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Furnace Blower Fan Won't Turn OffFurnace Blower Won’t Turn Off - Hello Richard, I have a problem with my furnace where the indoor blower will turn off. My furnace seems to be heating okay it is just the blower will not shut off. It is an older furnace and I have checked the thermostat settings. It is set to automatic with the fan switch part set to automatic so I know that is not the problem causing the furnace blower not to shut and run continuously. So what else do I need to check and is this a problem I can fix myself or do I need to call a service and repair company to make a repair of some kind? Please help!! Thanks!!

Furnace Blower Fan Won’t Turn Off

This is a common occurrence and can be a result of the following:

  1. Fan select switch on the thermostat is in the manual “ON” position
  2. A control wiring problem where the insulation on the wiring is worn off
  3. A bad relay, fan sequencer relay, fan limit control or circuit board with contacts sticking (this depends on the type and age of the furnace)
  4. A problem with the limit switch or a problem that is causing the limit switch to open

Fan Select Switch - Furnace Blower Fan Won’t Turn Off

This problem is easily rectified by turning the switch to automatic on the thermostat. You indicated you have already done this so this is obviously not the problem. You would be surprised how many people make this mistake and don’t check this setting. It is the list of basic checks to make before calling for service.

Control Wiring Problem - Furnace Blower Fan Won’t Turn Off

Damaged Digital Thermostat Wiring Problem

A Bundle of 8 18 Guage Wires

This is basically what some people would call a shorted circuit. For one reason or another two wires are touching one another and keeping the circuit completed with no control. Older thermostat wire can have degraded insulation. When the insulation is gone the wires touch inside the thermostat wire bundle. In this case, the red and green wire would be touching somewhere in the bundle. For a quick check (and hopefully to resolve the issue) check the thermostat wire terminations at the thermostat and the furnace. If you are lucky, you will find the problem there. If not, then you will have to pull new thermostat wire from the furnace to the thermostat. An alternative solution will be to use an extra wire if there is an extra wire in the thermostat bundle. Simply change out the green or red for the extra wire (if one is available).

Bad Control Relay or Board - Furnace Blower Fan Won’t Turn Off

This problem will depend on what type of gas furnace you have. More modern gas furnaces will have a control board. Older furnaces will use either a sequencer relay or a simple peanut relay that is controlled by the thermostat and will turn the fan on and off based on what the thermostat is set to. All of these controls have contacts and a coil inside them. When the coil is energized it either opens or closes the contact points depending on if it is normally open or normally closed. The real solution here is to replace the control board or relay. With the fan limit control, it is possible it could also be sticking and needs to be replaced. Either way, something is sticking when it should not be stocking and the device needs to be replaced to correct the problem.

Limit Switch Issue - Furnace Blower Fan Won’t Turn Off

If the furnace overheats then a limit switch will open. This will cause the burners or heat source to shut off and the fan to run until the limit switch closes again. In some cases, the limit switch will stick open and this will cause the fan to run continuously. It will also cause your system NOT to heat so I doubt this is related to your problem. If a limit switch is found to be problematic as described here it is recommended that it be replaced. Before this happens, the overheating problem needs to be rectified or checked out. Things that lead to fun overheating can include:

  • Poor airflow as a result of a plugged or clogged dirty air filter. Change your air filter on a regular basis as a part of furnace maintenance.
  • Poor airflow as a result of too many supply vents closed off or blocked. I always recommend to my customers to leave all your supply vents open. The system is designed for a specific amount of airflow and closing these vents off will cause reduced airflow that can cause issues as described here.
  • Low or reduced airflow because of a collapsed duct somewhere.
  • A bad fan motor running at a reduced speed because it will soon fail completely. In this case, the fan motor needs to be replaced.

Conclusion - Furnace Blower Fan Won’t Turn Off

As you can see, these problems are furnace control issues. Some involve safety controls as a furnace can be dangerous if not controlled properly. For this reason, I always recommend that you call in an HVAC professional to properly troubleshoot and repair the problem. Never, ever override a safety control on a furnace even temporarily. Most HVAC technicians are trained in furnace repair and know about these safety issues and what happens if the controls are overridden. It is not safe and can lead to a catastrophe. Additionally, most HVAC technicians have replacements parts on the truck to make the repair quickly and efficiently. If they do not have the part, they can usually get the part on the same day. As always, we recommend regular maintenance for your heating system to ensure it is working properly and efficiently. Of course, regular maintenance cannot guarantee the system will operate without a break down during the heating season, but problems or potential problems can be found and rectified before they do become a problem and fail. Also, always make sure you check and change the furnace filter at least every thirty days. This will prevent a lot of problems that can occur that causes the furnace to fail. I hope this helps and you have fixed the problem. Good luck and happy heating!!!

Furnace Blower Fan Won’t Turn Off

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