No control voltage at condensing unit

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mark32

Senior Member
Location
Currently in NJ
A customer very recently had a small fire outside her home. This fire swept past her a/c condensing units and melted a section of the t-stat wire going to two of the five units. Luckily there was enough slack in both runs allowing me to cut out the damaged part and reconnect it. One of the effected units work, the other doesn't. I didn't have time to really look into this as I wasn't even there for this task and I had to go elsewhere. I did take the access panel off the unit and saw no voltage at the contactor's coil terminals (With the digital t-stat set to cool). In all my years I never had to troubleshoot such a circuit, nor have I had to replace many t-stats, like two in my whole career, so where to start trouble shooting?

I'm going back there tomorrow which leads me to another question: The pipe insulation on the suction line has some visible heat damage and the customer wants me to replace the blemished sections. What's the typical size suction line for a 30 amp residential unit?
 

growler

Senior Member
Location
Atlanta,GA
A customer very recently had a small fire outside her home. This fire swept past her a/c condensing units and melted a section of the t-stat wire going to two of the five units. Luckily there was enough slack in both runs allowing me to cut out the damaged part and reconnect it. One of the effected units work, the other doesn't. I didn't have time to really look into this as I wasn't even there for this task and I had to go elsewhere. I did take the access panel off the unit and saw no voltage at the contactor's coil terminals (With the digital t-stat set to cool). In all my years I never had to troubleshoot such a circuit, nor have I had to replace many t-stats, like two in my whole career, so where to start trouble shooting?

I'm going back there tomorrow which leads me to another question: The pipe insulation on the suction line has some visible heat damage and the customer wants me to replace the blemished sections. What's the typical size suction line for a 30 amp residential unit?

There is a controller board in the air handler that provided this low voltage. If you are lucky it may have just blown a fuse. Inside the door of the air handler is a schematic of the control circuit and most units I think there is a seperate control transofrmer that may be the problem.

I would check the tranformer first and then if there is a fuse on the controller board I would check this . If you don't find it quick call an AC tech.
 

kwired

Electron manager
Location
NE Nebraska
Could have blown the control transformer if it shorted out once the insulation burnt off.
Many newer units do have a control circuit fuse and you maybe just need to replace that.

Typical units will have full control voltage present on control terminals "R" and "C" If you don't have 24 volts between these terminals you either blew control circuit fuse, have an open circuit in the control transformer, or have no input voltage to the transformer for some reason. As lucky said, after the fire bare conductors at the unit likely shorted together and caused an overload condition.


Add: Terminals R and C may only be present at the air handler unit, but they typically are directly connected to the control transformer (R may have a fuse before hitting the transformer)
 

mark32

Senior Member
Location
Currently in NJ
Thank you so much for the help guys. I was really looking forward to fixing this issue but I just got off the phone with the owner. She had an HVAC guy out and he told her the t-stat was set too high so it wouldn't come on. Her English isn't so good, but I believe she said it's now working. I still have to replace that pipe insulation so I'm going over there anyway, we'll see. Thanks again!
 
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