Wiring thermostat to be shared by 2 indenticlal furnaces

Thread Starter

izon

Joined Mar 17, 2013
217
We have a situation where we are using two identical home furnaces. Don't ask why but think of the second one as a ready to go "back up" unit. There is just one thermostat. When it is necessary to make use of the alternate furnace, AC power is switch off of the not working one and the power is switch on to the back up unit. Then there is a switch box which selects which unit will receive the 24VAC from the transformer by choosing "west" or east" to indicate which furnace is which.
If you are not familiar with thermostat wiring, there is a common method as far as color codes of the wires to know which terminal connects to which terminal on the furnace connecting strip. Starting with red, it is the top side of the 24VAC transformer which the thermostat then connects to the other terminals as called for. White is for heating, yellow is for cooling ( AC compressor), green is for fan. That covers most common units but there is one additional connection which is "common" with contact "C". If that is not connected, they will have an internal battery to supply power for the thermostat.

So the thermostat takes the red wire (24VAC) and connects to let say a heating situation.... connects the 24VAC to the heating module and also the blower or fan for circulating the heat. The voltage path is from the red terminal to the item being supplied with the voltage and then from it to "common". to complete the circuit.
I have found that ground and common are not the same so cannot be connected together. I do not quite understand from the wiring I'm looking at, as to how and what are connected so as not to cause a problem or potential difference if I were to combine the frames for ground or if I were to connect the commons between the furnaces.

My goal is to get rid of the internal batteries so as not to forget to change them which results in no heat if they go dead. With the common at the thermostat connected to the common on the furnace hookup strip, this should complete the circuit to supply power for the thermostat from the 24VAC supply.
All I can see on the thermostat switchover box is what appears to be a double pole double throw switch.... 2 screws on each side with those on same side making or braking with switch action. Switch has all red wires except one also has a blue connected with a red... makes no sense as the blue wire from all cables are tied back and not connected anywhere.
Could anyone offer a schematic of how this can be safely done so I can get rid of the battery issue? So far I see that all whites are tied together, all greens the same, all yellow together ....... only the red wires are connected to the switch... the top of a 24VAC transformer.
Thanks so much !
 

Reloadron

Joined Jan 15, 2015
6,204
Earlier thermostats merely acted to switch 24 VAC to either a heat or cool function and there was no need for a 24 VAC common wire. Newer thermostats have a few flavors, some are powered from the furnace 24 VAC and rely on the common or C wire as they call it while others are powered by their own internal batteries. Things like backlight, WiFi and other functions. The battery powered thermostats do not use the C wire and if it is in the bundle it is normally just tucked back and not used.

The color code scheme usually plays out as follows:

  • Blue or BlackC – Common wire, may be unused by your existing thermostat. Enables continuous power flow from the Red wire.
  • Red – R – 24VAC power from the furnace’s transformer
  • Red – Rc – 24VAC (dedicated to heat call)
  • Red – Rc – 24VAC (dedicated to cooling call)
  • Green – G – Fan
  • White – W – Heat
  • Yellow – Y – Air conditioner
I also do not see why both furnaces could not share the same common of their 24 VAC transformers. The color scheme is not cast into a standard. The above are generally accepted but any creative installer may use any scheme they want.

Our thermostat uses batteries which I change when I change smoke detector batteries. Yes, found out the hard way one winter that when they need batteries they don't work. Actually credit to my wife who actually read the book. When the system went in I was not paying any attention.

Ron
 

Reloadron

Joined Jan 15, 2015
6,204
you must be understanding something im not... what is using the batteries?
The batteries merely run the electronics and things like the display and small relays. The relays do the switching of the 24 VAC for furnace functions like heat, cool, fan. At least that is how mine works.

Ron
 
The batteries merely run the electronics and things like the display and small relays. The relays do the switching of the 24 VAC for furnace functions like heat, cool, fan. At least that is how mine works.

Ron
that what i thought but other guy confused me. so he should be able to put a resister in depending what batteries were in it to power up the thermostat and connect to the common and 24VAC
 

Thread Starter

izon

Joined Mar 17, 2013
217
Reloadron you have it exactly right. But to selecting which furnace will be responding to the thermostat, that would be wiring a 2 position switch which basically selects the appropriate 24vac transformer high side as the "commons" or low sides should be able to be tied together?..... do you agree? This way I can connect the thermostat common terminal to a (blue) wire in this case which is tied together at the two terminal strips of the two furnaces.... is that correct? I don't quite get the switch they are currently using with 4 screw terminals. I would think a switch on the 24vac line from the thermostat would connect to either one or the other 24vac furnace supply thus "energizing" that furnace to function. Am I right? Then why are they using a switch with 4 screws?
Thanks BTW...2 AAA batteries
 

MisterBill2

Joined Jan 23, 2018
9,795
The thermostat "commons" are the other side of the transformer and may be in phase or out of phase, which is why they do not get tied. Likewise the red wires.
My best advice is to spend a few dollars more and have two thermostats, because they are a likely failure item, anyway. That will give you a totally redundant setup and avoid problems.
Batteries will always run down when a furnace fails to provide heat because the heat control relay is battery powered. So for backup heating in a system that does have that common side of the transformer connected, that lead also needs to be switched, So really you ought to have a FOUR pole change over switch, unless there is also a second air conditioning system present, in which case a FIVE pole change over switch would be needed.
But really, two separate thermostats would be the better choice, so that the whole system will be redundant.
But rather than a complex switch, you could use a multi-pin connector and plug the thermostat into which ever furnace is in use at the time.
 
  • Red – R – 24VAC power from the furnace’s transformer
  • Red – Rc – 24VAC (dedicated to heat call)
  • Red – Rc – 24VAC (dedicated to cooling call)
You mean R, Rh and Rc; Rh and Rc allows a separate transformer for cooling, otherwise Rh and Rc are connected together.

