I have a 5 year old Frigidaire Model # L1RA 090N-16B furnace. The fan is stuck in the on position. Thermostat is set on auto, yet fan remains on unless furnace is unplugged. Is this a sign that the control board needs to be replaced?

The problem is in the thermostat wiring, likely it is grounding somewhere between the thermostat and the air handler/furnace unit.

Could be a bad blower motor resistor,mounted by the blower.

If your furnace is relatively new, as in 10 years old or less, there should be a field adjustable 'cool down' cycle. On the circuit board there should be a dip switch or series of dip switches with 30, 60,90,120 indications. These numbers represent the length of time the relay on the circuit board allows the fan to run AFTER the thermostat satisfies. You may be able to adjust the time frame down one position from where its positioned now.


The second possibility, if the blower never shuts off at all is that the relay itself could be stuck in the closed position and keeping the fan on.

Janet, the problem most likely is not in the thermostat. It is probably the fan relay on the circuit board in the furnace. Some furnaces have an individual fan control board, some furnaces have all the controls integrated within one control board. If your furnace is 20 years old or older, it could be the fan/limit control because at that time all furnaces did not come with a control board. If it is much older than 20 years, it is much more likely a stuck fan/limit.

The assumption is that you are talking about the indoor fan....If so, check the thermostat fan 'on-auto' position and make sure it is in the automatic mode. If so, there is a possibility that the fan relay on the fan control is stuck in the closed position. If you have an older electric furnace, the first stage heating element could be stuck closed because the fan comes on with the first stage heat on many of the brands over 10 years old.

As your unit had just 2 wires, Red and White, you had just heat only with no AC and no fan on as described as you have no green circuit wired up at the furnace or thermostat, you can add a relay if not equipped but that would make your unit very old.

http://www.nordyne.com/Literature/7103590.pdf

If the thermostat is set on Fan and not Auto the fan will continue to run. If the fan control is bad or is set at too low of a setting, it will blow cold air after the furnace has shut off. From what I guess of your explanation, your furnace is not producing any heat and the room air blower runs blowing cold air. You may have two issues happening. Make sure your thermostat is calling for heat by setting it to a high setting. Inspect the thermostat to see if it is set at Auto. Go to the heating system and inspect it to see if it is heating. If you can't view any flames carefully touch the exhaust pipe and see if it is hot. If not, you have a no-heat condition. A 23 year old furnace may need some TLC so call a technician you trust. He/she may need to replace the ignitor or thermocouple to get it going. They will also check out why your fan is running when it should not.

Depending on the model of the furnace, the blower is either a 2 or 3 speed. When the furnace operates in heating mode with the thermostat in the auto position, the furnace starts the fan when the furnace reaches a specific temperature or after a set time delay, but it operates the fan in low or med low speed on heat mode, when you put the thermostat fan switch in the "ON" position, the furnace operates the fan in "HI" speed, the same speed used for cooling mode. You may have a blower moter that is starting to short. I would recommend having a professional replace the motor before now before it becomes an emergency in the middle of the night during a cold spell.