Thermostat wiring

[jeromedawg]

Hey all,

I'm hoping to pickup an Ecobee3 soon at a highly discounted rate but wanted to figure out the wiring beforehand. My existing thermostat is battery-powered and the installation job is awful (I posted a thread about that a while back and may need to do some patch-work before replacing).

First, I'm not sure what that bent nail is doing in the drywall - there is some speculation that it might be holding the wires in place so they don't slip back through the hole:


The pic of the thermostat isn't very good but these are the terminals that are connected (can't see it from the angle I took the pic but they're etched in lightly): Y, G, W, RH and jumper from RH to RC


And at the furnace, from left to right: W1, R, G, B/C, Y:


The B/C wire is weird - there is a light brown, red and white wire going into that terminal. Also, the Y terminal has a yellow wire AND green wire connected to it (not sure why the green wire is connected).

So does all this wiring look correct?

I followed the wiring out and the light brown wire is twisted together with a dark brown one that terminates at the thermostat but is unconnected/exposed (can't see this in the picture since it's behind the plate) - since that's ultimately connected to "B/C" on the furnace, I'm not sure if this would connect to "O/B" or "Rc" or "C" on the Ecobee (I was just looking at some installation vids).

Otherwise, I'm assuming the Y, W1, G and R wires that are currently connected would map 'as-is' to the Ecobee plate (well, I think R would connect to Rh based on watching the vids... so not sure what Rc would be for)

Anyone have experience with installing this particular thermostat or can recognize if anything is incorrect even with the current setup? Everything works as it currently is (I have a Trane XL80 furnace and central heating and cooling + a fan - it's a 2-stage)

[zolotiyeruki]

Yeah, that all seems pretty normal.  The B/C on the furnace end (with the light brown wire) is the 24AC common wire, which will connect to the C terminal on the ecobee.

Rc and Rh (and R) are the power supplies to the thermostat, one for heating and one for cooling.  For most people, it's a combined system, which is why Rh and Rc are jumpered.  Everything should transfer straight over to the ecobee.

FYI, that second set of wires (red, white, green) attached to the B/C and Y terminals goes out to your A/C compressor, to tell it to turn on.

[jeromedawg]

Yeah, that all seems pretty normal.  The B/C on the furnace end (with the light brown wire) is the 24AC common wire, which will connect to the C terminal on the ecobee.

Rc and Rh (and R) are the power supplies to the thermostat, one for heating and one for cooling.  For most people, it's a combined system, which is why Rh and Rc are jumpered.  Everything should transfer straight over to the ecobee.

FYI, that second set of wires (red, white, green) attached to the B/C and Y terminals goes out to your A/C compressor, to tell it to turn on.

Thanks - yea I figured the light brown (which changes to dark brown from  the wire nut junction to the thermostat), is what would likely go to C but wasn't certain.

What's the diff between the Rc and C in that case?

Also, from poking around and asking on the DIY forum, one person replied not knowing why there are two wires (besides the light brown) hooked up to the B/C going out to the compressor along with the one hooked up to the Y, totalling three wires. It seems like there are normally two wires that go out to the compressor - one on B/C and one on Y. He was speculating the second wire out of B/C might have gotten broken or something, or perhaps is a spare in case the other goes bad? I went out to the compressor and removed the wiring cover to try to trace everything back but all I see are large-gauge wires. I'm not sure where the wires from the furnace terminate and connect to the compressor...

[zolotiyeruki]

The Rc is just like the Rh, just for cooling instead of heat.  You see, if you have a separate cooling system and heating system (say, radiant heat and forced air A/C), each one will feed the thermostat from its own 24VAC transformer.  In the summer, the thermostat connects the Rc to the Y to tell the A/C to turn on, and in the winter, the thermostat connects the Rh to the W to turn on the heat.  Since many homes have a combined forced air furnace/airconditioning, there's just one system feeding the 24VAC to the thermostat.  And that same feed goes to both the Rh and Rc.

