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Old 07-05-2013, 02:20 PM   #1
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Tstat - furnace rewiring to accommodate Wifi tstat


Long-time lurker here.

I’m looking to replace my current programmable thermostat with one that has a Wi-Fi connection. This will allow my basement tenant to control the temperature when I am away on vacation. I have forced air with a gas furnace and A/C.

I took my current thermostat off the wall, and noted the following wiring behind it (see photo). I was hoping I had a C wire, which these new thermostats require. See the first photo showing what I saw.

Wire colour – thermostat connection
Green – G
Yellow –Y
White – W
Red – R
Blue – Rc
R and Rc are jumpered

There is no C wire, but I'm immediately thinking that there might be a redundant wire here, and that I can repurpose the blue wire from Rc to become the new C wire for my new thermostat.

Minor complication is that there is some splicing right behind the thermostat because the ones coming out of the wall were a little too short and the wires change colours. Please see the second photo.

Thermostat side of splice – furnace side of splice
Green - green
Yellow - orange
White – white
Red – red
Blue – black

So the blue wire of interest now on the tstat Rc terminal now becomes the black wire of interest that heads to the furnace. So far, so good.

Looking inside my furnace, I see the following (see third photo):

Wire – furnace connection
Green – G
White – Y *
Orange – Y
White – W
Black – C *
Black – R
Red – R

So there are two more wires here marked with *. The white on Y and the black on C come in together from a two-wire cable. The other five wires come in together on a five-wire cable.

My questions:
1. What is the purpose of the pair of wires that come in together and go to Y and C on the furnace board? I suppose this is something to do with the A/C.

2. For my proposed modification, I imagine I simply disconnect the blue wire from the Rc connection on the thermostat, and move the black wire that’s currently connected to R on the furnace to join the other black wire that’s currently on the C terminal on the furnace. Are there any potential complications to this? I have my multimeter standing by and ready to go. What should I check?

3. Why would my existing setup have this redundant wiring? Is there something I’m overlooking here?

Thank you!
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Tstat - furnace rewiring to accommodate Wifi tstat-tstat1.jpg   Tstat - furnace rewiring to accommodate Wifi tstat-tstat2.jpg   Tstat - furnace rewiring to accommodate Wifi tstat-furnace1.jpg  

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Old 07-05-2013, 04:33 PM   #2
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Tstat - furnace rewiring to accommodate Wifi tstat


Remove the blue wire from the R terminal of the furnace, and connect it to the C/common terminal. then use it for the common for your new stat.

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Old 07-05-2013, 08:48 PM   #3
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Tstat - furnace rewiring to accommodate Wifi tstat


Quote:
Originally Posted by MisterMike View Post
Long-time lurker here.

I’m looking to replace my current programmable thermostat with one that has a Wi-Fi connection. This will allow my basement tenant to control the temperature when I am away on vacation. I have forced air with a gas furnace and A/C.

I took my current thermostat off the wall, and noted the following wiring behind it (see photo). I was hoping I had a C wire, which these new thermostats require. See the first photo showing what I saw.

Wire colour – thermostat connection
Green – G
Yellow –Y
White – W
Red – R
Blue – Rc
R and Rc are jumpered

There is no C wire, but I'm immediately thinking that there might be a redundant wire here, and that I can repurpose the blue wire from Rc to become the new C wire for my new thermostat.

Yes, that will work.

Minor complication is that there is some splicing right behind the thermostat because the ones coming out of the wall were a little too short and the wires change colours. Please see the second photo.

Thermostat side of splice – furnace side of splice
Green - green
Yellow - orange
White – white
Red – red
Blue – black

So the blue wire of interest now on the tstat Rc terminal now becomes the black wire of interest that heads to the furnace. So far, so good.

Looking inside my furnace, I see the following (see third photo):

Wire – furnace connection
Green – G
White – Y *
Orange – Y
White – W
Black – C *
Black – R
Red – R

So there are two more wires here marked with *. The white on Y and the black on C come in together from a two-wire cable. The other five wires come in together on a five-wire cable.

My questions:
1. What is the purpose of the pair of wires that come in together and go to Y and C on the furnace board? I suppose this is something to do with the A/C.

The two wires go outside to the AC unit to bring it on when there is a call for cooling.

2. For my proposed modification, I imagine I simply disconnect the blue wire from the Rc connection on the thermostat, and move the black wire that’s currently connected to R on the furnace to join the other black wire that’s currently on the C terminal on the furnace. Are there any potential complications to this? I have my multimeter standing by and ready to go. What should I check?

You are correct and no, there are no potential complications as far as operation goes.

