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  • Mar 14th, 2018 1:05 am
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Thermostat wiring

I have done some reading online and tried to trace the wiring from the furnace to the thermostat and I just can't seem to figure things out. I think I may have some splices in the wall somewhere as the wire colors and connectors between furnace and thermostat just doesn't match. I see the C post occupied on the furnace but not connected at the thermostat. Anyway, the wiring at the thermostat seems to be in accordance with color convention. Can someone savvy with furnace controls explain to me why my thermostat is wired this way. My thermostat is a Honeywell RTH-7400D. There are 5 wires going to the thermostat. The 5 wires are attached to posts labelled R (blue wire), Rc (red wire), W (white wire), Y (yellow wire), G (green wire). From what I have read, it seems like 5 wire arrangements will always have the C wire connection. The online description for the R post is 24VAC from furnace transformer. Isn't that the same thing as the common feed? I am so confused.
Last edited by will888 on Jan 7th, 2018 4:47 pm, edited 1 time in total.
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vkizzle wrote:
Jan 7th, 2018 4:57 pm
Don't over think it.

Image
I am trying not to overthink it. My thermostat is not wired per this diagram. The common post is empty. The blue wire is on the post labeled R. I also have the red wire on post labeled Rc. The thermostat manual says that if a wire is to be placed on both R and Rc, then the jumper should be removed. Otherwise, the jumper is placed between R and Rc. In my case, I have a wire going to each of R and Rc. Again, nothing going to C.
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Your furnace and air conditioner are controlled by 24 volts AC. This is supplied from a transformer which converts the standard 120 volts AC down to the required 24 volts AC.

One side of the 24V transformer is connected to the "C" (common) terminal at the furnace, and the other side is connected to the "R" terminal at the thermostat. This terminal is sometimes labelled as RH to signify the Heating supply.
When the thermostat requests heat it connects the "R" terminal to the "W" terminal, which sends the 24VAC back to the furnace and triggers the furnace to start producing heat.

You air conditioner also runs on 24VAC, and if it has it's own transformer, one side is connected to the "C" connection on the air conditioner, and the other side is connected to the "RC" (cooling supply) at the thermostat. When the thermostat requests cooling, the "RC" terminal is connected to the "Y" terminal which sends the 24VAC back to the AC unit to produce cooling.

In most homes the furnace and AC unit share a single transformer, so power is supplied on the RED wire connected to the "R" terminal and a jumper is placed across the R/RH and RC terminals so that the voltage can be used to power both systems.

In the old days thermostats were simple mechanical devices that didn't require any power. Since most thermostats now have a built in processor of some sort, they require power. To solve the problem the "C" wire is usually extended to the thermostat, so that the 24VAC output of the transformer is available to power the thermostat. In your case the "C" wire was never extended.

It is unusual for a residential home to have separate transformers for both the furnace and AC. If you can determine if your system only has a single transformer, you can remove the blue wire at both ends and connect it to the "C" connection at both ends, Then place a jumper between the "R" and "RC" connectors at the thermostat. If this is your situation you may find that the current blue and red wires already have a jumper at the furnace ( "R" and RC" terminals).
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Rick007 wrote:
Jan 7th, 2018 5:25 pm
Your furnace and air conditioner are controlled by 24 volts AC. This is supplied from a transformer which converts the standard 120 volts AC down to the required 24 volts AC.

One side of the 24V transformer is connected to the "C" (common) terminal at the furnace, and the other side is connected to the "R" terminal at the thermostat. This terminal is sometimes labelled as RH to signify the Heating supply.
When the thermostat requests heat it connects the "R" terminal to the "W" terminal, which sends the 24VAC back to the furnace and triggers the furnace to start producing heat.

You air conditioner also runs on 24VAC, and if it has it's own transformer, one side is connected to the "C" connection on the air conditioner, and the other side is connected to the "RC" (cooling supply) at the thermostat. When the thermostat requests cooling, the "RC" terminal is connected to the "Y" terminal which sends the 24VAC back to the AC unit to produce cooling.

In most homes the furnace and AC unit share a single transformer, so power is supplied on the RED wire connected to the "R" terminal and a jumper is placed across the R/RH and RC terminals so that the voltage can be used to power both systems.

In the old days thermostats were simple mechanical devices that didn't require any power. Since most thermostats now have a built in processor of some sort, they require power. To solve the problem the "C" wire is usually extended to the thermostat, so that the 24VAC output of the transformer is available to power the thermostat. In your case the "C" wire was never extended.

