Two Furnace House with one shared AC unit setup question

[DCRyan]

Hello! Currently moved into a new house and have upgraded to some nests. Little background on the setup:

There are 2 furnace units in this house due to the size. The main unit provides heat to the second and third floors while the smaller furnace provides heat to the 1st floor (basement).

There is one 4 tonne AC unit that is plumbed into both furnaces BUT it is only controlled by the main furnace and thermostat. The smaller furnace has a damper that can be open or closed to allow cool AC air to enter into it.

I have no issues with the upstairs nest controller. Everything works as intended.

The issue I have is the basement nest obviously doesnt have the means to control the AC (or in this case the damper to allow AC air to enter the unit)

Is there a way to run this setup on the nest? The stock honeywell thermostat has a "cool" option that obviously just opened the damper and turned on the fan.

Appreciate any feedback!

[Davidl9999]

You should be able to connect the basement Nest thermostat the same way the basement Honeywell thermostat was connected.

It wasn't clear to me if you had a basement Honeywell and if it ran the basement furnace and damper of if you only had one Honeywell thermostat that ran all four components..

Assuming that you had two Honeywell thermostats: basement R would be 24v ac power from the basement furnace and the damper should then be connected between the Y and C (or T), the damper should open when the basement thermostat demands cooling. The basement fan would go the the basement thermostat G. Ideally, you'd have a fifth wire between basement thermostat C and basement furnace C so the Nest wouldn't "power steal" - which can be annoying to some owners.

[GarrettDS]

Hey there, 

Just wanted to jump in here to check to make sure that you saw the response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours. 

Have a great day. 

Garrett DS

[DCRyan]

Hey guys! Sorry for the delayed response. let me clarify a few items as well as provide some pictures:

So we have furnace 1 (F1 in diagram) which is the main larger furnace that feeds the upstair floors. I attached the wiring of the nest currently as it was a direct install from the existing thermostat (ie no jumpers, everything transferred over, etc). This thermostat will control the AC unit turning on and off.

Then we have the smaller furnace (F2) which was not a direct wire in (there was a jumper between R and Rc, no c wire, but there was a spare black wire that i connected to the c on the furnace board and the c terminal) - see picture. Following the google nest app install, it asked to remove the jumper and install the c wire which I did.
Now when I control the F2 Nest, there is only an option for controlling heat. I do not believe there is a way to make it control the damper as there is no "cool" option available.

ive attached pictures of the physical furnace setup showing the damper.

Please let me know if there is any other pictures needed:

 Imgur Link 

In the picture you can see where the AC ducting enters the main furnace and then the secondary furnace with the damper installed inline.

[Davidl9999]

Ryan. A photo of the F2 control board connections would be helpful.

If you jumper the F2 thermostat's Rh to G, do you get the F2 fan running or does the damper open? IMO, what you'll want to do is trace the two wires from the damper motor (which is likely 24v) and see where they connect. My suspicion is that one wire will go to F2 control board Y and the other to F2 Control board C (Assuming there IS an F2 (Y) - if not, there's another way to skin this cat.

However, it's okay if the damper goes between F2 thermostat Y and F2 control board C. That does essentially the same thing. The Nest should then sense that there's "cooling" equipment on Y and the damper should work. Here's one version of the possible circuit.

F2(C) --- (damper) ---- (Y)Nest2(Rh) --- (Rh)F2

Once the damper works, there's a separate issue to resolve - what to do with a cool demand from F2 if F1 is heating? There should be an interlock somewhere to prevent the damper opening unless F1 is cooling (or off), but there may not be. I'll stay on this with you as long as you'd like, but it might be less frustrating to have an HVAC tech show up and just get it done.

Hopefully helping here. Best,

D.

[DCRyan]

Hello!

I went and took a picture of F2 furnace (see below). 

looks like the damper valve is wired to (W) and (C). I’ll have to do some testing doesn’t that mean the valve is activated when it calls for heat it seems? 

https://imgur.com/a/7fvNw0z 

[Davidl9999]

Yes, it means exactly that.

However, I'm wondering if the damper is actually open when the F2 heat is off, permitting F1 to cool the basement, and the damper closes on a F2 heat demand. That makes more sense to me than the damper opening on a F2 heat demand. This also means that F1 could heat the basement on a F1 heat demand, which might not be ideal.

I'm not sure if I read correctly if you had an evaporator coil in both F1 and F2. If so, there should be an interlock to prevent F2 from heating while F1 is cooling. If there's only an evaporator coil in F1, it makes sense that F2 could close the damper if F1 was cooling and F2 needed to heat the basement. Hope this is helping.

[GarrettDS]

Hey there, 

Just wanted to jump in here to check to make sure that you saw our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours. 

Have a great day. 

Garrett DS

[Jake]

Hey all,

I wanted to reach out and let you know that I will be locking the thread at this time. If you have any questions from here, please feel free to create a new post.

Best regards,

Jake

[Hilary]

Hey there, 
Just wanted to jump in here to check to make sure that you saw our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

Have a great day. 

Hilary