Another where to get C wire from

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  #1  
Old 12-07-16, 01:41 PM
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Another where to get C wire from

I am trying to install new Honeywell RTH8500.It needs a C (common) wire.My old therm only has R+W wires.I do have two wires in the wall NOT connected that run down stairs close to boiler. Have a hot water base board system,5 zones. I have a Taco SR504-exp switching relay. In the picture I found a 24 common terminal. It is in both of the pictures. Would it be BETTER to get the common from that point or go to boiler and get common from transformer in the boiler. I figure from the relay is better so C is from in source and not from different power source. Not sure which is best.
 
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  #2  
Old 12-07-16, 01:51 PM
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You MUST get C from the Taco unit.
You must observe the W and R polarity on the Taco unit as well as the stat.
 
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Old 12-07-16, 03:05 PM
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Is this your exact unit ?
Do you just have T T id's for stat wiring ?
Do you have a meter ? We need to label the T T terminals W and R.

I had a labeled pic but it got lost.

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  #4  
Old 12-08-16, 10:46 AM
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Yes I have a meter. Second the T T's are the wires (red-white,one is black but color coded) that attach to board at the top right of the pictures. When looking at terminals the white is on the left,the red is on the right .

PS:I do not think you need this but I have shown (second panel pic) you the secondary taco board that I have.
 
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Old 12-08-16, 04:34 PM
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Hi, what is the model of that Taco control? it's hard to tell which terminals are which,maybe we can look it in the Taco website.
 
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Old 12-08-16, 06:34 PM
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Taco did an injustice as they kept offering different modified units that looked alike. They didn't identify each model with a particular model number.

I can't see the markings on the top board older board but from what you are saying the terminals are clearly labeled W on the left and R is on the right...... is that correct ?

Is it the same for the lower/newer board ? If it just has T T.... it needs to be checked.
 
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Old 12-09-16, 09:42 PM
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I do have White on the left and Red on the right.The black wire is also color code the same. I am enclosing the wiring diagram from the box of the relay.If that does not help I enclosed any making of the board.

PS:Was having a difficult time uploading pict to site.
 
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Old 12-10-16, 06:14 AM
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Hi, the new boards have the common terminal across the to where the stat wires connect ,you board does not have that option however you should still be able to pick the common up from the onboard transformer,with your voltmeter , check between each wire that goes into the 24 volt connector from the transformer to the Red thermostat wire on the top of the board, the one you get 24 VAC on can be used for your common,now use a IDC to jump onto it.
 
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Old 12-12-16, 02:57 PM
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OK I have tried to get 24v from the board.I am having trouble doing this.I have spoke with Taco about this and they are not sure (?).
So How can I use an external 24 v transformer to power the thermostat.From what I have rad if I connect the two sides to RC and C of thermostat it should work . Yes it would be better if 24v came from circuit board that the thermostat come off. So the question is would this work,RC+C to external 24v transformer??
 
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Old 12-12-16, 05:53 PM
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Not surprised at Taco's answer,not to beat a dead horse,but if you stick the probes from your volt meter into the terminals for the plug with the red and what looks like yellow wires that come from the onboard transformer you should get 24VAC.
 
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Old 12-13-16, 06:12 AM
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The board that you see the red and yellow coming out of the transformer is my expansion board. It is hooked into the main board where I want to put the new thermostat and need a C(common) to run the new therm. I have 5 zones base board heat. I have been trying to get 24v from main board. Can I take it from the expansion board,this is were you see the red and yellow wire. OR can I just add an external 24v transformer.Nothing is ever easy. Thanks for the help.
 
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Old 12-13-16, 08:23 AM
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I can't see the markings on the top - older board but from what you are saying the terminals are clearly labeled W on the left and R is on the right...... is that correct ?
I see the way your wires are connected.

My question is.... is the board labeled R and W ??????
 
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Old 12-13-16, 09:23 AM
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All it says is T above each wire.T above a Red wires,T above a White wire.I have tested both of my voltage meters with a old transformer and they are working.Just wanted to make sure they both are working. From what I have read I should be getting 24v out of the transformers that are on the two circuit boards. I am not,Why ,I really do not understand. But like a great many thing I will work around it. Hot water boiler is working fine and the 5 thermostats (zones) are working fine.
I understand it is not the best but how do I connect an external 24v transformer to the Honeywell RTH8500.
 
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Old 12-13-16, 04:54 PM
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Doesn't make sense,try sticking a couple of paper clips or needles down into that plug and check again,also read between the TT terminals and see what you get there.
 
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Old 12-13-16, 07:11 PM
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Ok.... so your stat wires are randomly installed on the TT terminals. We'll need to convert those letters.... TT to R and W. I will help you. it's very easy. In exchange for my help..... you will tell me which terminal is R and which is W. I will then label my diagram and the confusion in the future can be eliminated.

First off.... this is 24v AC not DC. Since this is AC... probe polarity is not important.
We will be working with the board that the stats are connected to.
This is important. We will use this picture.

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1) Check for 24v at the live R and live C terminals I have labeled in red and blue.
Those are set screws and easy to put probes on.
2) Remove the two wires from the TT terminals on the port the new stat will be on.
3) Leave one probe on live C and touch the other probe to either TT terminal. The one that shows 24vac is the R terminal. The other one is W.
4) Put the probe on live R and the TT terminal you tested as R. There should be 0v.
5) Leave the probe on live R and move to the W terminal. You should measure 24vac.

You're done. Tell me which terminal is which.
 

