The Forced Air Furnace Doesn’t Turn On

There are a lot of reasons for this so I will pick the 8 most common reasons for failure of a forced air furnace to turn on.

  1. The furnace door where the filter is not shut all the way. At the top of the filter compartment towards the burners there is a safety push button or rocker switch. This turns the power off to the furnace in the event that the blower compartment door is left open. If the fan motor were to turn on with the door open and the burners burning the vacuum could pull the flames right out of the firebox and cause serious problems in the home.
  2. The furnace is unplugged. This is a common occurrence with maids and housekeepers. They unplug the furnace to plug in a vacuum cleaner and forget to plug it back in.
  3. The circuit breaker has tripped. To reset the circuit breaker turn the handle all the wall off first and then all the way back on.
  4. The thermostat is battery operated and the batteries are dead. If this is an electronic display thermostat most, but not all, will tell you that the batteries need to be replaced.
  5. The 24 volts thermostat controls wires are broken. Many times if you have an electronic thermostat it will read -24 and this means that 24 volts is not reaching the thermostat.
  6. The furnace is in lock out mode. If a modern fails to ignite the burners three times in a row the circuit board stops all operations so that you have to call out a repair person to find out why gas ignition system is malfunctioning. Unplugging and plugging the furnace back into the wall resets the circuit board and the process starts all over again. When resetting this action stand back from the furnace as if this problem was created by delay ignition you could get a small explosion.
  7. The temperature setting on the thermostat is lower than the house temperature. This is quite common. Turn the temperature on the heat setting on the thermostat at least 2 degrees higher than the house temperature. If this doesn’t turn the furnace on try turning the thermostat all the way to see if the furnace turns on. If it does, replace the thermostat.
  8. You may have turned the fan switch to on thinking that this turns the furnace on. Most thermostats have air conditioning controls including a fan on/off/auto switch. Many furnaces are not equipped with these controls so the thermostat has essentially dead switches that don’ control anything. Make sure the thermostat is in heat mode and the temperature setting exceeds the temperature of the indoor air.