New thermostat


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Old 01-13-03, 10:07 PM
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New thermostat

I'm going to replace my old analog thermostat with a new digital wire. In the process I'm also replacing the old cable (4 wire) with a new 5 wire cable. Based on the setup the old T-stat didn't have a common wire, but the new one does require it. I looked on the transformer and I believe I located all of the places where I need to make my connections (5 places). I found the W, G, Y, C and R marked on the transformer. Currently the C is only connected to the AC (along with the Y). The thermostat intructions call for one wire to hook up to RH, however, there is no RH. Only R. I assume that's ok since I can turn on the heat if I connect the white and Red wire together.
The question I have is regarding the color of the wire. Is there a code or rule saying which color is supposed to connect? Like red to R, white to W, and so.

Thanks,
 
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Old 01-14-03, 02:53 AM
bigjohn
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It's more like an industry standard. R=red, G=green, Y=yellow, W=white, then whatever color is leftover, use it for C. The color coding has this significance:

R terminal is power from the transformer
G terminal is the fan relay circuit
Y terminal is the cooling circuit
W terminal is the heating circuit
 
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Old 01-14-03, 05:59 AM
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And....... Rh refers to a thermostat that has a separate circuit for the heating portion of the stat.
If your stat had an Rh terminal it would also have an Rc terminal for the cooling side.
Because yours has only R it means that heating and cooling are combined in one circuit.

The C terminal is the other line side of the 24 volt transformer.
Your stat calls for it so that it can have a constant supply of 24 volts between C and R to power itself.
 
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Old 01-14-03, 11:37 AM
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I don't have the T-stat with me right now, but I do know it has a RH and a RC terminal.
I did wire the T-stat for heat only (white and red wires), but I can't for the life of me remember where I connected them ( on the T-stat). It is working fine though.
What I ment to say is that the transformer only has a R terminal (red is connected to it). I'll do the rewire today when I switch the old cable to the new one.
 
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Old 01-14-03, 11:42 AM
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I have another question. When the heat is on (the T-stat calls for heat, or the red/white wires are jumpered) the furnace turn on and off the gas. It is a cyclical operation, about 30 seconds off, 2 minutes on. The blower doesn't stop during this time. I believe it's a Rheem unit, but I could be wrong. It's about 10 years old. I haven't notice any other furnace to do this. It seems like it's designed to do this (based on the clockwork cycle) so it will save gas. Any idea?
 
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Old 01-14-03, 12:46 PM
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Assuming that the red wire goes to R teminal in the furnace and the white goes to W teminal in the furnace, jumpering should kep the burners on. It sound like the burners are cycling on the limit switch Chcek for dirty filter, registers closed,retun grille blocked, obstucted flue pipe. Are you using pleated filters? If s, take them out, they reduece the airflow too much.
 
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Old 01-14-03, 01:41 PM
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trinitro:

bigjohn could be right that the furnace may be cycling on the high temp limit.
Check what he suggested.

If your old thermostat had only the R terminal you should jumper Rc and Rh and connect the R wire from the transformer to either terminal.
If you don't, your airconditioner won't work.
Some stats even give you a short little wire to do this.
 
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Old 01-14-03, 02:10 PM
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I believe the new thermostat does have the Rc and Rh jumpered. The furnace has a regular filter, and the returns are clear (as far as I can tell). The furnace is in the basement, and it heats the second floor. The air is pretty warm. I'll have to open up the panel and look at the blower motor.
I have 2 identical furnaces (one for each floor), I'll look and see if the other one is doing the same.
 
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Old 01-17-03, 07:16 PM
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I finally had some time to look around. No time to replace the T-stat, will do that tomorow.
I looked at both furnaces this evening (they are identical Heil units), and they both cycle on and off. I didn't have a timer, but it's cyclical. It seems unlikely that both units would be faulty.
 
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Old 01-17-03, 08:15 PM
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What you need to do is check the temperature rise on each furnace aginst the nameplate. With an ACCURATE thermometer, measure the temperature difference across the supply and return air. If you have cooling, take the supply temp ahead of the cooling coil. Then compare that with the temperature rise range on the furnace nameplate. Have the furnaces always done this? If you have cooling, are the cooling coils dirty? See if you can determine what control is opening. If they are Rheem furnaces, post the model and serial numbers. If they are from the same production run, it's possible they have a faulty part. Also, see what speed the blower motors are set for in heat.
 

Last edited by bigjohn; 01-17-03 at 08:40 PM.
 

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