The last thing you want is your furnace giving up on you in the middle of a cold Wisconsin winter night. Though you will hate this scenario, it may very well happen if you don’t test and troubleshoot your furnace before the cold season sets in.
This furnace repair & troubleshooting guide will teach you how to diagnose and repair the most common problems occurring in a modern gas fired furnace. You can use these tips and save the more than the $100 you have to spend on an average service call. However, this is only a guide, and will not provide all the details for all situations. Only you can decide if you can perform this task.
You can avoid many common furnace problems with proper maintenance. So before you start troubleshooting your furnace, it’s essential you perform the suggested maintenance regimen.
This furnace troubleshooting guide is meant for both natural gas and propane furnaces in the
Milwaukee, Waukesha & Racine areas. The electric furnace troubleshooting guide can be used for forced air electric furnaces or heat pump systems and the oil furnace repair guide for furnaces that burn fuel or waste oil.
First, the thermostat!
Start your troubleshooting by ensuring the thermostat is on and its set point is above the room temperature. It’s better if the thermostat is at the highest setting while troubleshooting and the furnace fan in the ‘on’ mode if it is not running.
If the furnace fan is not running, check the fuse, breaker and the furnace switch. If the breaker has tripped, leave it off and see if there are any loose connections in the furnace wiring. Also check the control board for burn spots and replace any board spots if required. Reset the breaker after all the necessary repairs have been made.
If the breaker or fuse hadn’t tripped, check the furnace’s blower compartment’s inspection window for a flashing green light. If there is neither a green or red light, it may indicate a problem with the transformer, blower motor, thermostat, furnace control board or the run capacitor.
The reservoir of furnaces with condensate pumps shouldn’t be full. Most units have a float switch preventing the furnace from running on a full reservoir. If the reservoir is full, check the unit’s power and replace the pump as required.
Place the fan back in the automatic mode at the thermostat once the blower motor starts running.
Furnace pilot light
The standing light of the furnace (if there is one) should be lit with the flame touching the tip of the thermocouple. If it isn’t, use the thermocouple replacement guide for further help with troubleshooting.
If the pilot is lit but the main burners do not turn on, touch the furnace side. If it is cool, it means there could be a problem with the furnace control board, thermostat, and limit control or gas valve.
The heat on furnace side
If the furnace side is warm, just let it cool down and you can continue with your furnace troubleshooting. Check to ensure the inducer motor of the furnace (if it has one) is running. If it isn’t, there could be a problem with the inducer motor or furnace control board.
Pilot or main burner ignition
If the unit uses spark ignition, you will hear the pilot or main burner trying to ignite with a rapid clicking noise. Check if the pilot or main burner tries to ignite with the inducer motor running. If it doesn’t, it may mean the flue is blocked, a bad furnace control board or some limit control.
If the pilot or main burner tries to light, but tends to go out, it may indicate a dirty pilot assembly or flame sensor. Use our guide to learn how to clean them. If your furnace does not use a spark igniter, check if the hot surface igniter comes on or not. There will be a bright glow in the burner compartment if it’s trying to come on. If not, either the furnace control board is bad or the igniter has to be replaced.
Main burners turning on
Check if the main burners light once the pilot or igniter is on. If it doesn’t, check to see if gas reaches the furnace by ensuring all manual isolation valves are on. If gas does reach the furnace, there may be a problem with the furnace control board or gas valve. If the burners turn on and stay only for a few seconds, the flame sensor could be dirty or the furnace control board may have gone bad.
If the burners of condensing furnaces come on for more than 5 seconds to turn off before the thermostat is satisfied, the problem could be a plugged condensate drain. There may be built-up sediment in the built-in trap of most of these units that leads to the abnormal functioning of the furnace.
Does the blower start?
The blower should start after the burners stay lit for one to three minutes. If it doesn’t, a run capacitor or the motor may be the problem.
Observe a complete heating cycle of the operating furnace to complete your furnace troubleshooting. The fan should typically run for 2-3 minutes after the burner shuts off. If it doesn’t, the fan and limit control of older units may be sticky and need replacement.
Also check the temperature increase in the furnace by using a thermometer to check the temperature coming out of registers and comparing it with the average room temperature. There is a label plate on the furnace inside telling you the desired difference between temperatures. While it may be over 100 degrees on older furnaces, it may be as low as 35 in newer efficient units.
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