Two problems that could cause your fan to run without stopping include: A bad fan limit switch that needs to be replaced. Bad thermostat wiring.
Common causes for fans not turning off when they should include faulty thermostats, bad wiring, and stuck relays. These problems are not easy DIY repairs and should be left to an HVAC professional.
The most common cause of a furnace fan running continuously is that the blower setting on your thermostat is set to “on.” This means that the fan will run constantly as long as there's electric power. This will happen regardless of whether or not your furnace is actually heating.
On your thermostat, look for the fan settings. There should be two, “ON” and “AUTO.” If it seems like your furnace won't turn off, the thermostat may be set to “ON” for the blower. Toggle the switch to “AUTO” so your furnace's blower fan only runs during a heat cycle.
Three possible reasons the gas furnace is blowing cold air and won't shut off is the gas supply is not feeding the furnace, or it could simply be shut off. A third reason could be due to a blockage in the gas line your furnace is not receiving an adequate supply of gas.
First, check the thermostat to ensure it is set correctly and functioning properly. Replace batteries if it's battery-operated, and make sure it's set to 'heat' and not 'fan' mode. Next, inspect the air filter; a dirty filter can cause airflow issues that make the furnace run continuously.
Two problems that could cause your fan to run without stopping include: A bad fan limit switch that needs to be replaced. Bad thermostat wiring.
The cause of your furnace not blowing hot air could be the thermostat configuration, a dirty air filter, the high limit switch, a clogged condensate line, insufficient fuel, leaky ducts, ignition problems, a blocked burner, flame sensor issues, gas valve malfunctions, a cracked heat exchanger, or a clogged coil.
Running it continuously may lead to more frequent maintenance and repair needs. However, the added comfort and air quality benefits often outweigh these concerns. Noise: If you're sensitive to background noise, you might find the constant hum of the furnace fan bothersome.
Fan limit switches work by turning on and off based on the temperature inside your furnace, but a faulty switch could cause it to run continuously instead of on and off when temperatures are cool in your house. If this is happening to you, this could indicate a bad fan limit switch in play.
Wrong Thermostat Setting
If your fan won't shut off, the first thing you should do is check the thermostat to make sure the fan is set to AUTO. On this setting, the fan will only run when your furnace or AC unit is on. If you have the fan set to ON instead, it will run continuously.
The fan limit switch controls when the furnace blower fan turns on and off. This control also provides a safety limit that turns off the burner or furnace if temperatures at the furnace get too high. Fan/Limit switches are used on all types of heating furnaces.
Engine Temperature and Cooling
In many modern cars, the engine control unit (ECU) monitors the temperature, and if it detects that the engine is still too hot after shutting down, the fan will stay on until the temperature lowers to a safe level.
Problem 1: Incorrect thermostat settings The most common reason for a furnace fan that won't shut off or a furnace blowing cold air and won't shut off is the wrong thermostat settings. The solution: Check the thermostat settings. The fan setting should be on AUTO. If it's set to ON, the furnace fan won't shut off.
First, turn off the power supply to the furnace by turning off the breaker in the circuit box. Locate the reset button on the furnace and press it to reset the furnace. It is usually located inside the blower compartment on the side of the blower motor. Be cautious, as the housing may be hot.
Less frequent starts and stops of the fan can reduce the stress from startup, and potentially help extend it's lifespan. Leaving the fan on 24/7 ensures cleaner air, as the air is pulled through the filtration or UV light system. This can be better for people with allergies.
One of the most common issues is the thermostat. The wiring, switches, or heating sensors can fail on your unit. If this is the case, then you'll need to replace the thermostat with a newer model. A damaged primary controller can also cause the furnace to stay turned on.
Typically, you'll find it inside the air handler, either next to or above your furnace. In most newer systems, the button is often directly next to the blower motor, while in older systems, it may be harder to spot as it is sometimes underneath or behind the motor.
Mind the safety lock. When resetting your furnace, make sure to press the button only once to avoid engaging the safety lock. Reach out to professionals. In most cases, the safest course of action is to contact your go-to HVAC company if your furnace doesn't work after a power outage.
Typically, there's a 10 to 15 minutes allowance for this. However, if after this time it's still producing cold air, then there's an underlying problem that needs to be addressed. It would be wise to turn the furnace off and get in touch with a pro immediately for help dealing with the issue.