3rd Gen Nest Set Up For Gas Heater- Error e77 not found when searching the site

[bferguson]

I'm trying to set up a Nest 3rd Gen on a gas furnace. The previous thermostat was a Honeywell. There are only 3 wires. My furnace was just serviced a few weeks back and they said it should work with a Nest.

The Honeywell was originally set up with older wires (not plastic coated):
Red wire: R
White wire: OB
Blue wire: W2

When I configure the setup on the Nest:
Red wire: RH
White wire: OB
Blue wire: W1 (also tried W2 but had it correct to say to use W1)

It shows I am missing a Y1 and the error message is E77, however when I go to nest.com/e77 and have to retype it into a search box (Why??) it says that error id is invalid.

Photos: https://photos.app.goo.gl/yoNLDaYCpMEjwKBV6

[Patrick_Caezza]

@bferguson 

Post a picture of the original thermostat's wiring if you have it.

[bferguson]

 @Patrick_Caezza 

[Patrick_Caezza]

@bferguson 

Look inside the furnace and see where the thermostat wires are connected.

There is no reason for the O/B wire with a heat-only system.

I wonder if it was for the blower and if it got moved at some point.

[GBD · Accepted solution]

Hi @bferguson here is a link to the Nest Pro installation guide for the learning thermostats, you'll find your error code E77 listed in there. http://support-assets.nest.com/images/pro-faq/Nest-Pro-Installer-Guide.pdf

I also took a minute search for your thermostat model with the numbers on it from your photo and I believe it's a Honeywell T4 Pro programable thermostat. Here is a link to the installation manual for that thermostat, https://digitalassets.resideo.com/damroot/Original/10011/33-00187EFS.pdf?rv=aad05089f92145a08d5059f4.... 

When I took a look at this guide for the T4 pro install, it doesn't match your wiring so I'm not even sure how your system is working at all. According to the Nest and Honeywell manuals, the O/B connection is the reversing valve for a heat pump (switches the heat pump between heat and cool) and that's why you're getting that error on the Nest. It is detecting the wire on O/B but nothing on the Y which is the output for the compressor on a heat pump. In addition, W2 is for 2nd stage (aux) heating to supplement a heat pump in colder temperatures.

Personally, I would check where each of those wires terminates at the furnace because if it's gas furnace as you say, there should be no O/B terminal in it. My guess is that the red/white wire will terminate at a R or Rh terminal, the blue/white should terminate at a W or W1 terminal and the last wire (Can't see the colour in you photo) would terminate at either a G or C terminal. Once you can confirm where each of those wires terminates in the furnace, just wire the Nest to match the furnace and ignore where it's wired in the Honeywell/

Anyway, I hope this information helps you figure it out.

[bferguson]

Thanks, @GBD I'm also not sure how it was working before, but the guide you shared helped. I now have the blue wire connected to G, the white wire to W1, and Red to Rh. Everything is running and heating properly.