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HVAC Guru
HVAC Guru, HVAC TECH
Category: HVAC
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Experience:  15yrs in the HVAC industry with certificates and degrees.
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Carrier won't output 24vdc, now thermostat is blank

Customer Question

carrier won't output 24vdc, now thermostat is blank
Submitted: 1 month ago.
Category: HVAC
Expert:  HVAC Guru replied 1 month ago.

Hello, one minute while I get some information together.

Expert:  HVAC Guru replied 1 month ago.

Hello and thanks for using this service! If you are a JA Member I would like to personally thank you and welcome you back! I am HVAC Guru an extremely experienced tech on the site and I try to get to as many customers as I can. NOTE it takes time for me to type research and add links so please wait for me to respond back I can only do this so fast. Secondly, I do not know your skill level so please try and let me know if I am going beyond your knowledge or comfort zone thanks! I strive for a 5 star rating from everyone especially our JA members.

When you use a volt meter across R and C thermostat terminals on the board do you get 24 volts?

Customer: replied 1 month ago.
No. I am not now getting 24VDC, and there was before, when the thermostat lit up. That's why I don't think it's the thermostat, which must be supplied 24VDC. There is no float switch in this air handler of this model, that I can find, anyway. I have replaced the transformer and the controller board. The drain pipe is dry, appears unobstructed, and has been redone about a year ago to allow more air to escape over its run.
Customer: replied 1 month ago.
This is a Carrier 9200 58mxa060-12. I pulled the hose off of a pressure switch and it was dry, with no water.
Expert:  HVAC Guru replied 1 month ago.

So the LED light on the board is on?

Customer: replied 1 month ago.
The red LED is on steady, no blinking. I have done nothing with the big capacitor on the outside of the blower fan.
Customer: replied 1 month ago.
I have no 24VDC on the controller board itself, the 2nd one that's been in there. The earlier one had an LED on steady. I tried just replacing the transformer first, keeping the old controller board. Then I replaced the controller board with a newer one. Red LED on steady the whole time. There is no 24VDC, even when the thermostat wires are taken off.
Expert:  HVAC Guru replied 1 month ago.

Do you have 24 volts out of the transformer? You may have a power issue to the unit itself.

Customer: replied 1 month ago.
That would 24VAC, off the secondary, right? I didn't check. I need to go back & take it apart, and that might be a while before I can get at it. BTW, I did check the fuse. An power problem upstream would make more sense, from having the same problem with 2 boards and transformers kind of view, but then why does the board light up with the red LED at all?
Expert:  HVAC Guru replied 1 month ago.

I think you have a main power issue to the unit check the supply voltage

Customer: replied 1 month ago.
I pulled off the light switch face plate of the switch on the side of it, and found 121 VAC. It's after midnight here, so I won't be able to follow it downstream tonight. But at least I have a place to start. I well understand that I might have to call someone in. It's all just so puzzling.
Expert:  HVAC Guru replied 1 month ago.

Double check the power on the line side of the transformer and the output side and see what you read

Expert:  HVAC Guru replied 1 month ago.

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Customer: replied 1 month ago.
Here's why I'm unhappy: My original problem was that I couldn't get 24VDC at the thermostat. I shouldn't have been looking for that. I just called a furnace guy in, and he found 24VAC! This was so puzzling, and my goal was bad. Anyways, that was $130 I probably shouldn't have had to spend, because I came here first, and accurately reported I didn't know what to look for. Can you understand?

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