Tstat Common Wire

DaveC

The common terminal on the thermostat should be connected to the common terminal of the same transformer that provides the R terminal. The Rc and Rh terminals are only seperated and individually connected if seperate transformers are used to control the heating and cooling functions of the same zone. In that case your instructions should indicate whether to use the cooling common or the heating common.

PTN

Tstat Common Wire

i want to install programmable thermostats. one each for the upstairs and downstairs zones in my 2000SF raised ranch.

right now i have the basic honeywell round tstats.

the one downstairs has two wires which are not labeled. the upstairs handles AC and heat and has the following wiring;

separate wires to R and Rc with an additional jumper wire between the same two terminals. Also, W, Y and G.

I have 2 Taco zone valves (3 terminals) and no C wire (constant)to either tstat. However, i would like to use the extra wire already installed at each tStat which is not connected to anything to bring constant voltage to be used for the new programmables.

Could you tell me where i need to hook to at the boiler/transformer end to get the common voltage to power the new Tstats?

The boiler is a new yorker with domestic coil. i could get pics of the wires if needed...

Thanks.

JJ_4

Can't answer everything but....

G = Fan (forced air), but it could be used for a pump in a HW system. Likewise, W = heat, R = hot to contact for heat, Rc = hot to contact for cool (usually jumpered off of R), Y = cooling (Y1, Y2 for 2 stage).

Hopefully someone else can fill in the rest.

Norm Harvey

The C terminal is if your applying a constant 24v to do things like run without batteries, power a constant backlight, etc,..

its not necessary.

for your heat only tstat one wire to R, jumpered to Rc and the other wire to w and your all set

as far as the AC tstat the terminals should be labeled on your old one, again make notse of what wires go where and connect them the same way.

Again the C terminal is not necessary

bruce_21

Not all programmable stats require 24v power, many have batteries to run the electronics and maintain the memory. Check the wiring diagram of the ones you're thinking of getting before you invest.

You may have a problem finding a C terminal (for a return path to complete the 24v power circuit to the stat's C terminal) on your boiler control. Where does the power for the zone valves come from? A separate transformer or control box?

PTN

thanks for the replies.

just to be clear, i know what each of the connections stand for i just wanted to present my acxtual wiring for all to see. the terminals i listed are all connected.

i am also aware that some tstats work with batteries. i want to use one with constant voltage to run the backlight etc...

as far as the wiring, the wires come off a 2 wire transformer on the boiler then to the 3 wire Taco zone valves and then to the tstats. i am just unsure of where to connect the wire for a "C" terminal?

also, why is there a jumper between R and Rc but no connection to Rh?

Thanks.

PTN

maybe i'm not getting the picture here but i have both AC and heat in the upstairs zone. one wire from ac to Rc terminal on tstat and one wire from heat to R terminal on tstat. No wire to Rh. Additionally, i have a jumper connecting the two individual wires from R to Rc.

So the jumper must mean that there is only one transformer. If that is the case, then i know that there is a heat transformer but is the common terminal on the heat transformer marked as "C"?

Also, why no wire to Rh? i thought Rc for cooling and Rh for heat.

DaveC

mystery

Most likely R and Rh are also a common terminal so that if you put your red wire under either the R or Rh terminal it will serve the same function. You can determine this by removing all wires and jumpers from both terminals and use an ohmmeter to determineresistance between the two. So far you have explained the connections at the thermostats and how many terminals are on the zone valves. Now tell what is at the other end of each wire. For instance G on upper stat to G in Air Handler. Don't assume that since two wires are jumpered together means there's only one transformer. Your installer could have gotten away with running two in parallel.

PTN

I will check more closely and report back. i can take some pictures as well.

PTN

Also, when you say "gotten away with" for running two transformers in parallel, why would someone wire them that way? would it make it easier? what would be the point of running them parallel? i would think it would be easier to connect each wire to a separate terminal and leave it at that, no?

Airman Mikey

common

Easier, more common and correct. However just when you trust that it's done the right way is when you start finding wisps of smoke coming from parts no one in his right mind would think of using. That's why you trace every wire to where it came from and understand where positive starts, what switches it goes through, what load is activated and how the common side of the load gets back to the negative or neutral or common or just the opposite terminal of the power source.

jim lockard

Jumper

If you have a seperate A/C and heat then pull the jumper between R and RC. What stat are you using Honeywell focus pro 3000 or 5000? You do not need a common wire just batterys. R is the same as Rh. Good luck be kind to the transformers. J.Lockard

PTN

ok, i don't have all three (R, Rh, Rc) connections. i have Rc and Rh - the h on Rh was almost rubbed away and i didn't see it until i looked very closely. why i thought i had all three i don't know.

According to Climatouch, the stat can take a max of 4 amps. i have two Taco valves at 0.9 each - no problem.

According to Taco, sometimes a resistor is placed between terminals 2 and 3 on the zone valves to fool the stat into thinking there is constant voltage and keep the display working. they say that there is a time period where terminals 2 and 3 do not connect ( about 20 seconds or so intermittantly). This is one way to do it, but if you can bring a constant 24volts to the stat's C terminal then you don't need the resistor. According to them, these are two different ways of doing the same thing.

i looked for a second transformer in the AC equipment and found nothing so the jumper is ok.

