The Reasons Why Gas Furnaces Won't Light

Thermostat

• One of the simplest reasons that a gas furnace won't light is that the thermostat is set to a temperature that is too low, or the heat function is not engaged. Electronic thermostats may require both a "heat" setting, as well as a temperature setting, before the furnace will light and begin producing heat. A manual thermostat that is not connected to the furnace or set to a low temperature will also prevent the furnace from lighting. Resetting the thermostat and checking its wiring and connections can solve some gas furnace lighting problems.

Ignition

• Some gas furnaces rely on pilot lights, which are small flames that burn continuously inside the furnace's ignition housing to light the gas once it begins to flow. If the pilot light goes out, the furnace will not be able to light. Other gas furnaces use sparks for ignition. Again, if the ignition fails, the furnace won't light even when gas is flowing. Faulty spark units may need replacement or cleaning, while a pilot light can be reignited manually.

Gas Supply

• Even with its pilot light and thermostat in good working order, a gas furnace won't light unless there is a sufficient flow of gas. If you use a shut-off valve to stop the flow of gas during the summer, you'll need to turn the gas back on before lighting your furnace for the first time each winter. Your utility provider may also shut off your gas if you fail to pay your bill or move into a new home and don't set up an account.

Cleaning Needs

• Gas furnaces have regular maintenance needs. Neglecting basic cleaning can cause a furnace not to light. Before starting your furnace for the year, clean out or replace the air filter to ensure that the flame has a steady supply of clean, oxygen-rich air. Check the sensors near the burner for carbon and dust deposits. Enough debris in any of these key areas can prevent a furnace from lighting, or cause it to shut down spontaneously after lighting.