Nest Thermostat wiring question--two thermos into one

Hi there!
I recently installed a Nest learning thermostat and in the process combined two thermostats into one. The original thermostats were each exclusive to their function ie. one controlled heat and one controlled AC. We have baseboard via heat pump for heat (Viessmann Vitodens100) and an over 20 year old American Standard air handler (TWE030C140) that is used just for central AC with an AC condenser outside.

When I connect both heat and AC wires into the Nest, the RH is not recognized. Current wiring has white to W, red to RH for heat, and red to RC, green to G and white to Y. My understanding is that this issue may be related to needing a common wire/more power. When I looked into the wiring in the air handler, there is no C terminal. The connections I have into the air handler wire panel are red to R, red to B, green to G. I have 2 pics and 2 videos here https://photos.app.goo.gl/KWljsqkobgJJ4EH42 of the wires and a wiring diagram that I found with our handler but I am not sure if this is the configuration of our unit. I read some different strategies to get a common wire online like move the G wire on air handler to B (doubling up on wiring at the B) and connect to C on thermo. Or i could add a new 18 wire and connect it from B to C? I'm not sure if this will resolve the issue we're having with the RH not registering. I don't want to break anything...Any help or advice you can give would be appreciated!

Thanks so much!
Ashley

 

Yes the B at your air handler is a common which would be C at the Nest.

Does the a/c work? Sometimes with Nest you tell it to ignore errors like the one you have.

 

Thanks so much for your reply. Would I rewire the G to the B/C or run a new wire from the B to the C? The AC works but the thermostat ran out of power once and disconnected from the wifi the other day so I bet the common will fix that. The heat didn't seem to be working when the AC was connected so I've just been connecting one system at a time until I can fix this.

Once I hear from you I'll make the wiring change and see if it fixes the RH issue. Thanks!

 

Running a new cable would be the best long term solution. Short term/proof of concept yes, move green wire to C/B and jumper G to Y at air handler. Some of your pics show a wire on Y1 but others don't.

 

 

The picture is upside down compared to the video. Sorry, I didn't notice this earlier and got things reversed.

Yes, Move the green G to B on the air handler and green G to C on Nest. Then take the two whites that are spliced under the wire nut (one goes to Y on Nest the other goes to your outside a/c compressor) and move both to the G terminal on the air handler.

 

 

Good luck. Make sure to cut power (circuit breakers) first to avoid any issues.