the boiler casing and are safe for an owner:

1. Thermostat and temperature

Set the thermostat above the current room temperature (for example, 20°C when the room is at 19°C). Vaillant suggests testing the thermostat by turning it up and down and seeing whether the boiler responds.

2. Power, circuit breaker and boiler switch

Check whether the rest of your home has electricity. If other appliances are not working either, the boiler won’t run until the power is back. Check the consumer unit for a tripped breaker and make sure the isolation switch next to the boiler is on.

3. Control panel settings after a power cut

Vaillant notes that an overnight power cut can reset the boiler’s programming or time settings, so it may not come on in the morning as expected. Check that the control panel settings are what you expect.

4. Gas supply

Check whether your gas cooker or hob works – this tells you whether gas is reaching the home. Without gas the boiler cannot ignite until the supply is restored.

5. Pressure gauge

According to Vaillant, pressure that is too low or too high can cause problems. How to check it and what to do is explained in our guide on boiler pressure dropping.

6. Error code and reset

If the display shows an error code, look up its meaning in your boiler’s manual. You can try the reset button as your manual describes, but Vaillant advises that if the boiler doesn’t relight after 2 or 3 attempts, you should call a qualified technician rather than keep resetting.

What only a technician should do

Do not open the boiler casing. Vaillant warns never to attempt to mend any part of your boiler on your own. Cleaning or replacing the burner, checking and repairing the fan, gas valve and electronic control board, and any work on the gas pipe must be done only by a qualified technician. Vaillant also says that if you suspect a faulty fan you should not turn the boiler on, and a faulty gas valve can let gas escape and pose a fire hazard. If you smell gas, do not use open flames or electrical switches, open the windows and contact the gas emergency service immediately.

When to call a boiler technician

  • Power and gas are on and the thermostat is set correctly, but the boiler still won’t turn on;
  • The boiler doesn’t relight after 2–3 resets, or the error code keeps coming back;
  • The central heating won’t turn on and the pressure is also too low or too high;
  • The boiler makes unusual noises or you smell gas.

A MasterMan technician will find out on site why the boiler won’t start and repair it – see our boiler repair page. All our services are listed on the HVAC services page.

Frequently asked questions

According to Vaillant, the most common reason is that the room has already reached the temperature set on the thermostat. Set it above the current room temperature. If the boiler still won’t start, there may be an internal fault – call a technician.

Vaillant advises that if the boiler doesn’t relight after 2 or 3 attempts, you need a qualified technician. Before resetting, look up the error code in your manual.

Check the breakers in the consumer unit and the switch next to the boiler. Vaillant notes that a power cut can reset the boiler’s time settings, so check the control panel.

No. Vaillant warns never to attempt to mend any part of the boiler yourself. Work on the burner, gas valve, fan and control board is a job for a technician only.

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