Hi guys,
I know there are other threads about non-stop fans/blower motors, but I am really stumped by mine and havent found a good solution or suggestion.

Out of the blue my furnace won't shut off once the heating cycle is complete, the fan stays on constantly and blows cold air now. What has really puzzled me (and it could make total sense to someone with more knowledge than myself), but I pulled first the red wire from the Tstat from the furnace control board... fan stopped. It also stopped with the green removed and the red back in place. What I dont understand is why it will continue to blow when I remove the wires at the other end--those attached to the t-stat.


I've check everything I can think of.. .tried changing out the t-stat to one that I know works.. tried disconnecting all wires from t-stat... made sure the fan was always set to "auto," cut the power to the furnace for an hour just as a long shot.. checked the circuit board over and all the wiring appears fine, solid,no corrosion, no sign of burned connections etc.

By the process of elimination, I'm thinking it almost has to be the control board at this point. The only problem for me is I'm a college student... I know I'm going to have to call in an hvac specialist, but I want to know if there is anything else to check before I do so. Any advice would be greatly appreciated!

Thanks guys... sorry if some of this seems vague.. im not very knowledgeable about heating and a/c

 

80X is a trade name. To better be able to assist you, please post the model name. Trane normally uses a two letter designator like XE80, XL60, XR80 and etc.

Is the red status light on the control board flashing? If so, the flash code may help determine a possible cause for your issue.

Many times when a blower won't turn off it is due to a stuck relay on the control board. Another possibility is limit switch.

 

thank you for your reply. My appologies on my previous post... it was after midnight and I was at wits end. My furnace is an XR80. The light on the control broad flashes slow and steady with no apparent gaps or pauses in the pattern. I tried lightly tapping on the relays after reading that solution last night but still have a problem. How would I test the limit switch?

 

We still need the model number to help identify the control board number. The model number should be located on a metal tag inside the furnace and begin with something like TDD or TUD. Flashing slow and steady indicates a normal, no call for heat condition. Based on that flash code it does not point to a limit switch problem but more likely a stuck relay on the control board. Once we get the model number someone will be better able to assist with probable cause(s) of your issue.

Here is a link to some information about your XR80 that may be of assistance to you: http://www.blue-phx.com/pdf/Trane%20XR80%20Product.pdf

 

Thanks for the quick reply. My model number is:TUD100C936K4, constructed 08/05. The control board is a white-rodgers model

 

just a note, my control board looks exactly like this:
Trane American Standard Control Board CNT05165 CNT5165 D341396P05 50A65 476 | eBay

 

More than likely the blower relay on the control board is sticking. I have attached a photo of the control board and circled in yellow where the wires for the blower motor should be attached. You should have a blue wire on the terminal marked Heat-H and a white wire on the terminal marked Cir-N. These two terminals are where the control board powers up the blower motor when there is a call for heat. I have circled three black relays in white. One of them is for the blower. I am not sure which one it is because the wiring diagram does not show it. I suggest you shut off power to the unit at the switch and then tap several times on each of those relays with a butter knife. Then, with the thermostat set to OFF, restore power to the furnace and see if the blower has turned off. Then set the thermostat to where you normally set it to check operation. If the blower is then operating correctly it would point to the fact the relay was indeed stuck. If so, you will need to eventually replace the board as it will more than likely stick again. You might get lucky, who know?

 

I followed your advice and tapped each relay in turn and checked the wires.. still nothing. The minute the cover is placed back on the furnace (or I depress the switch which indicates that the cover is on) I hear a short click, then another (strange--as if relays were being activated), then the fan blows.. and blows some more... heating part works totally fine, shuts off when it is supposed to etc.. but the fan carries on..

 

The clicking noise you hear is the relays activating. However, the relay that is in question (blower relay) may not be the one clicking. It is really difficult to tell. These are electro-mechanical relays that control the blower, inducer fan, gas valve and etc. I usually remove the control board and turn it over to look for burnt marks underneath the relays. If that blower relay is actually stuck there is no way (other than turning power off or removing one of the leads to the blower) to get the blower to shut off. In a case like this, HVAC technicians will replace the control board. I have actually successfully replaced a couple of these electro-mechanical relays on these boards to get them to work. However, these boards control flame safety and I would never recommend they be repaired for that reason.

 

good lord. So... this whole thing was caused by a switch in my kitchen pantry which, when flipped activates the furnace fan.. Thank you for your time.. I'm very frustrated, but glad that it didnt cost me anything

 

Wow! That is good news and glad you got it figured out.