Google recommended wiring issue

[u/NYSports1985]

Trying to install third Gen Nest thermostat. I contacted Google after the app said my existing wiring needs their assistance. After following their instructions, I turned on my ac/furnace and it turned on, then immediately turned off. It did this a couple more times before I was able to get to my panel to shut down my furnace and prevent any damage. Pictured is my existing wiring, as well as the instructions from Google on how to wire the nest.

I have oil heat, forced air, with one AC condenser. Upstairs is controlled by slave thermostat (currently nest 3rd gen as well). This attempt to install was on my first floor thermostat.

Any idea if I can hook up a nest or am I stuck?

[u/ebusch73]

Yeah, that's not correct. In some cases (like Trane) the B wire is the common, but in your case it looks like your B is actually B (reverses cool to heat) and you also have an O wire (reverses from heat to cool). Most heat pumps use a single wire for O/B, and it configured as one or the other. I don't think Nest supports heat pumps that need individual O and B wires.

[u/NYSports1985 → u/ebusch73]

Okay. I’m surprised Google told me, on two occasions, that my wiring would work. There is an additional wire in the wall, as well as one at the panel on the furnace with an open C terminal, but anytime I put a C wire as an option on nest app it says it won’t work anyway.

[u/ebusch73 → u/NYSports1985]

I think you'd likely need some help from an HVAC tech to see if its possible to reconfigure your system to work with a single OB wire, but that may be an expensive operation as it would likely require a replacement changeover valve.

[u/NYSports1985 → u/ebusch73]

Thanks. I might just stick with a dumb thermostat if that’s the case. Appreciate your input

[u/AStuf]

Having both O and B wires is the issue. Find the zone controller near the furnace which is where the thermostat wires connect. If it is an EWC Ultrazone ST-2E or ST-3E then best to replace the zone controller. If something else then post pictures - sometimes wiring can be fixed.

[u/NYSports1985 → u/AStuf]

It’s an ultrazone. Unsure how to add the pic after the fact, but it sounds like this is above a quick diy solution either way. Thanks for your help

[u/AStuf → u/NYSports1985]

There are a couple of kludgy options. With some small wiring changes you can get your system working in a/c mode using the Nest. Then move a wire in the fall to change to heating mode. EWC used to sell a switch for this but you can use other three way switches.

So today you can:

1. Remove the orange wire from O at the Nest
2. Remove the orange wire from O at the EWC
3. Move the black wire from the B at the EWC to one of the C connections
4. Install a short jumper wire between R1 and O at the EWC

Then to change to heating in the fall the jumper would go between R1 and B instead of O at the EWC.

[u/NYSports1985 → u/AStuf]

Interesting work around. Thanks for taking the time to make this simple to understand what my options are.

[u/AStuf → u/NYSports1985]

Over the years this has come up quite a few times. Going with the manual wiring changes or a 3-way switch works but is a hassle. I've suggested getting some relays to do it automatically but I don't think that anyone has done it.

Most do end up replacing the board with a more modern version. It can be DIY but does require patience and attention to detail as there are a bunch of wires to move.

[u/sdp1981 → u/NYSports1985]

I'd just upload it to imgur and share the link in a comment.

[u/sryan2k1]

Need to see the wiring at the other end. Having both O and B isn't supported but also isn't common.

[u/Yayman123]

I can confirm the Google tech is wrong here... Though some thermostats (cough Trane) label the C Wire as B, this is not the case here so their instructions are wrong. The weird thing is the O and B wires. Normally they're the same wire? I don't think Nest supports this particular setup. The Ecobee compatibility checker mirrors the Nest one usually and also says it's not compatible. I think you'd need to change the controller side of things for it to work.

[u/NYSports1985 → u/Yayman123]

Yeah that seems to be the consensus. Thanks for the input