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Tim H.
Tim H., HVAC Professional
Category: HVAC
Satisfied Customers: 3971
Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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After completely disconnecting the thermostat of our HVAC

Customer Question

After completely disconnecting the thermostat for one of our HVAC zones, the AHU blower continues to cycle on and off every 20 seconds or so. What could be causing this?
Submitted: 1 year ago.
Category: HVAC
Expert:  Tim H. replied 1 year ago.

Hi, I'm Tim and I'm an HVAC expert here to help answer your question(s). Please provide all the information I ask for since I'm not onsite. At the end of our dialog you will be asked to rate my service, but please wait until then and all your question(s) have been answered satisfactorily. Thanks for your business!

I'm sorry to hear about your problem with your unit. I know this can be frustrating. In order to help, I would need the full model of the unit. That way I can pull up a wiring diagram. The blower is controlled in different ways depending on the unit and age. For example, it could be a bad board causing this in some units.

Customer: replied 1 year ago.
Carrier 48+FD007-501 (rooftop package unit)
Expert:  Tim H. replied 1 year ago.

Carrier does not use a "+" in their model number. Could you have read that wrong?

Customer: replied 1 year ago.
I sent one of my techs to the roof to get the exact model number.
Expert:  Tim H. replied 1 year ago.

OK, thanks.

Customer: replied 1 year ago.
Carrier 48TFD007---501--
Expert:  Tim H. replied 1 year ago.

Sounds to me like a bad board. The IFM (indoor fan motor) is controlled by a contactor which is controlled by the board. Have you tried flipping the board over and inspect it for burned spots or damage?

Expert:  Tim H. replied 1 year ago.

Is this a 460 3 phase unit?

Customer: replied 1 year ago.
3 phase, yes. Though I think it is 208/230 VAC
Expert:  Tim H. replied 1 year ago.

can you verify that?

Customer: replied 1 year ago.
All three legs of our circuits here are 110VAC
Customer: replied 1 year ago.
Sorry, 208Y/120 VAC
Expert:  Tim H. replied 1 year ago.

Look on the tag on the unit. It should tell the supply voltage.

Customer: replied 1 year ago.
We will have to go back to the roof. I will also check the control board for scorch marks, etc.
Expert:  Tim H. replied 1 year ago.

OK, inspect the back of the board, too. Thanks.

Customer: replied 1 year ago.
Ok, no obvious scorching and definitely 230 VAC. LED-1 was blinking 8 times per cycle
Expert:  Tim H. replied 1 year ago.

8 flash code means " IGC (board) has sensed internal hardware or software error" This would seem to verify it is a board problem.

Customer: replied 1 year ago.
Is there any chance the problem could be caused by clogged air filters? (i.e., some pressure differential switch?)
Expert:  Tim H. replied 1 year ago.

Nope, not with that flash code.

Expert:  Tim H. replied 1 year ago.

Let me know if you have additional questions. Otherwise, good luck! Thanks for using Just Answer and please don't forget to rate my service as that's how experts get paid (thanks in advance for any bonus left). Otherwise experts work at their own time and expense. Feel free to post back with any additional questions.

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