Now, my question is... when a C terminal is powered with constant 24volts, typically, where is the constant voltage pulled from? it was said... from the transformer where R is hooked up. since i am now dealing with Rh and Rc and no specific R i don't understand where to get a constant 24 volts. this is what i want to do since i have the extra wires in place already. i just need to figure out the source for the constant voltage.

i have attached two pics. one of the transformer at the boiler and one showing the zone valve wiring.

the wires with the white sheath that has red x's on it is the same bundle shown in the transformer pic. the transformer pic shows this bumdle and the one comming from the boiler control.

i understand how it is wired and it is correct as per Taco wiring diagram. i am not sure where to get my constant 24 volts. Help....

Joe Mattiello

To keep a constant 24volt power supply to the thermostat, you can place a resistor between terminals 1, and 2 on the taco zonevalve. The Taco 3 wire zonevalve is an intermitting type interrupting the 24 volt to the thermostat every 30 minutes. The ohms of the resistor is dictated by the thermostat mfg. Honeywell, normally includes a resistor with the thermostat.

PTN

joe mattiello,

so you are saying the resistor makes the constant voltage possible and no additional wire is needed to be hooked to the C terminal on the stat?

unfortunately, in speaking with climatouch they don't have an answer as to what resistor will work. they said they haven't worked on this particular application as yet. i pressed them (tech support)into discussing this issue with the engineering dept and they said they might have some info later this afternoon. we'll see.

i really had to work on the guy i spoke to (spiro) and told him i couldn't believe they didn't have an answer to the problem considering the prevalence of Taco zone valves. i hope its not just lip service.

Airman Mikey_2

common

The red wire on the transformer is the positive. It should go directly to the R terminal on one stat. The white wire from the stat which is wire nutted by the transformer is the call for heat from that stat. It shold go to the top terminal of the zone valve that serves that zone. The blue wire on the transformer is the common and should go to the center terminal of all the zone valves that are served by that transformer. Connecting to that terminal at the transformer or the zone valve gives you the common you are looking for. I would still be concerned about the stat that also serves the cooling section. Try turning off the power to this transformer and see if you can still make the fan or the air conditioner start.If you can then there is a second transformer tied into the arrangement and those R terminals should be seperated. The bottom terminal on each zone valve should go to a relay that starts the pump and boiler. Hope this helps.

zeke

I agree with airman, but the zone valve seems to have terminals 2 and 3 jumpered.; if so, then you would be constantly powering the circ pump sourcing that zone valve. Can you verify that circ pump on always and if so, why? Normally it should NOT, unless you are using a bypass system.
Also, it appears that the top wiring from the transformer is going to a thermostat and the blue wire (common) should be or once was available at the Tstat. And the bottom wire white is the w from the tstat and blue are probably going to the zone valve. Is that correct?
In any case, the blue wire is the common you are seeking.

PTN

thanks for all the help. i now see that the blue wire is the common i need.

Airman, i think the way you are describing the terminals is that the bottom terminal is #3 and the top terminal is #1. right? if i have your descriptions correct, then my wiring follows what you say. the Taco valve terminals are numbered 1,2,3 starting on the left of the picture and working to the right side closest to the copper piping.

The only thing that i should clear up is that the white wire you see nutted at the transformer isn't comming from the stat. it goes to terminal #3 at each valve and the other end of this wire is comming from the boiler control. This is the way you described the wire going to the relay to start the pump and boiler so this is ok.

(the transformer pic shows two bundles of three wires wrapped in white sheathing with red x's, the one going to the top of the transformer goes to the valves and the one going to the bottom of the transformer goes to the boiler control)

i will definatley use your method to check for another transformer so i can verify id the jumper should go.

Zeke, there is no jumper across terminals #2 and #3 on the zone valves and the circ pump is not always on. You are correct that the top wire from the transformer goes directly to each stat.

However, the bottom white wire at the transformer (as described above) goes to the boiler control and not to the stats.

Just to be clear, the two brown sheathed wire bundles in the pic with the valves are the stat wires. The W from the stats are the whire wires connected to terminal #1 on each zone valve.

Thanks for all the help guys.

PTN

well, i tried to turn on the fan and AC using the upstairs stat after turning off the breaker to the boiler.

I was able to turn both the fan and AC on.

If this means i have another transformer at the AC somewhere, i don't get why anyone would decide to jumper the Rc and Rh at the stat? What would be the purpose for this since you guys say that this is wrong?

zeke

While tieing the Rh to Rc is incorrect, it won't cause any problem so long as the commons of the two transformers are not connected. So it would be prudent to untie them, but not urgent.

zeke

While tieing the Rh to Rc is incorrect, it won't cause any problem so long as the commons of the two transformers are not connected. So it would be prudent to untie them, but not urgent, even if you bring up the heating transformer common.

PTN

ok, so its only a potential problem but it still baffels me. the AC was added to this heating system at a later date. Somewhere about 3-5 years ago.

Can you think of any specific reasons why RC and Rh would have been tied together during the AC install? Something went through the installers head when they put in the AC and i imagine they thought it made sense at the time. i just don't have enough knowledge to come up with some ideas.

Anyone?

Airman Mikey

guessing

I noticed that all the wires I see are 3 wire cables of differing ages. Most likely when the air conditioning was added they either used the existing stat or got a new one. The jumper is factory insalled since that is the most common use of thermostats. Since everything worked with the jumper installed they just left it there. As Zeke mentioned certain unlikely changes would have to occur before there would be a problem. When you install your new stats simply remove the jumper, connect all the wires to their same terminals and add the commons. Everything should still work as before with the added protection of not parallellong two thermostats. Since your installers didn't leave you a wiing diagram make your own. The red from the transformer goes Rh on both stats. A call for heat closes a switch to the W terminal Which goes to the 1 (top when mounted on top of the pipe) of that zone valve and so on.

PTN

Thanks for all the help. I got everything wired and all is working well.