It is unusual for a residential home to have separate transformers for both the furnace and AC. If you can determine if your system only has a single transformer, you can remove the blue wire at both ends and connect it to the "C" connection at both ends, Then place a jumper between the "R" and "RC" connectors at the thermostat. If this is your situation you may find that the current blue and red wires already have a jumper at the furnace ( "R" and RC" terminals).
How do I go about determining how many transformers are in play here?
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will888 wrote:
Jan 7th, 2018 5:45 pm
How do I go about determining how many transformers are in play here?
Rick007 wrote:
Jan 7th, 2018 5:25 pm
If this is your situation you may find that the current blue and red wires already have a jumper at the furnace ( "R" and RC" terminals).
Edit: How Can I Tell 1 or 2 Transformers for Heating Cooling System
Quote:" if your system was installed as an integrated heating and cooling system even if it is a split system air conditioner or heat pump then you should only have one transformer. If not then something is really weird about the installation. "

How old is your house? How old is your furnace and AC units?
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Rick007 wrote:
Jan 7th, 2018 6:39 pm
Edit: How Can I Tell 1 or 2 Transformers for Heating Cooling System
Quote:" if your system was installed as an integrated heating and cooling system even if it is a split system air conditioner or heat pump then you should only have one transformer. If not then something is really weird about the installation. "

How old is your house? How old is your furnace and AC units?
Mid 1960s.
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will888 wrote:
Jan 7th, 2018 7:19 pm
Mid 1960s.
At that time only mechanical thermostats were available, and that is why the "C" wire was never extended to the thermostat. It is also probably true that the AC unit was added years after the house was built, so it may have been connected as a separate device to the thermostat (requiring both the "R" and "RC" wires).

On the other hand it is almost certain that the original units have been replaced since the mid 60's, and most likely less than 20 years ago. I would assume the installers used a single transformer and just left the existing wires in place since they would still work.

Did you check for a jumper at the furnace between R/RH and RC?
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Rick007 wrote:
Jan 7th, 2018 7:34 pm
At that time only mechanical thermostats were available, and that is why the "C" wire was never extended to the thermostat. It is also probably true that the AC unit was added years after the house was built, so it may have been connected as a separate device to the thermostat (requiring both the "R" and "RC" wires).

On the other hand it is almost certain that the original units have been replaced since the mid 60's, and most likely less than 20 years ago. I would assume the installers used a single transformer and just left the existing wires in place since they would still work.

Did you check for a jumper at the furnace between R/RH and RC?
The Furnace and AC have all been replaced within the past 10 years. The wiring at the furnace looks like a mess. I several cables coming into the connectors. The wiring colors are different than what I see at the thermostat. This is why I started this thread because I am pulling my hair out trying to understand the wiring at the furnace. I will have to study it in greater detail. Given the extreme cold, excessive down time is not possible. I will resume this when the weather is cooperative. Thanks for the feedback.
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Okay, I will give this one more try.

I agree with you that if the colors are different at each end, then there is probably a splice somewhere in the middle. Either that or you are looking at the wrong place in the furnace. You may actually be looking at the cable that goes outside to the AC unit.

It really doesn't matter what the colors are at each end. Since the furnace is working, we can be safe in saying that each connector on the furnace is properly wired to the correct connector on the thermostat.

Since your furnace and AC are fairly new, you don't need a wire going to both the "R" and "RC" connectors on the thermostat. The fact that you have two wires is just a leftover from the original 60's installation. You can find which one is required, and relocate the extra wire to the "C" terminals at each end.

Let's start by answering these questions.
Have you found the terminal block on the furnace that has terminals labeled "R,RC,C,W,Y,G". If so, it would be really helpful to answer the question"is there a jumper between the "R/RH" and the "RC" terminals. It would also be a big help if you could list the color of wire attached to each terminal on the furnace, and list any jumper wires that may be connected between terminals.
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This is a picture of the thermostat wiring.

Image

Reading from top down, green wire on the G post, yellow wire on the Y post, white wire on the W post, red wire on the Rc post, blue wire on the R post.



This is a picture of the furnace wiring.

Image

On the furnace side there are 3 cables supply the wires to the control board. Two cables have black and white wires only. The other cable has 4 wires, red, white, orange and green. These are off to the left and not visible in the picture. The thick yellow wire in the picture connects on the numbered post (24) below the screw on posts. Reading from the top down, the two white wires are on the W post, orange and black wires are on the Y post, green wire is on the G post, black and white wire are on the C post, red wire is on the R post. One of the two wire cables has a white wire on the W post and the black wire on the C post. For the other two wire cable, the white was connected to the C post and the black to the Y post.