3. Why would my existing setup have this redundant wiring? Is there something I’m overlooking here?

Why was it done that way? You'd have to ask the person that did it.

Thank you!
Good luck.
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Old 07-06-2013, 07:32 AM   #4
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Tstat - furnace rewiring to accommodate Wifi tstat


Thanks for the help. Will update when I do the work.
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Old 07-12-2013, 10:03 AM   #5
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Tstat - furnace rewiring to accommodate Wifi tstat


Quote:
Originally Posted by beenthere View Post
Remove the blue wire from the R terminal of the furnace, and connect it to the C/common terminal. then use it for the common for your new stat.
Is this the Beenthere from HVAC Talk? Sorry to the OP for the diversion.

Al
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Old 07-12-2013, 03:38 PM   #6
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Tstat - furnace rewiring to accommodate Wifi tstat


Quote:
Originally Posted by Methodical View Post
Is this the Beenthere from HVAC Talk? Sorry to the OP for the diversion.

Al

Yes, I am.
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Old 07-12-2013, 04:14 PM   #7
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Tstat - furnace rewiring to accommodate Wifi tstat


Also, fix that wiring for those screws on the circuit board. Who ever hooked them up under the screws, did not loop them around. They need to be looped around the screws, or have crimp spades on them, so they will make a good sound connection.

Now as for that wire, is it nailed inside the wall, that you cannot pull a new one, to get rid of that junk wire nut & taped job at the thermostat?

Also, which Wifi thermostat are you installing?
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Old 07-15-2013, 03:59 PM   #8
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Tstat - furnace rewiring to accommodate Wifi tstat


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Originally Posted by gregzoll View Post
Also, fix that wiring for those screws on the circuit board. Who ever hooked them up under the screws, did not loop them around. They need to be looped around the screws, or have crimp spades on them, so they will make a good sound connection.

Now as for that wire, is it nailed inside the wall, that you cannot pull a new one, to get rid of that junk wire nut & taped job at the thermostat?

Also, which Wifi thermostat are you installing?
Thanks for the tip on the wiring. I will loop them around when I do this installation in a few days.

I'm going to go with the Honeywell RTH8580WF1004. I looked at the Nest, but the only upsides that I could identify were the monthly usage report and the fact that it looks flashy. I already receive a monthly usage report (my gas and electricity bills) and I don't care about looks. We have an irregular schedule so I don't think the Nest's learning feature would do us much good. I also don't like the fact that it seems to have a three degree tolerance on your setpoint (e.g., if you set it to start heating at 70 degrees, it won't actually come on until the temperature hits 67).

Honeywell has a fancy new unit too, but it doesn't have very many user reviews yet. People seem to really like the RTH8500 series.
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Old 07-15-2013, 04:19 PM   #9
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Tstat - furnace rewiring to accommodate Wifi tstat


Yeah, the Nest is more of a gimmick than a tool. I have the 3m-50 and ise the Our Home Spaces/Cloudbeam site with their radion in it, for control.

Tracking it I am not so much looking at that, then being able to control it from afar through the Internet or iPhone app.

I do have a Ted5000 for tracking electric use, so I can judge peak use periods and low use periods.

My next step to save money is getting a couple of the Plg & Play Solar panels. Place two on my garage, and twoaybe on the side of my house.

Would definitely help during the Summer with the high electric bills.
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Old 07-16-2013, 11:07 AM   #10
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Tstat - furnace rewiring to accommodate Wifi tstat


We got the Nest a year ago, and there are two things that I love about it.

The first is that you can turn it on with your smart phone to cool the house down before you get home from a summer vacation. Neat trick.

The second is that it has really improved our comfort in winter. We have an old boiler and radiator heat. No circulation pump, which is unusual today. The water circulates by convection. So the boiler is slow to respond to commands and really used to overshoot with our old thermostat. We would be freezing and then hot. Freezing then hot. The Nest learned what the overshoot was and it shuts the boiler off long before the target temperature is reached. It also calls for heat before you need it because it knows how long it takes for the heat to reach you. It's pretty smart the way it predicts after a while. We're more comfortable and have saved a little money. (compared to the previous winter.)

Not sure what other thermostats can do these days, and if the Nest is still unique, but we're real happy with it.
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Old 07-16-2013, 12:07 PM   #11
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glatt you can do that with the 3m-50 or the Honeywell that the OP got, or even the Ecobee. The Nest is all hype and for those places that you have a lot of traffic going back and forth by the thermostat, not a structure that no one is in there.

The way the Nest can only properly work, is if you have enough traffic walking past it for its sensors to keep tabs on what is going on, so it can adjust accordingly. Otherwise it is an overpriced gadget.

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