Heating & Air Conditioning Electrical Wiring

Help with wiring Nest on my system

3 years ago

last modified: 3 years ago

Hello all,

I recently tried installing a Nest Learning thermostat with my system, but encountered some incorrect behavior once complete. Wondering if anyone can assist.

Info

My system is a 16VMB-4HW variable speed system.

Link: https://firstco.com/product/vmb-hw-variable-speed/

I called the manufacturer today and they said the system does work with Nest.

System provides heat and cooling.

Problem

After following the Nest instructions on how to wire my system, I was able to get the system working, and the heat would come on, but not turn off. The heat continued pumping out well after it hit the temperature setting, and it would also not turn off if I turned the mode to 'off'. It just kept going until I took off the Nest faceplate.

Pics

Picture of my system.

Picture of the wiring for my old thermostate (Honeywell)

Picture of the wiring for my Nest.

Other

I saw some reddit posts online where people fixed a similar issue by moving the OB wire to W1, but I don't want to do this as I know my system needs to be able to change between heat and cooling, which this connection handles from my understanding.

Any help appreciated and cheers!

Phil

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Comments (7)

wdccruise
3 years ago
Assuming these are the correct installation instructions, figure 1 (page 7) shows a wire to "W", not "O". If you have access to the air handler, you might want to look at the low voltage connections inside the unit to confirm they are the same as those in the instructions.
Lyphe 416
Original Author
3 years ago
I thought about this based on a reddit post that suggested using W insted of O, but my worry is that O is needed to change the valve from heat to cool and vice versa. I'm not educated enough to know if I'm interpreting it correctly, but I know the old wiring was using the O.
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wdccruise
3 years ago
last modified: 3 years ago
Did you look at figure 1 of the installation instructions?
Lyphe 416
Original Author
3 years ago
I did and it definitely seems like I should roll with the W. The only hesitation I have is that my old Honeywell used the O instead of the W.

Might try it tonight though. Will report back if I do ...
Lyphe 416
Original Author
3 years ago
last modified: 3 years ago
Well ...

Went for it. All seems to working aok on the heating side. And based on the manual, as you noted, it certainly seems like it should be the W1 slot. I also took a closer look at my old Honeywell and noticed that the OB and W1 slot are the same ... no idea how I didn't notice that before.

The next test will be to see if it correctly toggles the valve when I swap it to cooling. I may wait for my HVAC guy to be present before I try that ( need him to come in and add a control panel to allow Nest to control my 120v 2nd furnace ).

For now, it looks like all is well. Cheers wdcruise and ty.

EDIT: Adding an edit for anyone that might be reading this with a similar issue. I noted above that the OB and W1 slots were the same, but it should have read, 'I noticed that the OB and W1 slot are on the same line with each other, although each with a unique connector. In short, I misread where my W1 wire was connected and it was in fact in the W1 on my old thermo.'
wdccruise
3 years ago
last modified: 3 years ago
W and O are not the same. W is for heat. O/B is for a heat pumps' reversing valves; in most cases O is powered for cooling and not for heating. For some reason, your Nest is configured for a heat pump and to power O in heat mode. The heat (white) wire should be connected to W1.
Lyphe 416
Original Author
3 years ago
last modified: 3 years ago
Good point on the clarification. It should have read, 'I noticed that the OB and W1 slot are on the same line with each other, although each with a unique connector. In short, I misread where my W1 wire was connected and it was in fact in the W1 on my old thermo.'

Thx again wdcruise.