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DanD 11-10-2008 03:25 PM

One thermostat, 2 furnace?
Hereís my situation; I have 2000sq feet of open (no dividing walls) garage/warehouse with two natural gas, forced air, ceiling mounted, unit heaters.
One I believe is a 60,000 BTU Reznor and the other is 100,000 BTU Sterling. Both units use a low voltage, 24 Volt thermostat; one for each unit.
Both thermostats are from LUX and are/seem to be working fine; they are mounted side by side, on an interior cement block wall with a block of wood between the stats and wall.
What I would like to know is, how do I or if I can use just one thermostat, to control both furnaces?
Iím forever playing with the temperature settings, trying to get both furnaces to come on relatively close together.
Iíve tried two different sets of thermostats (different manufactures) with the same results; one furnace will come on well before the other; one time itís the smaller unit and then itís the larger that will start up first.
Iíve tried offsetting the temperatures but then one furnace will run forever (well it seems like) and the other will maybe not even come on or run for a much shorter time.
I understand that the blower fans of the two different furnaces will likely not shut off at the same time because they are different makes; plus the fans thermal couplers will likely cool at different rates because of their size difference.
I would at least like to hear both power vents come on at the same time, with the burners lighting shortly afterwards.

If Iím there at the thermostats and hear the one furnace start up and I then turn the other up, making it come on. The warm up time of the garage is less then half; compared to when I let them play hopscotch.



Yoyizit 11-10-2008 03:51 PM

Are the 'stats identical?

beenthere 11-10-2008 03:54 PM

Either have to add a relay system, or convert them to both use the same transformer.

Keep in mind. The shorted run time, is not using less fuel, then the longer run time.

DanD 11-10-2008 04:31 PM

Yes both stats are identical; I can even change the control heads back and forth between the two. When I first moved into this building, the stats were of the old mercury tube and by-metallic spring style and I tried switching these stats between the furnaces, ending with the same result. The problem/issue didnít follow or change with the stats.

Iíve look all over for a wiring diagram showing how to install a relay system and I havenít been able to find anything. I found a couple, showing two stats controlling one furnace but not the other way around.

The one transformer sounds interesting; would you be willing to explain how I could go about doing this (wiring diagram). Iím an automotive mechanic; so reading an accurate diagram isnít an issue. Well most times itís not. LOL

The fuel & electric isnít my concern with this; but having one furnace shut off, for the other to come on or having just one running, for what seems like forever; is driving me crazy.

I just canít believe there isnít a thermostat manufacturer out there that doesnít offer a stat that is designed to do this?


beenthere 11-10-2008 05:35 PM

1 Attachment(s)
Not an accurate diagram. More of a concept.

Just remove the transformer from one of the units.

And run a jumper from the transformers low voltage terminals to wires that the other one was connected to.

Along with a jumper from the R and W terminals from one furnace to the other.

Yoyizit 11-10-2008 06:36 PM

I think you can just parallel the two 'stats as far as commands given from the 'stats to the furnace, but there may be a problem with the phasing of the 24v xformers that each furnace has.
If they're phased incorrectly you'll get zero volts to the 'stat. And if you leave one disconnected the other xformer has to supply both furnaces.

But if the 'stats display or accept signals from the furnace, let's say trouble codes, I wouldn't parallel them without additional circuitry.
My 'stat only sends signals to the furnace.

So, what can you tell us about what functions the 'stats perform?

Can you post schematics for each furnace as far as the 'stat inputs and the 24v transformers?

Black Adder 11-10-2008 11:34 PM

Why not use one tsat with a relay for the control of the other unit? 24 volts comes through the tstat goes to a relay to switch the 24v on the other unit then back to the first unit. Fires both furnaces at the same time..

DanD 11-11-2008 03:59 AM

Is this what you mean by installing a relay?
If so, would I have to be concerned with the relay's pull-in coil's current draw?
As in using an automotive relay for a fuel pump; it uses less then an amp for the pull-in?
One last question, the 24 volts from the transformer is it AC or DC voltage?
Thanks again

Just had another look at this diagram and its not going to work the relay would always be energized. LOL

8 Ball 11-11-2008 05:01 AM

Try separating and moving the stats closer to the heaters. If one area is too cold now, by tying both units together, one area may be too hot when your done.

Separating and moving the stats is a lot easier than twinning, and the heat accross the entire area will be much more even and efficient.

Drop a 2ft piece of conduit from the handy box on the furnace mount a 2x4 handy box on the end of it, then mount your stat on the box.

If one heater is 60,000btu and the other is 100,000btu, the sizing reflects two different and dramatic loads in a 2,000sqft space, probably a garage door somewhere?

Are the heaters mounted back to back, or in separate locations?

DanD 11-11-2008 06:26 AM

The thermostats were originally on opposite sides (longer walls) of the rectangular garage; one on an outside wall (mounted on a block of wood). With them there the furnaces were taking turns coming on; one would finish its cycle and then the other would start, within minutes. One or the other furnace always seeded to be running; with different run times.
With the stats together they are turning on closer together but still not in sinc.
I guess what I want out of all this; is that both come on, make there noise and then shut off. Now it seems as though, Iím always listening to one furnace or the other
The furnaces are also on opposite sides of the garage (above where the stats were), both are facing the garage door; one (100,000) about half way the long length of the shop and the other 3/5s the way back from the door. I canít do anything about the placement of the furnaces; itís a leased building and the landlord wonít hear of it.
There doesnít seem to be any real cold spots in the shop; it feels pretty equal. I also have 5 paddle fans running at very low speeds, hopefully pulling the heat off of the 20 foot high ceilings?

kennzz05 11-11-2008 10:45 PM

you need a double throw single pole relay, pick one unit to be the master split red from that furnace one side goes to relay coil and one side goes to open set of points (in other words the same wire at both coil and points 2 reds in one spade term than to coil with other red) wish i could draw it but...
ok on the other side of those points the white wire gets hooked
now you will have to run a wire from the furnace, unless theres more than 2 wire. to the common term at the transformer that will go strait to the other side of the coil on the relay
you should have 2 unused terminals left on the relay that are switched the same as the previous set. on these you will put the two wires that go to the other tstat
which should be red and white so when relay kicks in power is sent thru both sets of points and both furnaces will fire good luck

beenthere 11-11-2008 11:42 PM

Pulling one transformer, and using the other to power and control both furnaces is quicker and easier then adding a relay.

kennzz05 11-11-2008 11:58 PM


Originally Posted by beenthere (Post 183693)
Pulling one transformer, and using the other to power and control both furnaces is quicker and easier then adding a relay.

except my way if one xformer goes he still can have heat might have to do a little rewireing if its the control one that blows but basicly were splittin hairs it can be done either way and both ways are correct

beenthere 11-12-2008 01:19 AM


Originally Posted by kennzz05 (Post 183698)
except my way if one xformer goes he still can have heat might have to do a little rewireing if its the control one that blows but basicly were splittin hairs it can be done either way and both ways are correct


My way, he just puts the saved transformer he removed in for the one that went out, and has his heat back.
Since either way he would need a transformer. My way saves more money.

DanD 11-12-2008 05:50 AM

Thanks guys.
I'll try out these suggestions and get back to you, on how things worked out.
The garage has got busy here and I'll have to wait for the weekend to play with this. Got to make some money fixing my customer's cars; so when I burn out one of these transformers, I'll have the money to buy a new one or two. LOL
Thanks again.


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