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Discussion Starter #1
Hi earlier I found help regarding wiring for 2 furnaces using 1 thermostat but lost the thread.
With the scan shown below, I would like to be referred to the thread about this or better yet,
the member that posted it as it looks like what I want to do and he/she sure could help me.
Let me know if you are the one or know who posted this helpful drawing.
Thanks !
 

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Just use the search.

Are your furnaces by any chance twinned - on the same duct system?
Are they heating different spaces? (...if so, why would you want to do this)
 

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Discussion Starter #3
Hi & thanks. I didn't know the exact search terms so went through 25 pages and did not find.
The two furnaces are identical standing side by side and are ducted together.... one being a back up.

I want to use one of two possible methods of connecting the common thermostat to one furnace at a time.

I am sure they have bonded grounds since the same metal ductwork is shared which is what grounding screws
basically are tied to. My concern is isolating the "common" portion of the furnace electrical / electronics. The common
is not same as ground just to make that clear. I've attached a drawing.... not like your professionally done beautiful
artwork. I'm showing all W, Y, G from each furnace tied together.

For the switching between the two furnaces (only one at a time gets AC power to it) I've set up this DPDT switch
which selects which furnace common and which "R" or "hot" side of transformer is selected. I believe this should
work and also makes their common and "hot" isolated from each other so as to not worry about phasing or other
connections. I assume if the commons were at the same potential, you could tie them together and not need a
switch for that part but guess it would be safer to keep them separated.

Hope this answers your questions and would appreciate your response as well as others that may have knowledge on this.
Thanks again!
 

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As is, do both furnace blowers come on when only one calls for heat?
When the units are twinned on the same supply ductwork, this is a must or supply air leaks through the unit that isn't on and back into the return.

Manufacturers have specific instructions for twinning them.

Why do you have two furnaces when one is just a backup?
It's very odd as it raises the cost.
Usually two units are put in when one doesn't have enough capacity for the space.
By backup, do you mean a second stage of heat?


I definitely would not tie G, Y, W together like that. I would have the power to the stat coming from one furnace only and use isolation relays.
 

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Discussion Starter #5
There is a long history of the heating/cooling system at our 125 + year old church. Various contractors have serviced it as some have gone out of business or are no longer here.
A basement was dug several years ago necessatating another furnace for that area which is probably why we have two furnaces there. Along the way a changes were made (different folks ideas and different men "in charge". ) Professionals were always consulted and did any major changes. At some point it must have been found that a single furnace could handle both upstairs and basement which is maybe why we ended up with two. Again, this has been working without any known issues for decades. Just the battery issue.

BTW I didn't know you had replied till now because there was no email received that you had posted even though your earlier post created and email message to me..... seems like I have the box checked that requests to be notified when a new post is available so don't know why this happened.

I don't think these furnaces are "twinned". Currently the Y,W,G are tied together with no problem.......again my goal is to get rid of the battery because someone always drops the ball on replacing it and we end up with it being cold Sunday morning. Backup in this case means if Sunday morning it is found the one furnace is not working, there is not time to get a contractor out and try to fix it..... so the second one (back up) can be switched on instantly and be comfortable in 15-20 minutes and allowing people to stay and worship. Has been a great option more than once.

Sooo my wiring proposal seems the simplest, cheapest and most practical solution. Nothing will really change except switching the 24vac from one transformer to the other when changing the furnace being used and doing the same with the "common" connection. Whichever furnace is not used will have it's 120vac supply turned off so I can't see any negative effects or interactions..... no isolation issue.

Hopefully this still makes sense and sorry if I have been poor at the explanations.
 

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Discussion Starter #6
........Just have to add this information, I discovered another possible solution to my problem of eliminating the battery (2x1.5 = 3v) . On Ebay they are showing using a simple AC adapter which is providing 24vac with it being plugged into a wall outlet.

It shows one wire connected to "R" and the other to "C"..... would that work. Only issue is finding an AC outlet available then could run the 24vac line up the wall with the thermostat wiring and tie in while keeping the wire out of sight ?

Any thoughts? Here I'm going through all kinds of trouble thinking and planning how to accomplish a solution when it is now right in my face !!!!! Duh
See the Ebay page: 24 Volt C-Wire AC Adapter Transformer for Thermostat 25 Ft For Nest, Ecobee 24v 744759308757 | eBay
 

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If you have central A/C on those furnaces. Then that ebay diagram won't work.
 

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Discussion Starter #8
Well there is AC on one of the furnaces. Could you explain why that external 24vac won't work? I'm not sure why and is there any option similar.
Why couldn't an AC adapter delivering 3 volts dc be tied into the circuit in stead of the batteries by wiring it in similar fashion?
Thanks...
 

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A. It takes 24 volts AC to the R or RC terminal, not 3.

B. The RC terminal needs to get 24 volts from the same source that will be powering the A/C.
 

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Discussion Starter #10
A. Regarding this I would not connect 3 volts DC to R terminal. I would have to open up the circuitry to the point of where the battery connects and then apply the
external 3 volts to those points.... that is what is powering the processor to deliver the commands so is independant of the 24vac..... this is an assumption at this
point as I haven't tried to get into the stat to see if that is practical.

B. The unit that is running in the summer for the AC will be the source of the 24vac (otherwise the entire "furnace" with AC would not be functioning. So I'm not clear
if you were thinking along another line as it seems to be right to me but I can be quite a meathead at times, ha.
 

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Well, I think your making this more difficult then it is.

I'd just put a 50 or 75 VA in one of the furnaces, and disconnect the other transformer. Then mirror the wire connections from one furnace to the other.

Then one thermostat controls both furnaces.
 

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Can you post some pictures?

I'm unclear of if these furnaces are actually twinned - meaning two furnaces on one duct system. don't know if if there are dampers to prevent supply air from leaking into the return through the inactive furnace.

You said both furnaces are on the same duct system -> is that inaccurate?

The electrical part is straight forward - correct isolating relay and a toggle switch.

But I'm not making any definitive recommendations, not being familiar with your setup.

If you're doing all this to avoid having two sets of t-stat batteries to change don't bother -> the batteries only need to be changed every 1 to 2 years.
 
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