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Hoping I am positing this in the appropriate forum - please see the image below:

https://imgur.com/upWbb6g

Thermostat is a Honeywell TH4110D and furnace is a late 1980's installed Lennox G8D2. What is confusing to me is why the "R" and "Rc" terminals are bridged at the thermostat, and if they should be bridged in this configuration. I ask because I am interested in upgrading to a smart thermostat ("C" is available through an unused wire in the bundle).

https://imgur.com/SNkmxpK

https://imgur.com/a/MHlaNdQ

Any help is appreciated, thanks.

  • What make and model is the new thermostat you plan to install? – ThreePhaseEel Oct 16 '18 at 4:04
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Many thermostats can figure it out on their own these days

Modern "smart" thermostats such as Nests and ecobees have electronics inside them that figures out if they're being powered from Rh, Rc, or both, and handles the situation accordingly (usually giving preference to Rc if power is present both places), which means that they don't need a Rh-Rc jumper.

If your new 'stat does need the jumper, it will come with it

If your new thermostat isn't that smart (i.e. needs a Rh/Rc jumper), it will come with one, similar to the one on your current thermostat. This jumper is needed because the thermostat doesn't have the electronics to figure out which terminal to draw power from on its own, and some situations (such as boilers, wall furnaces, and the likes) require the thermostat to keep the heating side power (Rh/W) rigorously separated from the cooling side power (Rc/G/Y).

  • Wow I didn't know that, very informative. I like how they're implementing precautionary measures for the calls for heat – user70085 Oct 17 '18 at 0:08
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R terminal is your 24volt control power. Rc stands for 24v Cooling.. Rh stands for 24v heating. Some older cooling systems have their own transformer inside the condensing unit. If this is the case then the 24v to stat needs to be separated. if this is NOT the case then you must have a jumper wire between the Rc & Rh to get proper control voltage to both pieces of equipment. Some newer digital T-stats have that separation built in. The stat themselves can determine what voltage is needed. NOT all digital thermostats can do this. Its always recommended to read AND understand the instructions that come with your T-stat.

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How do you know if your thermostat is bad when calling for heat, get a piece of wire and jump R to W, that says I want power to my furnace without the thermostats switch

The thermostats wiring just says R red is power from the furnaces transformer, RC is power from the ac units transformer, W white is a loner until the switch closes for heat then power goes down it into the furnace, Y is also a loner until the thermostat switch closes for cool air, then green is your fan - is it coming from your furnace or your ac unit or both, if you have 2 fans you can hook one up to a light switch and only turn it on when you want a boost

  • I thought your terminology was funny but what you said was accurate. Gotta give the + to 3phase for the most complete answer. – Ed Beal Nov 3 '18 at 15:52

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