[jeromedawg]

The Rc is just like the Rh, just for cooling instead of heat.  You see, if you have a separate cooling system and heating system (say, radiant heat and forced air A/C), each one will feed the thermostat from its own 24VAC transformer.  In the summer, the thermostat connects the Rc to the Y to tell the A/C to turn on, and in the winter, the thermostat connects the Rh to the W to turn on the heat.  Since many homes have a combined forced air furnace/airconditioning, there's just one system feeding the 24VAC to the thermostat.  And that same feed goes to both the Rh and Rc.

What would happen if I wired the C wire to Rc instead then? Which wiring eliminates the need for battery-power? I'm still waiting for my Ohmconnect approval to go through before pulling the trigger on the Ecobee (a supposed $150 discount on the unit which is $269 on their site). It's either that or I wait until my local power company adds Ecobee to their discount program (which they claim is coming soon) where I can get $125 back... what I'd be interested to know is if I'd be able to get both the Ohmconnect discount AND utility discount - most likely not.

[zolotiyeruki]

The Rc is just like the Rh, just for cooling instead of heat.  You see, if you have a separate cooling system and heating system (say, radiant heat and forced air A/C), each one will feed the thermostat from its own 24VAC transformer.  In the summer, the thermostat connects the Rc to the Y to tell the A/C to turn on, and in the winter, the thermostat connects the Rh to the W to turn on the heat.  Since many homes have a combined forced air furnace/airconditioning, there's just one system feeding the 24VAC to the thermostat.  And that same feed goes to both the Rh and Rc.

What would happen if I wired the C wire to Rc instead then? Which wiring eliminates the need for battery-power? I'm still waiting for my Ohmconnect approval to go through before pulling the trigger on the Ecobee (a supposed $150 discount on the unit which is $269 on their site). It's either that or I wait until my local power company adds Ecobee to their discount program (which they claim is coming soon) where I can get $125 back... what I'd be interested to know is if I'd be able to get both the Ohmconnect discount AND utility discount - most likely not.

Wiring C to Rc could very well cause a short and Bad Things.  Rh and Rc connect to R.  B/C connects to B.  Y to Y, W to W.

[topshot]

Which wiring eliminates the need for battery-power?

Normally the C wire is what the newer tstats need to not need battery. The ecobee likely has some warning that you need to hook up certain terminals for power. Many older homes only have 4 wires to the tstat.

Yes, typically the compressor only takes 2 wires so I'm not sure why you'd have 2 going to C. Outdoors those are normally run with the copper lines so you can see where the wires branch of from the lines and enter the unit.

[jeromedawg]

The Rc is just like the Rh, just for cooling instead of heat.  You see, if you have a separate cooling system and heating system (say, radiant heat and forced air A/C), each one will feed the thermostat from its own 24VAC transformer.  In the summer, the thermostat connects the Rc to the Y to tell the A/C to turn on, and in the winter, the thermostat connects the Rh to the W to turn on the heat.  Since many homes have a combined forced air furnace/airconditioning, there's just one system feeding the 24VAC to the thermostat.  And that same feed goes to both the Rh and Rc.

What would happen if I wired the C wire to Rc instead then? Which wiring eliminates the need for battery-power? I'm still waiting for my Ohmconnect approval to go through before pulling the trigger on the Ecobee (a supposed $150 discount on the unit which is $269 on their site). It's either that or I wait until my local power company adds Ecobee to their discount program (which they claim is coming soon) where I can get $125 back... what I'd be interested to know is if I'd be able to get both the Ohmconnect discount AND utility discount - most likely not.

Wiring C to Rc could very well cause a short and Bad Things.  Rh and Rc connect to R.  B/C connects to B.  Y to Y, W to W.

Thanks... did you mean "B/C connects to C" at least in the context of Ecobee? You mentioned this earlier: "The B/C on the furnace end (with the light brown wire) is the 24AC common wire, which will connect to the C terminal on the ecobee"

[topshot]

Thanks... did you mean "B/C connects to C" at least in the context of Ecobee? You mentioned this earlier: "The B/C on the furnace end (with the light brown wire) is the 24AC common wire, which will connect to the C terminal on the ecobee"

Correct