I purchased online a $400 Honeywell remote controlled batteryless thermostat to upgrade the HVAC battery powered thermostat in my home, and when I tried to install it I easily wired the R, G, W, and Y terminals in the new thermostat with the same wires that were connected to those terminals on my old thermostat, but the new Honeywell thermostat also has a "C" terminal, which my old thermostat did not have, and the Honeywell manual says a 24VAC wire should be connected to the "C" terminal, and that the thermostat will not operate if the "C" terminal is not connected. My furnace (15 - 20 years old) does not have a "C" terminal either. The thermostat cable between my furnace and my thermostat has only the four wires, R, G, W, & Y, and because the walls and basement ceiling are finished, I cannot fish a new 5 wire cable, and even if I did, and connected the 5th wire to "C" on the new thermostat, what would I connect the other end to at the furnace terminal board. Can I jump the R or the Y contacts on the thermostat to its "C" to make the thermostat run?

I just read the install manuel on your new high dollar tstat
I`m afraid you have bought more tstat than you can handle,1st its more expensive because it was built for a heat pump with only four wires ,you don`t have (but it could still be used)
And you have got to have the 5th common wire to make it work as it was built for.
Maybe some else will come along and have a better way other than what the instructions are saying

C is actuall common. You will need to run an extra wire for this stat for sure. As for where to hook it in the furnace. You would simply tie it into the common on the low voltage (24v) side of the transformer. If you use a voltmeter, and check between the two low voltage wires, you should get roughtly 24v. Now, to see wich is hot and which is common, test from one to ground, and then the other to ground. The one that gets a reading of 24v between wire and ground is your hot. The one that gets o or nearly 0 between wire and ground is your common. The common is the one that will need to be hooked to , and run to the c on the thermostat. You could always hire an electrician to pull you a new five wire stat cable, and hook everything up as needed. Some of those guys can really work magic in pulling new wires, or at least opening minimul cieling/wall to pull new wire. Good luck and please let us know how things work out.

Yes, I realize this. But it seems that I should nonetheless be able connect the overqualified thermostat up so it will control only my furnace, a/c, and humdifier. Only two output wires come from a stepdown transformer. Both of these are connected somehow to the R,G,W,&Y terminals on my furnace. I think the hot side is on the R terminal. The ground side I bet is on the Y terminal. If I jumped from one of these to my thermostat "C" terminal shouldn't it satisfy the 24VAC connection the thermostat literature indicates should be connected to "C".

R is the only hot, or power at all from the tranformer to the stat. If you add a c on the common side of the trans, it will send a true 24v to the stat to power the stat itself. With just a red, the power only goes to the stat, and than back out of the stat to either g (to power the blower circuit), w( to power the heat circuit), y(to power the a/c circuit). If you jump y to c, you stand a good chance of damaging the thermostat, burning up a transformer, or blowing a low voltage fuse.

Try this, Google 'add a wire', and see if it is something you might want to try. I have never tried one, but sounds like it might be an option.

To Mygirlsdad77: Thank you for your responses. Yes, I measured the voltage at the furnace terminals (which match the R,G,W,&Y terminals at the thermostat) and learned that R was the only wire which had any voltage. It has 24 volts. I am aware that the R wire carries the 24V to the thermostat and that the thermostat circuitry then directs this as needed to the furnace, a/c, and fan. You say "add a C terminal to my transformer". I haven't actually looked at my transformer (haven't even located it yet) but how could I add any terminal to my transformer? Also, since you and I are agreed that the R wire carries 24V to my thermostat, and my thermostat literature says a 24VAC connection is needed on the "C" terminal to power the circuitry of the thermostat itself (it has no batteries) why couldn't I jump from the R terminal on the thermostat to the C terminal on the thermostat?

The manual is a little decieving. It actually means you need a common, (or neutral) to the c terminal. It needs the common to power the stat. Think of it this way. You can grab hold of the hot wire coming of the low voltage side of the transformer and not get shocked, but if you touch the hot wire, and the neutral (common) wire at the same time, you will get shocked. Kind of like you could run one hot wire to a light bulb, but it won't work without the neutral wire also. Basically, your specific thermostat requires a true 24v to operate the themostat itself. It is not just acting like a light switch, wich would just send the hot back out, but needs the neutral to power the stat. As for adding a terminal at the transformer, you would just cut and strip the wire and wire nut it together with an extra wire to run to the stat.

