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Old 04-15-2013, 09:33 AM   #1
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1 Thermostat, 2 furnaces


In the middle of some renovations, installed a gas fireplace unit as an alternate heat source. Small 2 level home one heating zone. I have a standard wall switch roughed in as of now but I was wondering if there was a easy way to have the fireplace and existing oil furnace on the one thermostat. I have tested with success wiring both units to a 3 way switch, then to the thermostat. So when the thermo kicks on I can manually switch from fireplace to furnace.

The fireplace is a simple 2 wire connection and the furnace is as well. The thing is the oil furnace should have died 20 years ago...its really old. So thinking ahead when the time comes to replace it, possibly with an A/C system, will the simple 3-way switch setup still work with the modern wiring? It's a forced hot air system with blower. Would a multi zone thermostat be better? Trying to avoid major re-wiring and patching a hole in the wall if the switch won't work.

Thanks

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Old 04-15-2013, 09:36 AM   #2
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1 Thermostat, 2 furnaces


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Originally Posted by xjstever81 View Post
...installed a gas fireplace unit as an alternate heat source.
Would a multi zone thermostat be better?
A fireplace is NOT a central heating appliance and certainly not in the
sense that a furnace is. Don't even try to combine their controls.

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Old 04-15-2013, 11:01 AM   #3
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Originally Posted by TarheelTerp View Post
A fireplace is NOT a central heating appliance and certainly not in the
sense that a furnace is. Don't even try to combine their controls.
Even with the fireplace being furnace rated? An optional thermostat is sold as an accessory.

We use the furnace as the main heat source, when we are not home and overnight. Once the house is up to a comfortable level, like when we get home form work, we maintain heat with the fireplace.

Last edited by xjstever81; 04-15-2013 at 11:11 AM. Reason: additional info
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Old 04-15-2013, 05:20 PM   #4
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1 Thermostat, 2 furnaces


the stat option would be considered a stage of heat as in W1 and W2.if your doing an either or set up go with a separate stat and isolating relay(N.O. contacts and R-W1) if you want to do a 7 day programmable with 4 temp settings per day wake/leave/return/sleep
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Old 04-16-2013, 12:19 PM   #5
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Originally Posted by biggles View Post
the stat option would be considered a stage of heat as in W1 and W2.if your doing an either or set up go with a separate stat and isolating relay(N.O. contacts and R-W1) if you want to do a 7 day programmable with 4 temp settings per day wake/leave/return/sleep
So I'd need a second stat with the relay? Kinda new to HVAC setups so pardon my slowness.

Ideally I'd like to have both the furnace and fireplace hooked up to one thermostat with the ability to switch from one to the other when I wanted. Weather that be programmed or manual switch. The furnace is very old and only has a 2 wire hookup just like the fireplace. Not sure if most are like that or not, this is my first house. I'm trying to plan ahead for when the furnace dies and I get a new one with modern electronics.

Wasn't sure if was worth trying to plan ahead and get the right stat or whatever now, or wait to see what I need with the new furnace. The guys who service it say it can go any time.

Thanks
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Old 04-16-2013, 07:04 PM   #6
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if there is no transformer in the furnace yes you need that for one thing if your controlling a fan to throw the heat off the box then a relay to control that fan.....how many wires do you have from the fireplace...115V...24V? get back i'll tell you how to control the fan..
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Old 04-17-2013, 11:05 AM   #7
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The furnace's control voltage to the thermostat is 24 volts AC. The fireplace contol works off of millivolts .... approximately 300mv. You can not combine the two without damaging very expensive components in the fireplace not to mention a potential for that damage making the fireplace dangerous.

The switch you've installed is only a good idea if the two sets of wires are 100% isolated from one another. If they aren't you run the risk of the above concerns.

I would suggest two thermostats as the simple solution. For the more complicated option you would need to isolate the fireplace from the furnace with a relay circuit using the 24vac from the furnace to power the coil of the relay.

Experiment carefully or it will become expensive in a hurry.
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Old 04-19-2013, 12:28 PM   #8
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The furnace's control voltage to the thermostat is 24 volts AC. The fireplace contol works off of millivolts .... approximately 300mv. You can not combine the two without damaging very expensive components in the fireplace not to mention a potential for that damage making the fireplace dangerous.

The switch you've installed is only a good idea if the two sets of wires are 100% isolated from one another. If they aren't you run the risk of the above concerns.

I would suggest two thermostats as the simple solution. For the more complicated option you would need to isolate the fireplace from the furnace with a relay circuit using the 24vac from the furnace to power the coil of the relay.

Experiment carefully or it will become expensive in a hurry.
Sounds like I can really screw things up if not careful. Might just leave the standard wall switch for now and try to combine the two when it's time for a new furnace. Will probably be getting a new tstat anyway and will have to do a little rewiring.

Thanks for all the info guys.

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