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Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 22001
Experience:  40+ yrs. experience as a licensed oil & gas technician.
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I believe that the fan relay on my furnace controller is

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I believe that the fan relay on my furnace controller is stuck closed. What can I do to confirm that, and is it reasonable to believe that it will open if I tap it?

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

If you disconnected the thermostat leads at the furnace and the furnace is not in an open limit circuit condition then you probably do have a stuck fan relay. Tapping on the relay won't necessarily free it up.

Customer: replied 25 days ago.
What is the next step?

Replace the relay

Customer: replied 25 days ago.
I believe this is the manual: have not been able to determine which relay it is. Is that something that you can help me with?
Customer: replied 25 days ago.
Or should I replace the board?

Give me a few to review the manual.

The manual shows an ECM motor. This muddies the water. Do you have an ECM motor on your unit?

Customer: replied 25 days ago.
I don't know - how can I tell?

Refer to page 19 in the manual. It shows an eight pin molex connector on the fan control board which also has the thermostat terminals strip.

Customer: replied 25 days ago.
I see only text on page 19. It does mention K3 turning on the ECM...
Customer: replied 25 days ago.
OK, was looking at the site, rather than the manual page number. Yes, I see the diagram.
Customer: replied 25 days ago.
the 8 pin molex connector

Ok, now see i you have the same thing on your unit

Customer: replied 25 days ago.
mine has the daughter board, if that's what you mean.

Ok then if your unit has the same board you have an ECM motor. Diagnosing an ECM motor issue is not a DIY project.

Customer: replied 25 days ago.
Hmmm, so I should give up and call a professional?

Given the cost of the components invloved (an ECM control is sevral hundred minimum) and the expertise invleved you're better off leaving this to a pro.

Customer: replied 25 days ago.
Why suspect the motor? Why not suspect that the fan relay is stuck, and the motor is OK?

ECM motors are a different animal. The fan controller is what controls the fan.

Customer: replied 25 days ago.
You are certain that the motor has a problem?

That's what it looks like from here based on what you're telling me.

Customer: replied 25 days ago.
If K3 is the fan relay, perhaps I should just replace it - or just pull it off the board to see if the fan stops. What do you think Rick?

You can't replace a relay on the board.

Customer: replied 25 days ago.
Why not? Or I could replace the board. I think it's ~$160. It would be a shotgun approach, but hiring a pro could be $550+, no?

You're just thowing parts at it. A diagnostic by a pro is typically around $100.

Customer: replied 25 days ago.
How do I find someone I can trust? Do you know vendors in the Boston area?

I can't make a referral. Try a referral service like There is no charge to you for their service. Specify your furnace brand in your service request

Customer: replied 25 days ago.
Thank you for your help Rick!
Customer: replied 25 days ago.
Have a great day!

You're welcome

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