No AC power at thermostat


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Old 10-16-16, 07:43 PM
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No AC power at thermostat

Hello all. I am new to the forum. I am a general handyman by trade and able to do most anything. This is a personal issue I am having with my HVAC. It is a Goodman Unit. My wife said it started blowing hot air one day while in Cool mode only. I was at work and had her flip the breaker and she said it was still blowing hot air, so I am doubting that was it was doing, since it was the correct breakers. Anyway, now the stat says no ac power. Here is where im confused and sure it is something simple that I dont understand about the units.
On the transformer, I get 120V on the 120V line on the transformer using the ground wire to the chassis for my neg lead. I also get 120V on the common terminal of the transformer using the same negative. Is that supposed to happen on the common? i then have 27V coming off the low voltage terminals of the transformer. If i check voltage on the Red going up to the stat using the chassis as my negative I get 27V.
Broken wire? Bad transformer? Losing ground? i know this has to be something simple. Thanks all
 
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Old 10-16-16, 08:32 PM
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Welcome to the forums.

I also get 120V on the common terminal of the transformer using the same negative.
Are you saying from the 24VAC common terminal to ground you measure 120v. ?
How can that be if it's a jumper between the transformer and ground ??

You haven't given us any info on your system. If I had to guess I'd say you have a heat pump system. The reversing valve in the outside condensor shorted out and blew the 3A fuse on the control board.

You need to measure for 24VAC (27vac is ok) between the R and the C screws terminals on the board in the air handker. If it's not there then check the 3A plastic fuse.
 
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Old 10-17-16, 02:10 PM
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Hi Pete, thanks. Yes is is the split unit with outside condenser. The 120V on the COM side is on the line in side of the transformer on the same side it says i think 120/240 terminal. The Red has 24V to it, where the installer tied the stat wire in; but only if i use a separate negative than the common wire.
Also on the control board I was unable to locate any fuse. I was told before that was probably the problem and no luck. Basically its like it has 24V out but isnt getting the common anywhere for some reason. And if it sounds like a reversing valve caused the problem would it need replaced. If so that is above my pay grade. Thanks again

This is an image of the control board that is in the unit, not my actual board but identical
 
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Last edited by pirateslife73; 10-17-16 at 02:19 PM. Reason: adding image
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Old 10-17-16, 03:26 PM
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The only way 120 vac can be measured on both the line and neutral terminals of the transformer primary (with respect to ground) is if the there is an open in the neutral wire going to the breaker. This can't be your case since you state you have 27 vac across the transformer secondary terminals so your transformer is OK. In some furnace control systems, the 27 vac is floating and not connected to ground. Therefore, any 27 vac measurement must always be referenced to one side of the secondary terminals. If referenced to earth ground, the meter will indicate 0 vac. Hope this helps.
 
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Old 10-17-16, 05:45 PM
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That looks like the EBTDR board found in a 240 volt Goodman air handler. 240 volts is provided by two 120 volt legs. Measuring 120 volts from each leg to ground is expected.

That looks like the older board with no fuse but many now ship with a fuse. Check the fuse if one is present.

Another possible cause for no power at the stat is a tripping float switch. Do you see water in the pan under the indoor coil or two wires running to a fitting in the drain pipe?
 
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Old 10-17-16, 10:07 PM
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Split unit or heat pump ???

It sounds like pictures of your wiring there are in order. http://www.doityourself.com/forum/el...-pictures.html
 
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Old 10-18-16, 06:00 PM
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thanks all. itll probably be Thursday before I can get back into it to check it but will try to get some good pics of the wiring and what is going on.
 
 

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