Thermostats without a Common, can use batteries or can use a technique called "power stealing". When a contact is open, the thermostat can "steal" power and charge a supercapacitor as long as it doesn't draw enough current to activate the relay.

Let's note something that some thermostats have a programmable options:
1. Furnace controls the fan. Can be either option1 or 2 for heating.
The heater can shut of fan based on temperature,
The thermostst can turn off the fan based on time.
2. Thermostat controls the fan. This is common for AC

If it's not a power stealing design, then between R and C is 24 VAC all the time. There is a caveat though. One side of the flame detector (flame rectification) has to go to the right phase of the transformer, so usually C is connected to chassis.

the furnaces 24V supply in my opinion should be fused or the transformer be of an energy limiting design, Those trasformers will handle a direct short with no ill effects. Transformer are usually 40 VA.

With one transformer, you should be able to parallel the R and C contacts.
Connect W to G at furnace 1 only (no stat): make sure it starts.
remove connection
Connect W to G at furnace 2 only (no stat); make sure it starts,
if it doesn't, you have flip the wires R and C going toward the furnace.

There could be a problem with R and C. it's not one where it's going to blow up. It's just a case where one furnace might not work (flame detection). So, you swap R and C at the furnace.

You can use a mineature 4PDT locking toggle switch ON-none-ON. Your transformer should be able to handle the drain of two stats and you will switch W, G and Y with a simple switch.

When you have stages, you would have things like y1, y2 and w1 and w2 and maybe w3.

I don't know if it's a useful product or not called add-a-wire.


Let's say you ran an extra wire to the thermostat with a bi-color LED in it. the left-over poll flips the LED. You can sneak that out at the top of the stat an no one would notice. green for normal furnace. red for the backup.

1. basically suggesting a separate energy limiting transfomer for the thermostat.
2. Don;t use the ones in the furnaces.
3. the new transfomer has to be phased properly, Either flip the secondary wires or the primary wires.
4. Use a simple 4PDT switch to switch furnaces,
5. Use the extra pole to wire an indicator (somehow).

The only real problem that I can see is that you can't willy nilly flip the switch especially if cooling is involved. Start-up with full head pressure destroys fuse holders.


Something like:
1) Assure mode from thermostat heat-off-cool is OFF
2) Assure fan off and is not running.
3) Cut power to transformer for only a short time (10-20s), throw toggle switch and then thermostat back on
4) Thermostat would think it had a power glitch and would deal with it.
5) Don't do back and forth flips.

In a normal setting and you have a power glitch while the cooling unit is on, the cooling unit would wait the lockout time <5 minutes before commanding Y. It would not let you command Y until a certain time elapsed.

heat:
A power glitch while the burners are on and the fan isn't. Again, give the stat a glitch.
A power glitch when cooling down. gas off, fan is running until temperature is below some value.

You have AC and Heat things that happen behind the scenes as well as power up scenes. Switching systems, you have to provide a power-up condition and you would like to have "powered down" the old furnace before powering up the new on.

You want a key switch to control power to the thermostat, so it can deal with another furnace and power glitches.

==

Don't answer this:
You really don't want to know what I want to do? When the furnace does it's air filter use test, you want an independent damper full open. It's for 15 minutes around 1pm. Otherwise, just that damper is controlled by a thermostat. it would be in a laundry room. When you wash clothes, you want to bump up the heat or cooling just in that room for a few hours. That independent stat would not be able to turn on the furnace or cooling, but it MUST open it's damper during a duct test.
 

MisterBill2

Joined Jan 23, 2018
9,795
Presently I am wondering about the quality of the furnaces that a possible failure is worth that much effort and expense. Years back my dad bought a "high efficiency" furnace and had to have it serviced many times during the warranty period. Several of the failures were of the computer controller module, very expensive if not under warranty. My furnace has suffered one failure in 25 years, which was the blower motor burning out while running at max speed for cooling. Inconvenient but not a disaster. So my question is what brand of furnaces, so that we can avoid a similar situation?
 
Several of the failures were of the computer controller module, very expensive if not under warranty
Got one of those flunky 58MVB furnaces from Carrier. The coil was from Supercoil went south, but the entire AC unit was replaced with a Trane. It's very tough to get the leaves out of it.

I had to put an EMI filter at the furnace because the ECM motor was polluting the power line, It would make automation devices (e.g. X-10) fail and eventually die. The modules would not work while the blower was running, then die eventually. I have the technology now to measure, (scope) power line noise. http://www.gryphon-inc.com/PDF Literature/Power Probe - AC Line Viewer.pdf

Put a 24 VAC Bidirectional TRANSORB on thet power supply ASAP!

It had a backlight failure on the stat. The backlight was always on. I turned the furnace off for like 4 weeks and installed a transorb. Backlight is fixed and has been for a long time.

I do have to replace the thermostat (OOPS, the Infinity Control) because a button doesn't work. Backup is a supercap.
 

Reloadron

Joined Jan 15, 2015
6,204
I should not say any of this. Tempting fate is not a good idea. About 10 or 12 years ago during home renovations we finally added central air and replaced a 30 plus year old furnace. It has performed flawlessly and I am not even sure who made it. A few years back I had to replace the AC start capacitor but that is all. The Thermostat is a Robert Shaw battery powered and two AA cells power it a few years easily but I change them once a year when I do the smoke and CO2 detectors.

Further thought on the thread and thread starter. What you have in place for a backup furnace apparently works well. The only drawback being having to change batteries. I would run with it and just get in the habit of changing batteries once a year rather than open a can of worms for potential problems.

Ron
 
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