Last edited by PJmax; 12-18-17 at 02:58 PM.
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Old 12-14-16, 06:51 AM
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OK I have to admit I have a few days of being REALLY ,really DUMB . PJmax pointed out the problems (First off.... this is 24v AC not DC). For some reason I was always looking for DC,no wounder why I did not have the right voltage. A few replies back it was mentioned 24AC but I just did not read carefully enough. On the left side of the circuit board it says--- COMMON-24VAC So I have 28vAC from common to red wires. I will not just run the common up to the new therm.
Again sorry about not following instruction (reading).
 
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Old 12-14-16, 07:24 AM
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One statement you made,"I will NOT just run the common up to the new therm",what dose that mean?I thought that was the whole point of the post.
 
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Old 12-14-16, 12:54 PM
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I guess I am just continuing being silly.I WILL RUN COMMON TO C ON THE THERM. THANKS
 
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Old 12-14-16, 03:38 PM
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One other thing,at the Tstat set your meter to AC and check between the red and your new C to be sure you have 24 VAC,if not switch the C to the other terminal on the board.
 
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Old 12-14-16, 04:48 PM
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No Geo.....

I specifically asked for one thing from this member. To test and ID the TT terminals.
This step MUST be done !!!!!!!!
 
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Old 12-18-17, 02:42 PM
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PJMax, your a wonderful man! I've combed through many posts over 2 weekends and this one got me up and running! Let me explain... I built my boiler with my father about 15 years ago... must never had paid any attention to TT R/W assignments. After reading this I reversed my T-W / T-R assignments and boom! Lyric T5 booted. Thanks brother.
 
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Old 12-18-17, 02:49 PM
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Been there too on the AC/DC setting on my meter, the very first day I tried to install. Then after I checked the voltage on a battery it occurred to me... not DC...
 
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Old 12-18-17, 03:00 PM
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Thanks for the thanks.

I get a lot of feedback from this thread as the wiring is easy once the ID'ing is done. You actually did me a favor as the picture I had posted with my directions was AWOL. I've reloaded it.
 
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Old 10-08-19, 10:22 AM
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Another where to get C wire from - add'l info rqst.

Hi, I'm a newbie. Requesting a follow up on PJmax's advice dated Dec-13-2016 regarding adding C wire to power existing Honeywell TH8000.

I have existing Taco SR504 for oil burner with 2 heat zones, zone 4 is hot water tank & zone 3 is not used

When I check voltage across the 24VAC / COM terminal (labeled J3) it's good.

When I check from COM to the TT terminals I find ALL the white wires are hot and reds are not (unless thermostat is cranked up).

Should I swap around the reds & whites in their respective ports before I proceed?

Thanks,
Shawn
PS, the thermostat is wired red wire to R & white wire to W. There is no jumper between R & RC. The manual says the jumper should only be removed for 2 transformer systems. I only have one.
All the previous work was done about 10 years ago by a licensed HVAC contractor
 
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Old 10-08-19, 10:55 AM
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Welcome to the forums.

All the information is here to make these connections.
I now have pics stored of most of the zone controllers.

Post a clear picture of your zone board...... How-to-insert-pictures.
 
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Old 10-08-19, 12:06 PM
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Pete, thanks for the reply.

https://picresize.com/popup.html?ima...6182628918.jpg

Everything I tried didn't work. File is 4.5mb and it says too large.
Perhaps you can see it at this pic resize web page.
I will keep trying.

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Last edited by PJmax; 10-08-19 at 12:31 PM.
  #27  
Old 10-08-19, 12:30 PM
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Yeah.... 4.5mb is massive.
Your pop-up link isn't working for me.

If you measure from the left R to the four zone R's.... you should measure 0v.
If you measure from C to any of the four zone R's... you should measure 24vAC.
If you measure from left R to any of the four zone W's.... you should measure 24vAC with no call for heat or the W wire disconnected and 0v with a call for heat. .
 

Last edited by PJmax; 10-08-19 at 01:40 PM. Reason: wording whoops
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Old 10-08-19, 01:29 PM
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You got my picture!
I did all the tests & everything is as you predicted, except the very last test. When I disconnected the W, I still have 24v at the W post. (Checked zone 1 & 2)
 
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Old 10-08-19, 01:39 PM
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You are good. Everything is ok. I had my description wrong and corrected it.
No call for heat would be with the wire off.

I didn't realize that was your board. I thought it was a random one you grabbed the picture of.
 
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Old 10-08-19, 02:54 PM
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Now that I'm so good at cropping & attaching photos, here is a pic of my Honeywell TH8000 mounting plate.

You can see I ran a third wire to use as the common. (Should have just picked the unused green wire, as there is no fan). I'm scared to attach the other end to the COM screw on the 24VAC / COM terminal on the zone board.

Reason being, on the zone board my R wires are the white ones, and W wires are red. On the tstat plate my white wire is connected to W (& red wire to R). It seems the polarity is reversed.

Does that matter, or should I swap red & white?
 
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Old 10-08-19, 05:54 PM
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That's why I took the time to label your drawing.
Regardless of the wire color..... R must go to R and W must go to W.

So do yourself a favor..... put red on R and white on W at both ends.
Using the green shouldn't be any problem.
 
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Old 10-09-19, 06:29 AM
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OK, it's crystal clear now!
Photos of finished tstat wiring attached.
Everything works perfectly.
Thanks a million to Mr. PJmax for all the assistance.
Hopefully, this thread will help others as well.

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Last edited by PJmax; 10-09-19 at 02:48 PM. Reason: resized pictures
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Old 10-09-19, 02:49 PM
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You are very welcome. Good job there.
 
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