I have a two story house with two air conditioning units. One for the upstairs and one for the downstairs. I rarely if ever use the upstairs unit because the master bedroom is downstairs and no one lives upstairs.

I had some company in town the other day and when we cut the air on it ran fine. The next day I noticed that the fan was still running on it and it was blowing air but not cold air. I switched the thermostat to the Off position and the fan continued to run. The only way I could get the fan to stop running was to trip the circuit breaker for the unit. I thought that the thermostat may be bad so I switched with another one I had here at the house. When I flipped the breaker back on the fan started right up just as before. I'm not sure how the fan can run like that when the thermostat is set to Off and the fan setting is set to Auto.

I went up in the attic to see if I could find some kind of reset switch on the air handler but I didn't see anything. If anyone has any ideas on what's going on that would be great! This unit is rarely used so I know its not worn out, but maybe that's part of the problem.

Thanks in advance for your help!!

More than likely you have a A/C fan relay that is stuck OR the contacts are burnt together. The relay is activated by 24 volts when you turn on the A/C OR fan switch activation.

Thanks hvac1000! I'm pretty handy with most things around the house but I'm not an hvac technician. Is this something I can replace on my own or are special tools needed? Where exactly is the fan relay located?

The unit is a Comfortmaker Model# FBF050B12A3. MFG# NTG3050FBA3. Serial# L9909 68241.

Heil/ICP/Tempstar/Comfort Maker are all about the same as far as units/parts are concerned. I do not have a diagram of these units handy but if you look on the blower door there should be a wiring diagram there. Many times the fan relay will be located on the circuit board so it could be a expensive proposition. If you have to change the part after you check it to be sure it is defective I recommend you make your own wiring diagram of EXACTLY where the wires will be removed from so they can be put back at the same location on the new part.

As mentioned before I suggest you test the part to be sure it is defective before replacing. Other items like a defective/tripped limit control or a shorted thermostat wire can also cause the problem you mentioned.

I attached a picture of the diagram. The capicator for the blower motor is easy to spot because its pretty big and has two brown wires going to it.

I am thinking that the fan relay is the part on the circuit board that's titled "CIR BLOWER" on the diagram with black and orange wires going to it. Its located on the circuit board and its looks like a very small black box. If this is the fan relay, should I be able to pop the top off the box to see the contacts inside? Thanks again!

Just one more simple test to try before replacing fan relay... try unhooking the thermostat wires from the airhandler.(not from the thermostat, but from the air handler itself). Let us know if the blower turns off now.

I just unhooked all of the thermostat wires from the circuit board in the air handler and flipped the power back on to the unit and the blower is still running.

Does this mean the wires to the thermomstat are fine? I still don't really know how to check the fan relay. Thanks!

When you switch the thermostat to heat, does the exhaust fan come on? Or when you switch the thermostat to AC does the outdoor unit come on.

The reason I am asking is that if your transformer is burned up the fan will run continuously.

The exhaust fan and blower run all the time. They run when the thermostat is switched to off, heat, or cool and the outside unit doesn't come on in any of the settings. The fan is always set to Auto. Ideas?

I just unhooked all of the thermostat wires from the circuit board in the air handler and flipped the power back on to the unit and the blower is still running.

Does this mean the wires to the thermomstat are fine? I still dont really know how to check the fan relay. Thanks!

Yes, this eliminates the thermostat and wires as the problem.

I was under the impression that you just had a air handler. But now it sounds like you have a forced air furnace with ac, correct? If so, you may have a bad high limit, or a flame rollout switch that has tripped. Either of these could cause draft inducer and blower to run nonstop.

You are correct, it is a forced air natural gas furnace with AC. What is the best way to check they high limit and flame rollout switch? Could it still be the fan relay?

Best way to test limit and rollout is to take the two wires off them and connect the wires together. If this solves your problem, don't run unit (other than for trouble shooting) this way. I think for us to help any further, we will need a make and model of the furnace. Pics of the inside controls of furnace would help also. Let us know what you find.

A roll out in action. This usually wipes out the parts you are going to test.

Here are some pictures of the Comfort Maker unit, the full wiring diagram, and the model #s. In the picture of the unit you can see three sets of red wires all running to sensors and three sets of blue wires running to sensors. Two sets of the blue wires run to señors on the top and bottom of the blower motor in the back and the other set runs to a sensor right above the exhaust motor. Let me know if you need better pictures of the sensors. Are these the right switches/sensors to be testing? Thanks.

Sounds to me like you either have a bad control board, or a pressure switch stuck closed.

Best way to test limit and rollout is to take the two wires off of them and connect the wires together.

Sorry this was incorrect info. I should have said, the easiest way to check these limits if you don't have a multimeter is to jump the switch. The actuall best way to test the switch is to shut power off to furnace, remove wires from switch(limit) and do a continuity test acress switch to see if switch is open or closed. YOu can do this test on your pressure switch as well. (to see if its stuck closed). Hope this is all making sense.

Your rollout switch has the red wires going to it and is located at top side of burner area, there should be a little reset button in the middle of it, try pusing this button and see if it clicks. The high limit also has red wires going to it, it is located on the heat exchanger wall behind the gas valve. It has the bluish green sticker on it. Your model may also have a limit mounted on the blower fan housing, it should have red wires going to it also if you have one.

I had the same problem with my unit. The fan would not go off unless I turned off the circuit breaker. I replaced the RELAY Switch/board (cost about $15) and that fixed it.

Gentlemen, it looks like I may have the unit up and going again. Mygirlsdad, I did as you instructed and checked the rollout switch resets and sure enough one of them was tripped. You would have never known it by looking at it. Thank you all for your help!!

Glad you got it going. Please let us know if it trips again. Good job.

The fan relay could be stuck,fused/melted, in the closed position. I have the same problem but the relay is visibly melted. My relay goes for about $132 to $150, about half the price of a control board.

How much extra can this cost you a month.. the same thing is happening to us?

I run my blower 24/7/365 for the last 4 years. My electric bill dropped $20 a month when I did this and the house stayed more comfortable from air movement and mixing whole house temps. The blowers are made to run all the time with out damaging it. Its actually better on it to run than all the starts and stops it will do otherwise.

If I run my heater and the blower stays on, will that be safe? I have 4 children that's why Im asking. I see you said you have ran yours for 4 years like that.

Hi cindy. Are you saying you want to run your blower all the time (if so, yes you are perfectly safe doing this) or are you saying your blower is running constantly running even when you don't want it too? If that is the case, there is a problem with the fan control portion of the system, and although it may not be a health hazard, it should be fixed to allow proper operation of the system.

Your rollout switch has the red wires going to it and is located at top side of burner area, there should be a little reset button in the middle of it, try pusing this button and see if it clicks. The high limit also has red wires going to it, it is located on the heat exchanger wall behind the gas valve. It has the bluish green sticker on it. Your model may aslo have a limit mounted on the blower fan housing, it should have red wires going to it also if you have one. I am having a similar problem. But I can't find a rollout switch. Do all units have this?