How to test short in low voltage Y terminal

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Old 01-03-15, 11:38 PM
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Question How to test short in low voltage Y terminal

I have a York Guardian Single Stage Heat Pump, York Electric MV16 Furnace/Air Handler, and a new Honeywell RTH9580 Wi-Fi Smart Thermostat. I got the heat pump installed this past summer.

Recently, the 3A fuse started blowing out of the blue it seems, as the weather has gotten colder. I replaced it with a 5A and it still blew. I can only use Emergency Heat at this time.

As part of troubleshooting, I wanted to just energize "Y" to try and get the heat pump to function. I disconnected EVERYTHING from the air handler terminals (all low voltage wires from the Thermostat and Heat Pump) and started up the fan (by jumping R to G), all good, fan starts. Then if I touch the Y (yellow) wire from the Heat Pump to R... it blows the fuse immediately. I also did it with the C (Common) lines hooked up. I thought there could be a short in my yellow wire to the Heat Pump, so I switched to an unused wire between the AH and HP and it still blew.

Any ideas on why it'd keep blowing? It seems like it could be a short somewhere. Jumping R and Y should work, to test the Heat Pump compressor, right?

Thanks!
 
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Old 01-04-15, 12:48 AM
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Turn off the breaker to the outdoor unit and indoor unit.
Disconnect the two 24 volt wires to the contactor and get an ohm reading across the contactor coil.

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I'd expect around 12 to 16 ohms.
 
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Old 01-04-15, 12:54 AM
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Did you swap the y wire between the stat and indoor unit or between the indoor unit and outdoor unit?

The latter would be a better test.


Pressure switches are also on the Y circuit. The wire to these pressure switches sometimes rub a short.

 
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Old 01-04-15, 12:27 PM
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Hey Houston204 -

Thanks for the info... so I tested the resistance across the contactor and it was in the ~16 ohm range, I took a picture linked below in the Heat Pump folder, that shows 17 ohm.

To answer your other question, I tried a different wire for Y from the indoor unit to the outdoor unit (switched from yellow to tan) and it shorted out the same way.

I also disconnected the 24v yellow wire from the outdoor unit entirely (removed wire nut) and tested the voltage across it and it went to zero, so I don't suspect a short in it now.

Another bit of troubleshooting, when I test the V across R and C on the indoor unit, its 27 V. Not sure if thats within the acceptable parameters.

Here are pics of all three of my devices if it helps.

http://bit.ly/1IcRMHW

I appreciate your help.
 
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Old 01-04-15, 01:47 PM
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I also disconnected the 24v yellow wire from the outdoor unit entirely (removed wire nut) and tested the voltage across it and it went to zero, so I don't suspect a short in it now
I'm not sure that I understand this statement. Did you have 24 volts until you disconnected it?

Did you check the pressure switch wires for a short?

I would disconnect heat pump Y at the air handler and give the stat a cooling demand to see if the fuse pops.

If it checks out okay, reconnect heat pump Y to the air handler and disconnect Y at the heat pump and give the stat a cooling demand.

If the fuse does not pop it points to a problem inside of the heat pump.
 
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Old 01-04-15, 03:17 PM
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By that I meant, I was testing to see if there was a short (grounded) somewhere along the yellow wire.

So I disconnected the Pressure Switch wires in the outdoor unit, jumpered them, and the compressor comes on now, both for heat and cooling. It's working!

I switched to continuity mode and touched the black wire with one lead, bare metal on the unit with the other lead; got a tone. Did the same with the yellow wire; got a tone. So somewhere along those circuits we must have a short, either bare wire or water I guess. It's odd because this unit is about 6 months old. I am going to search for that now...

Do I have next steps, or at this point, do I need to call the original installer to correct it (hopefully under warranty)? If I need servicing, can I leave the pressure switch circuit jumpered until then (a week)?
 
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Old 01-04-15, 04:05 PM
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You should have at least a 1 year parts and labors warranty.

While I would leave it bypassed, you can void warranty bypassing pressure switches.
 
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Old 01-04-15, 06:15 PM
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So found the problem -- I popped open the top of my heat pump and looked around and it appeared one of the pressure switch lines got a little too close to one of the copper lines and ended up melting the sheath and shorting it out. I took a picture and posted it.

https://drive.google.com/open?id=0B2...bjA&authuser=0

I moved the wire, retested continuity, no tone. So I put some electrical tape on it, zip tied the the lines (and a couple others) so they'll never touch a copper line, removed the jumper and everything is working again.

Thanks for the help!!
 
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Old 01-04-15, 06:40 PM
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I have seen this happen many times.

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I am glad that you were able to locate the short.
The zip ties were a good idea.
 
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