1. The Furnace Won’t Start If nothing happens when the thermostat is set above the home’s current temperature, the most likely causes are these: The furnace switch (located on or near the furnace, looks like a common light switch) is turned off and simply needs to be turned on, though the furnace might have a short delay before starting The circuit breaker is off, and if the circuit continues to trip after it is turned back on, there is likely a wiring short in the furnace The thermostat isn’t set to “heat,” which is easy to check and remedy The thermostat isn’t working, and this is sometimes caused by a wire coming loose, so check the wiring connections The thermostat needs fresh batteries, if battery-powered, or it is bad and must be replaced, an issue that can be diagnosed using an electrical multimeter that shows if a circuit is complete The filter is dirty and must be replaced or cleaned, an issue that is especially true on newer furnaces that are equipped to shut down if the filter isn’t allowing adequate air flow, a condition that can cause serious mechanical failure The furnace’s main circuit board/control board is bad, an issue that can be diagnosed by an HVAC technician is nothing else solves the issue
2. Furnace Blowing Cold Air If the furnace won’t produce heat, you can check to see if it is getting power by moving the fan switch from the “Auto” position to the “On” position. If the fan comes on, then the furnace is getting power. The likely issues then are:
3. The Draft Motor Runs, Furnace Won’t Start The draft motor is also called the inducer motor. It pulls hot combustion gases through the furnace and out the exhaust stack. If it starts and then the furnace shuts down, the cause might be: The motor or the board that controls it has failed and must be replaced A blocked air intake or blocked exhaust chimney (snow, debris and bird’s nests are common causes) must be cleared One of the issues discussed above with gas, gas valve and ignitor must be addressed
4. The Draft Motor Won’t Start The problem, in this case, is either that the motor is bad or the capacitor that supports it is bad. This video will assist you in troubleshooting draft motor issues and what is causing them.
5. The Flame Starts, but the Furnace Shuts Down If the burner starts making heat but soon shuts off without warming your home, the flame sensor isn’t working. If the sensor has corrosion on it, sanding it might work, though replacing it is a longer-lasting fix.
6. The Blower Won’t Shut Off If the blower motor runs all the time, there are three common issues: The fan switch on the thermostat is set to “on” rather than “auto” Relays in the control board are stuck in the closed position, so the blower is getting a constant supply of electricity, and the control board must be replaced The thermostat also has contacts that control power to the furnace, and they can be worn, so the thermostat must be replaced. Here’s a video that covers this troubleshooting a furnace blower motor that won’t shut off.
7. Why Does the Furnace Keep Shutting Off If your furnace burner lights but the furnace soon shuts off, there are a few common issues that might cause this: The furnace filter is dirty and should be cleaned or replaced The flame sensor isn’t working and should be replaced The thermostat is failing While these are common, there are several other potential causes best diagnosed by a furnace technician. This video gives in-depth coverage to the problem of furnace short-cycling.
8. The Furnace Runs but Can’t Keep the House Warm If your furnace is properly sized for your home, then it should keep your home warm even in extreme cold. If it runs constantly but isn’t providing enough heat, the likely causes are: A dirty filter that must be washed (only if the washable type) or replaced The filter is the wrong type for your furnace and restricts airflow, so should be replaced The filter is installed backwards, an issue that can be checked by looking for the arrows on the filter frame that show proper installation direction with the flow of air Air ducts are leaking, sometimes producing telltale whistling noises, and it is time to have them sealed as recommended by Energy Star
9. The Furnace Makes Loud Noises A noisy furnace is most often the result of: The furnace being a single-stage model that runs on high capacity all the time, so it’s going to be loud A bad bearing in the draft motor or the blower motor that must be replaced A leak in the furnace cabinet, sheet metal plenum or ductwork near the furnace that must be tightened A dirty filter, which can also cause whistling
10. The Furnace Leaks when AC is Running Water condenses on the evaporator coil in your furnace, and if it leaks onto the floor, the issue is most likely a clogged drain pan or drain line, an issue covered in our AC Troubleshooting Guide.
11. The Air Coming from the Furnace Smells Bad Foul odors from the furnace are usually caused by: A very dirty filter that should be washed, if washable, or replaced A dead rodent or bird somewhere in the system, usually a job for a professional furnace technician
12. The CO Monitor Going Off
A carbon monoxide alarm should be taken seriously. Turn the furnace off at the thermostat. Don’t go into the room where the furnace is located. Open doors and windows, and go outside. Call 911 from a cell phone outside your home.