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Old 02-20-2020, 08:43 PM   #1
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Getting different Voltages at Thermostat


I have been trying to troubleshoot my HVAC and it's been very interesting but I've hit a wall. Maybe this is an easy issue before I spend hundreds to have a guy come out. So, here we go...
- It started with a blown 3v fuse. I changed that and it hasn't blown again.
- Now I've tested the transformer and I'm getting a steady 29v off the low voltage side of the transformer so that seems good.
- my wires at the HVAC unit are all getting 28.5v at the point where they're spliced to the wires running to my thermostat.
- The red light is on on the control board so the board is getting power.
Things get screwy at the thermostat wiring...
- R-C at the Thermostat is getting 20.8v
- R-G at the Thermostat is getting 15.3v
- R-Y1 at the thermostat is getting 28.5v
- R-W1 at the thermostat is getting 28.5v

All the wires run through the same bundle and come from the same place, right? How can I be getting these screwy voltages?
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Old 02-20-2020, 09:36 PM   #2
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Re: Getting different Voltages at Thermostat


sounds like you have a thermostat wire problem.
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Old 02-20-2020, 11:25 PM   #3
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Re: Getting different Voltages at Thermostat


It could also be a problem in the sub-base. Disconnect the wires from the base and measure again.

I would suspect the coil is shorted on the condenser unit. An amprobe can check that without disconnecting any wires. Lacking that, use the 10 amp function of your multimeter.

I've had that coil to go and it always takes the fuse out instantly. Never had one to fail with less than a dead short.

Last edited by surferdude2; 02-20-2020 at 11:40 PM.
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Old 02-20-2020, 11:26 PM   #4
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Re: Getting different Voltages at Thermostat


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sounds like you have a thermostat wire problem.
Yes, I'm working through different tests I can do to figure it all out. With the Power off I tested if there was a closed circuit between any of the wires. I found that there is a closed circuit between Rc and Y1. Tomorrow I'm going to disconnect the thermostat wires at the HVAC unit and test to see if the short is on the HVAC wiring or the circuit board.
I'm still trying to figure out why the Rc-Y1 and Rc-W1 are at 28.5 while the Rc-G and Rc-C are messed up. I think it must have something to do with Rc being shorted to Y1 but if that's the case, why would it be fine with W1 and low with the others. Probably something Sciency.
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Old 02-20-2020, 11:40 PM   #5
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Re: Getting different Voltages at Thermostat


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It could also be a problem in the sub-base. Disconnect the wires from the base and measure again.
Sorry, I'm a complete rookie. What is a Sub-base? I do plan to disconnect the Thermostat wires from the Wires coming from the Circuit board tomorrow to see if/where the short is. This will be with the power off. Then, with the wires still disconnected I'm going to turn the power on and test the voltage coming from the Circuit board to make sure it's still 28.5v from Rc to all the rest.

Bright side is I've learned more about HVAC in the last few days than I've known my entire life before this. It's very interesting stuff.
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Old 02-20-2020, 11:41 PM   #6
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Re: Getting different Voltages at Thermostat


if there was a active short the fuse would have blown again. With a high load causing low voltage, you would have seen low voltage right at the transformer.

Disconnect the wires on both ends, connect the R and C wire and ohm out that loop. Should be near zero resistance.

Good to start with that since you got a low reading between R and C.

You got lower voltages through G, Y, W etc because you were testing through the heating/cooling control circuits.

Has anyone by any chance done any picture hanging or anything else involving putting screws/nails into walls?
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Old 02-20-2020, 11:51 PM   #7
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Re: Getting different Voltages at Thermostat


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Originally Posted by user_12345a View Post
if there was a active short the fuse would have blown again. With a high load causing low voltage, you would have seen low voltage right at the transformer.

Disconnect the wires on both ends, connect the R and C wire and ohm out that loop. Should be near zero resistance.

Good to start with that since you got a low reading between R and C.

You got lower voltages through G, Y, W etc because you were testing through the heating/cooling control circuits.

Has anyone by any chance done any picture hanging or anything else involving putting screws/nails into walls?
The thermostat wires run directly from the unit, under the house, and straight up to the thermostat. No walls in between. I am suspecting a mouse or a rat though. When I test for a closed circuit I do get something between Rc and Y. All the rest of them are open when I test them.
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Old 02-20-2020, 11:55 PM   #8
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Re: Getting different Voltages at Thermostat


sounds like you need a new wire.
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Old 02-20-2020, 11:57 PM   #9
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Re: Getting different Voltages at Thermostat


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Originally Posted by surferdude2 View Post
It could also be a problem in the sub-base. Disconnect the wires from the base and measure again.

I would suspect the coil is shorted on the condenser unit. An amprobe can check that without disconnecting any wires. Lacking that, use the 10 amp function of your multimeter.

I've had that coil to go and it always takes the fuse out instantly. Never had one to fail with less than a dead short.
I was hoping that it was the coil but I haven't been able to find it. I've found the transformer, the Contactor, and a capacitor, but I haven't been able to find the condenser coil. I may have to take some more panels off to find it and check the Ohms on it to see if it's bad. I've seen that's one of the first things to check when that fuse blows. It looked a lot easier to find in all the videos that I've seen.
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Old 02-21-2020, 12:38 AM   #10
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Re: Getting different Voltages at Thermostat


condenser coil has nothing to do with this, is not an electrical component.

Did you do the testing showing something between Y and R (not open circuit) with the wires disconnected from the at the furnace/air handler?
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Old 02-21-2020, 07:49 AM   #11
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Re: Getting different Voltages at Thermostat


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I was hoping that it was the coil but I haven't been able to find it. I've found the transformer, the Contactor, and a capacitor, but I haven't been able to find the condenser coil. I may have to take some more panels off to find it and check the Ohms on it to see if it's bad. I've seen that's one of the first things to check when that fuse blows. It looked a lot easier to find in all the videos that I've seen.
I was referring to the coil on the contactor in the condensing unit. It sometimes shorts but as I said, usually a dead short and takes the fuse out instantly.

Now that you have indicated that the thermostat cable goes under your house, I suspect that's where you'll find the trouble. Some varmit has gnawed on the cable and managed to short a couple of the wires together.

Good luck under there, you may have company!
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Old 02-21-2020, 10:42 AM   #12
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Re: Getting different Voltages at Thermostat


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condenser coil has nothing to do with this, is not an electrical component.

Did you do the testing showing something between Y and R (not open circuit) with the wires disconnected from the at the furnace/air handler?
That's my next step today. I'm going to completely disconnect the thermostat wires from the wires coming off the board in the HVAC and verify that there is still a short in the thermostat wires. I also want to test the wires going into the board to make sure that the short isn't in the board somewhere. You've all helped me narrow it down. Now I think I just have to figure out if I really want to go under the house to replace the wire.
I haven't been a very handy guy in the past. This has been a great learning experience and step in the right direction. Thank you again for your help.
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Old 02-21-2020, 11:01 AM   #13
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Re: Getting different Voltages at Thermostat


If you disconnect the wire at both ends, you can purposely tie the conductors together and ring them out with your meter. Or leave them all apart and check for polarity conductor to conductor.
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Old 02-21-2020, 11:15 AM   #14
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Re: Getting different Voltages at Thermostat


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Or leave them all apart and check for polarity conductor to conductor.
*
I think you meant resistance not polarity.
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Old 02-21-2020, 11:37 AM   #15
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Re: Getting different Voltages at Thermostat


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I think you meant resistance not polarity.
Both, really.
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