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Old 06-01-2013, 01:32 PM  
NAWalton00
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Default New House, New Problems

I found this forum while trying to fix up my new house. I think inspections are really useless...

Anyway, it's been pretty warm out recently and so I turned on my AC for the first time. Sounded like it started but it ended up just being the blower on the furnace. The AC will run when the contactor is pressed down and the thermostat on Rc/Y is reading 24V. However, I cannot confirm a charge of 24V on the wire as it enters the AC unit. My question is what could be causing this issue? A break in the line? A bad thermostat? How do I diagnose?



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Old 06-01-2013, 02:35 PM  
kok328
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Does your system have separate transformers for A/C & Heat?
Can you verify 24VAC at the terminal strips inside the air handler?
Does your system have a condensate pump for the evaporator coils?
Have you confirmed continuity on the low voltage wires that lead from the air handler to the coil for the A/C relay?
A quick check and a reply will help me help you more efficiently.
Let me know what you find and we'll take it from there, post back if you need further clarification on what I'm asking.
P.S.- welcome to the site, we look forward to assisting you with all your DIY needs.



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Old 06-01-2013, 03:35 PM  
NAWalton00
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I guess this is where I am getting stumped. After the 24V leaves the thermostat, does the wiring follow the same path to the air handler in the furnace and then run off from there to the AC outside?

The air handler does have power and kicks on when thermostat demands cooling.

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Old 06-01-2013, 05:57 PM  
kok328
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it all depends on how the installer wired it but, in general yes, the T-stat is the switch to send forward the voltage from the transformer/circuit board. However, there are variables that can effect this circuit and that's why I need you to provide answers to all my questions so I can take the guess work out of it for you.
It's difficult to troubleshoot a system when I can't see it.
Your right in front of it so I need you to be my eyes.

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Old 06-01-2013, 06:10 PM  
NAWalton00
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Thanks for the quick replies. I will need to sit down and see what I can find on the transformers and the air handler.

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Old 06-01-2013, 09:11 PM  
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Shut the power off to your furnace. Get a volt meter and head outside to your a/c condensing unit. Shut off the disconnect and gain access to the electrical. Remove the two wires going to the contactor coil and seperate them so they arent touching each other. Now head to the thermostat, set it to cool, and turn the power back on to your furnace. Head back outside, take your voltmeter and check for voltage across the two wires you removed. You should have 24v. If you do then your problem is in the outside unit, most likely a bad coil on your contactor. If it does not have 24v, then work your way inside. Go to the furnace with your voltmeter, check between terminals Y and C for 24 volts. If you have it there then your problem is in the wire from the furnace to the outdoor unit. If you don't have it then it is a communication problem between the furnace/air handler and the thermostat. Just make sure the wire going from the furnace to the outdoor unit isn't connected to R and Y, it should be connected to Y and C at the furnace. Happy hunting!

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Old 06-15-2013, 11:57 AM  
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Ok sorry about the late response. Finally had time to look into the furnace. Here is what I found (attachment). I cannot seem to find a voltage at all on any of the lines.

image-1894395284.jpg  
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Old 06-15-2013, 12:02 PM  
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At the air handler the wires running outside are connected to Y/C. I cannot find a voltage there when AC is demanded. However, when I turn the heat on I do have 24v at R/Y on the handler.

At the thermostat I have 24v. Using resistance I see it drop when the furnace is turned on between W/RH. When I turn cooling on I do see a loss of resistance at Y/RC.

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Old 06-15-2013, 01:01 PM  
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With the door panel removed from the air handler, a door switch may be open and shutting down the whole system.
Make sure the door switch is closed and check for 24V across "C" & "R" at all times, "C" & "Y" when in A/C mode.
To check the stat, disconnect the yellow wire going to "Y" and bridge/jumper "R" & "Y" together. This will start the A/C cycle.

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Old 06-15-2013, 01:13 PM  
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Thanks. I think that was a problem when I couldn't find one of the voltages. Also one wire was loose. Now I have 24v going to out through wiring to AC. Need to recheck for 24v outside.



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