Connect separate Heat / AC systems to a Nest E

Question

I have oil/hydro zoned heat, and a separate AC system. I have Honeywell ProSeries thermostats that have some 16 wire connectors (apparently designed to handle a nuclear power plant or something). I recently found an offer through my electric company to purchase Nest thermostats dirt cheap and they are the 'E' model. I decide to buy them to get the remote control/app features, but the Nest E has more limited wiring options compared to it's big brother (Y, G, R, W, C, O/B).

With separate systems I have separate wiring (Rc, G, Y for AC / R, W for Heat). I am quite comfortable with general electrical stuff and after reading several of the questions and answers here, I can understand that there are two separate 24v feeds (R & Rc) and since they are coming from separate transformers they should not be combined. The Nest E model doesn't support my system configuration according to their installation app, but it's just a switch, right? I could probably figure out this puzzle, but I'm looking for some confirmation so I don't fry anything.

I was thinking that perhaps I could wire the thermostat so that the transformer from the heater is driving all of the 24v power and use relays at the AC to complete the AC loops (Y/G) while isolating the two 24v sources. Is that sound thinking? I've heard of using a separate transformer for a 'C' connection (I have none now) but it's unclear to me how 24v at C relates to the R and Rc I have now.

If I just put both 24v (red) wires together in the Nest E (with power off of course) and then at the ac I cut the thermostat lines and run them into two relays; the red wire now has 24v coming from the heater so I run R/Y to one relay and hook the AC R/Y lines to the other side, and the same for the R/G combo. Perhaps there is an alternative related to the C line, but not sure what that would be. Is my thinking sound?

Updating with more details and photos: I'm getting closer to making the changes based on my interpretation of the answer and diagram from @jwh20. My AC transformer is 40a and my heat is using dual transformers (unmarked) but lists the overall capacity at 80a. I have limited wiring and cross wiring capabilities as the boiler and control board is in the basement and the AC is in the attic, but it does appear that I have one free line in each.

AC Board:

Heater Board:

The heat board appears to wire the thermostat lines backwards (and I'm surprised this works ... seems that the thermostat doesn't care which is hot and which is neutral). I'll swap the wires on the first four zones to ensure consistent polarity since I'll be coupling the systems together.

Here is my plan:

1. adjust/correct heater thermostat polarity for consistency
2. use third wire from heat thermostat lines to connect to yellow (common) transformer line
3. disconnect 24v source (+/-) from AC board
4. connect 24v+ from heater (red) at thermostat to AC red line (to backfeed 24v power)
5. connect new common 24v (-) line from heater through extra line in AC thermostat bundle to the 24v (-/C) post on the AC board.

??. Thermostat currently works with no 'C' connection, is there a reason to connect C to the thermostat?

Answer 1

To make this work you need a common 24VAC source for both units. Chances are that either of your existing transformers will do the job but check the 24VAC requirements for each and make sure the capacity of the transformer you choose is adequate. If neither is, get a larger one and use it.

You'll also need a C wire which is the "neutral" side of the transformer, wired to your thermostat. Both units will need to share the C wire so you may need to do some wiring.

I drew the following sketch which I think should work for you using the connections you said were available on your two units.