Waking up to a cold home doesn't exactly give anyone a warm, cozy feeling. Fixing a furnace that won't start can range from easy to somewhat complicated, and sometimes it requires the assistance of a professional. There are a few things any homeowner can check before calling a furnace service pro, starting with the thermostat.
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Check the Thermostat Settings
Check your thermostat to make sure that the "FAN" setting is in the "AUTO" position. If it is in the "ON" position, the fan will just blow room-temperature air even when the furnace is not making heat. Make sure that the thermostat is set for heating (not cooling) and that the temperature setting is above room temperature.
Check for a Condensate Drain Pan
High-efficiency furnaces create water vapor that turns to liquid (condensate) that is drained from the furnace via a PVC pipe. The pipe may lead to a drain pan where the water is collected to be pumped away. If there's a clog or a problem with the pump and the drain pain overfills, a float switch is activated, interrupting the thermostat electrical circuit and effectively shutting down the furnace. If you have this type of setup, confirm that the drain pan is not full. Then crank up the thermostat to see if the furnace goes on — just don't forget to turn it back down.
Furnace Blows Cold Air
A furnace is made up of two primary systems: the burner and the blower. The burner burns natural gas, propane, or oil to deliver heat to the heat exchanger (electric furnaces have heating elements instead of a burner). The blower is a big fan that forces air through the heat exchanger and into the system's ductwork to heat the house.
A furnace blowing cold air tells you that the blower is working but the burner is not. Check to make sure there are not power issues. If the furnace is blowing, then it typically has power, but in the case of an electric furnace, there may be a separate breaker that has tripped. Try to reset any circuit breakers that are associated with the furnace.
Fuel-fired furnaces rarely have a separate power source for the burner assembly, so if the furnace is blowing cold air, then it has power. Check for any LED lights that may indicate some kind of failure. Both oil and gas (propane or natural gas) furnaces have a sensing device that monitors the flame causing what is known as a flame failure. Check to make sure there is actually fuel or propane in the tank. If you have natural gas piped into your home, make sure the natural gas is indeed on.
Resetting the Furnace
Oil furnaces have a reset button to push. Hit this reset button once and then watch as it establishes a flame. If not, then there's a problem with the oil burner itself. Do not press the reset button more than once; call a technician instead. This problem will require digging into the big three: spark, fuel, and air. These three things are the basis of combustion. Without the right combination of all three, the burner will not ignite on any type of fuel.
Gas furnaces, either propane or natural, also use the big three for combustion, but the reset is a little easier. To reset a gas furnace, simply turn off the power to the furnace and then turn it back on. This resets the controls, and the burner will try again.
Another control device is the limit switch. See if there is some sort of reset button on it. Some older gas furnaces have a standing pilot that may need to be relit to establish the spark part of the big three. If the pilot does not stay lit, then there may be an issue with the pilot assembly itself or the thermocouple that monitors the flame.
Fan Doesn't Work
After establishing a flame, if the fan doesn't work, this entails a whole other series of issues. One issue is that the power to the fan motor is being interrupted by some sort of safety device called a fan control. This control monitors the temperature of the air inside the furnace to make sure that the furnace doesn't blow cold air. This requires the help of a professional to resolve this problem safely. Other possibilities are motor parts or even the motor itself. Replacing or checking a fan motor and capacitor requires specialty tools and multimeters that every furnace professional should have.