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Getting e72, followed by e102 when turning on heat. 1 Recommended Answer 6 Replies 0 Upvotes
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I installed a Nest this summer when I upgraded my AC to new Carrier system. During the install, I added a C wire from the new Air Handlers to the Nest. The AC works fine, but as we get into some cold weather, the heat isn't working. 

When I enable heat, I get an e72 error. When the heat attempts to turn on I get an e102. The gas furnace clicks on and off, when the Nest attempts to turn the heat on.  To determine the cause, I tested wires (voltage seems fine) and also reinstalled an old thermostat that works just fine (R and W attached). 

Attached are pictures of the Nest. I've also included pictures of the gas furnace. Any thoughts?

All Replies (6)
Hi David,
Thanks for bringing this up. An E72 message means that your Nest thermostat isn't detecting any power wires and an E102 means that it's getting an overcurrent on the W1 wire. I noticed on the 2nd photo that the red wire is connected to W and white wire is connected to the R so it's possible that this has something to do with the message that you're getting. I would suggest doing the following steps to isolate the issue.
  • Turn the breakers off
  • Remove all wires on the cooling side (Y1, RC and C)
  • Swap RH and W1
  • Turn the breakers back on
  • Test heating
Please let us know how it goes.
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Thanks - I hadn't noticed that on the gas furnace before. I'll change the wires and report back.
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I tried switching the wires - it did not fix the issue. I still get the same errors. The old thermostats also still work - I've switched back to an analog thermostat upstairs without issue. Any suggestions or things I can check?
Last edited 10/18/19
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So my results - with VAC set to 250 - were as followed (with pictures). Is this confirming the RH and W1 wires are swapped?

1) Red lead on RH and black lead on W1, resulted in 24v

2) Red lead on RH and black lead on C, resulted in 0v.

3) Black lead on RH and red lead on C, resulted in 24v.
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Hi Dave,
Thanks for the update. I sent you a private message to further assist you.
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