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My Emerson WiFi thermostat stopped connecting but otherwise seemed to be working fine so I followed troubleshooting steps and replaced the batteries with fresh ones.

This failed to solve the problem, so I called Sensi support. They were great and guided me through testing various pins with the multi-meter. The thermostat was not getting power on the C-wire.

This could also be seen in the logs Sensi emailed me:

2021-10-19 20:37:39.000 America/New_York
2021-10-19 20:38:54.000 America/New_York

c_wire
lost_c_wire
battery_only

operational_status_vcap_voltage
30.578 V
6.685 V
0 V

The backlight and WiFi shut down when in battery_only mode - so that caused the symptom I initially noticed.

As a result, even though it was functioning properly (and I could hear the relays clicking) nothing was happening on the furnace side. Namely, it wasn't cycling on to provide heat.

I tried re-setting the furnace by shutting it down per the manual instructions, then powering it on again. That did not restore power to the C-wire.

Since I have American Home Shield (AHS) home warranty, and the manual said not to dig around at the electronics in the cabinet, I submitted a claim to have the furnace checked.

When the tech arrived, I provided the above info, and he proceeded to the basement and opened the furnace cabinet.

He fiddled with wires on the controller board and held the cover switch to bypass the safety and cycle the system manually.

At this point, he told me that it was clearly a bad thermostat - which didn't fit the info I had given him. He told me I was "wrong" and that he'd been doing this for years and was an expert.

When we came upstairs, the display on the thermostat seemed to be lit (problem solved!). But he pulled the face plate off and added the wire shown.

Wiring of Emerson WiFi thermostat wall mount.

He then declared that adding the jumper from RC to RH (shown in the attached image) was the solution to the problem. I checked the C-wire with a multimeter and it indicated that power was restored as well - 27 volts or so.

I asked the tech if he had reset anything in the furnace and he said "No, there is nothing to reset."

He also told me that without the jumper he installed, the heating system could never have worked. I told him that nobody had tampered with the thermostat and that the system worked last winter. His reply was "that's impossible".

So, my question to you all is two-fold:

  1. What did he do in the furnace cabinet to restore power to the C-wire?

  2. What does the jumper from RC to RH do and should I keep it?

Seems like he may have inadvertently tightened a loose connector - or he could have reset something on the furnace board and lied about it for some reason.

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    Extremely unlikely that there is anything he could reset in the furnace - a power cycle would take care of all that anyway. But "may have inadvertently tightened a loose connector" - very possible. Nov 4, 2021 at 15:58
  • @manassehkatz-Moving2Codidact Any thoughts on the jumper? Necessary, harmless, or worthless?
    – Bleak Morn
    Nov 4, 2021 at 18:09
  • 1
    AHS sending out a fool? Say it aint so! /sarcasm
    – dandavis
    Nov 4, 2021 at 19:38
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    @manassehkatz-Moving2Codidact Since Sensi support was so helpful, I'll call and ask them about proper configuration. I just don't want AHS to send the same guy out in 6 months and have him say "Oh, here's the problem - someone put this jumper in here".
    – Bleak Morn
    Nov 5, 2021 at 14:08
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    @dandavis I signed up because I just bought a house and the appliances are all kind of old. Every time I deal with them it is frustrating and time consuming - but on the other hand, the total cost of a washing machine replacement was $100. If you can bird-dog them, seems to be "worth it", but not for everyone.
    – Bleak Morn
    Nov 5, 2021 at 14:10

1 Answer 1

1

According to the Sensi manual, the thermostat has all needed jumper wires built-in. There is no need for additional jumper wires.

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    I don't see how this answers either of the OP's questions listed at the end of his post. Care to edit to elaborate on that?
    – FreeMan
    Nov 8, 2021 at 16:05
  • This is the OP.... In all the years I've been doing this I still don't know what that jumper does.
    – Mazura
    Dec 9, 2021 at 1:09
  • @FreeMan It answers question 2. The wire is unnecessary because the Sensi thermostat has a redundant wire internally already - as explained in the manual. As for Question 1, I doubt we'll ever know. I am assuming that there was a loose wire and he fiddled with it. I will say that I occasionally notice the thermostat reboot - perhaps this is some kind of power interruption from the furnace. I don't know if they normally reset like that but maybe it's an indicator of system trouble.
    – Bleak Morn
    Dec 17, 2021 at 20:26

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