Most trasformers are labeled with a 24v wire and a common wire on the low volt side. The 24v doesn't do any good without the common. If you jump are to c, you will damage the stat, as the c terminal is not actually looking for 24v, but needs the nuetral to create a true 24v to power the stat.

The only other way I can think of to describe this is to think of a car battery. A car won't start without both the pos and negative hooked up. Even if you jump the pos all over the car, it won't do a thing without the negative in place. Hope that makes sense. I really wish I could explain better. Bottom line, you need the common. If it were as easy as jumping are to c, I would save many hours and headaches from running new wires. However, nobodys is perfect, and I have been wrong before, so if you do find some literature, or someone who says you can make it work without the extra wire, please let me know. Im always interested in learning. Just because Ive been doing things a certain way all these years, does not mean there isn't a better way. Please let me know how things work out.

One last thought. Can you take the thermostat to a hvac dealer in your area and ask them their thoughts on this Let them know your situation, and see if they have any input. Good luck and take care.

To Mygirlsdad77. You have been very helpful, and I am thankful for your help. Naturally you don't know me nor know what is my level of skill & knowledge about electrical things, so I accept your comments about "hot" and "ground" (return, aka common), but I understand well the nature of "hot" wire and "cold" wire, and that grabbing only one or the other (assuming you are not grounded) will give you no shock.

What I finally am beginning to get through my thick head is that there is no "cold" wire terminal connected to my old style battery operated four terminal thermostat - only a "hot" wire, which the thermostat circuitry sends on to Rh, Rc, G, W, or Y. When I measured 24VAC at my thermostat between the R terminal and each of the G, W, & Y terminals, I jumped to the wrong conclusion that the G, W, & Y terminals had to be connected to the "cold" contact of my transformer, for how else could I get the 24V reading between R and each those other contacts? I now can see how it is possible that the G, W, & Y wires might actually be the "hot" feeds to three different equipment pieces, and when my multimeter measures 24V twixt R and those other contacts, it's not because those other contacts are direct grounds (or "colds", or "commons"), but because those contacts are "hot" feeds to those equipment pieces, which equipment pieces are connected to ground.

So, yes, I think if I threaded a separate wire from the ground side of my transformer to the "c" terminal on my thermostat, everything will be just fine (but it will require breaking into a wall at the thermostat, and at least one spot of a finished ceiling in the basement.

As for contacting my local HVAC service company, I have done this, and they kind of knee-jerk told me that "this thermostat would not work with my furnace" because my 15 - 20 year old furnace has no "C" terminal. I've asked them to call Honeywell for me (Honeywell refused to discuss the matter with me a nonprofessional) to ask if they could suggest a way to make the thermostat perform with connections from only a four-wire thermostat cable. I'm still waiting to hear back from them.

I have a Trane TUD080R9V3K2 furnace. I recently noticed that the thermostat (White Rodgers - 1F80-261 B) was indicating a measured room temperature that was approximately 10 degrees higher than the actual room temperature. This thermostat had 5 wires connected to it; red, white, green, yellow and blue. These were attached to the following terminals in the thermostat... red to RC (jumper from RC to RH), white to W, yellow to Y, green to G and blue to C. I've replaced the thermostat with an old Honeywell CT3300 that I had left from the previous (old) furnace. It has no C terminal and I've taped the blue wire back and insulated it so that it doesn't come in contact with anything. All these wires (yellow, white, green and blue have 24vac in reference to the red wire. What's the purpose of the blue wire that was connected to the C terminal in the White Rodgers thermostat? The system (heating / cooling) seems to be operating normally with the old Honeywell thermostat. Does this wire have anything to do with the variable speed control of the furnace? Is using the old Honeywell thermostat OK, or should I buy a newer thermostat that has the C terminal?
Thanks.

The blue wire is purely to power your thermostat. It is a common wire that connects to the Com terminal in your furnace. You can simply buy a t-stat that is battery operated and ignore the blue wire.