I am guessing that since there are two sets of cables with two wires coming into the play, that means I do have two transformers. Since there are only 4 wires on the third cable, that is likely what is going to the thermostat. Somewhere along the way, 4 wires from the furnace became 5 at the thermostat. This could explain why there is a wire to each of Rc and R at the thermostat. Those two are probably bound together somewhere, effectively reducing a 5 wire cable down to 4. When the weather is nicer and I can take a longer furnace outage, I guess I can do a continuity test and verify whether the blue and red wires at the thermostat are wired together as one.

All this effort is to ultimately try and install the ecobee thermostat. Given I have an effectively 4 wire thermostat installation, does it make sense that I should be able to use the power extender kit and make the ecobee thermostat work?
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will888 wrote:
Jan 8th, 2018 10:34 pm
This is a picture of the thermostat wiring.

Image

Reading from top down, green wire on the G post, yellow wire on the Y post, white wire on the W post, red wire on the Rc post, blue wire on the R post.



This is a picture of the furnace wiring.

Image

On the furnace side there are 3 cables supply the wires to the control board. Two cables have black and white wires only. The other cable has 4 wires, red, white, orange and green. These are off to the left and not visible in the picture. The thick yellow wire in the picture connects on the numbered post (24) below the screw on posts. Reading from the top down, the two white wires are on the W post, orange and black wires are on the Y post, green wire is on the G post, black and white wire are on the C post, red wire is on the R post. One of the two wire cables has a white wire on the W post and the black wire on the C post. For the other two wire cable, the white was connected to the C post and the black to the Y post.

I am guessing that since there are two sets of cables with two wires coming into the play, that means I do have two transformers. Since there are only 4 wires on the third cable, that is likely what is going to the thermostat. Somewhere along the way, 4 wires from the furnace became 5 at the thermostat. This could explain why there is a wire to each of Rc and R at the thermostat. Those two are probably bound together somewhere, effectively reducing a 5 wire cable down to 4. When the weather is nicer and I can take a longer furnace outage, I guess I can do a continuity test and verify whether the blue and red wires at the thermostat are wired together as one.

All this effort is to ultimately try and install the ecobee thermostat. Given I have an effectively 4 wire thermostat installation, does it make sense that I should be able to use the power extender kit and make the ecobee thermostat work?
Looks like you do have a 2nd transfomer.
Try moving the blue wire to the C terminal at the Ecobee thermostat.
If you have a fuse on the furnace mainboard, you will be safe from a short, as it's likely a 3amp auto fuse.
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will888 wrote:
Jan 8th, 2018 10:34 pm
I am guessing that since there are two sets of cables with two wires coming into the play, that means I do have two transformers.
One of the small cables probably goes to the AC unit outdoors. The second small cable was probably installed to provide more conductors to the thermostat. You may find that this 2nd small cable runs off in the same direction as the 4 conductor cable, towards the thermostat.
will888 wrote:
Jan 8th, 2018 10:34 pm
On the furnace side there are 3 cables supply the wires to the control board. Two cables have black and white wires only. The other cable has 4 wires, red, white, orange and green. These are off to the left and not visible in the picture. The thick yellow wire in the picture connects on the numbered post (24) below the screw on posts. Reading from the top down, the two white wires are on the W post, orange and black wires are on the Y post, green wire is on the G post, black and white wire are on the C post, red wire is on the R post. One of the two wire cables has a white wire on the W post and the black wire on the C post. For the other two wire cable, the white was connected to the C post and the black to the Y post.

I got lost trying to read that. I can see all the colors under the screws, but I can't tell what cable they go into.

Lets call the 4 wire cable "cable #1". One of the 2 wire cables is probably connected to the "B" and "Y" terminals, lets call that "cable #2". And finally the remaining 2 wire cable will be called "cable #3.

Please make a chart similar to this. Note that I just filled the chart in randomly, please fill in what you actually find.

Cable#1 Red to "R", Green to "G", White to "W", Orange to "Y"
Cable #2 Black to "Y", White to "B"
Cable #3 Black to "B", White to "R"

By the way, do you have access to a multi-meter to perform some voltage measurements to complete the analysis?
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Glad I went wireless thermostat so I don't have to deal with stuff like this. Sometimes it is better to invest the money now than to invest later.

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