Add C Wire with second transformer

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  #41  
Old 09-04-14, 02:27 PM
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Keep us updated. Did you see my last post on how to access the C wire connection. You were probably typing when I posted.
 
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  #42  
Old 09-04-14, 02:28 PM
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Yea. I would be hard to find that its up in attic with flat roof. Im looking for a place to get the exact same tape that is on the air handler. If so it will be easy to replace the tape when im done so it looks like no work was done.
 
  #43  
Old 09-04-14, 02:31 PM
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Another quick question dont really know if this is possible. I have seen a few posts saying all you need is the neutral, and that you could connect 1 wire from external transformer to C and to cap of the other wire does that really work?
 
  #44  
Old 09-04-14, 02:32 PM
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It has to come out of the air handler at some point. I was thinking you could splice it right next to where the thermostat cable for outside exits the air handler cabinet.
 
  #45  
Old 09-04-14, 02:36 PM
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https://www.youtube.com/watch?v=BgDseEF-4OY wouldnt that result in 48 volts.

He had a wire in Rh with jumper to Rc. But he also connected external transformer to Rc.

Another quick question dont really know if this is possible. I have seen a few posts saying all you need is the neutral, and that you could connect 1 wire from external transformer to C and to cap of the other wire does that really work?

Read more: http://www.doityourself.com/forum/ai...#ixzz3CNwxFjkQ
 
  #46  
Old 09-04-14, 02:50 PM
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I am not sure if adding just one wire to C terminal with the other capped off would work or not. It would be easy enough to try it and see. The guy in the video states that if you have your power wire going into RC you would put the second white transformer wire in free R terminal.
 
  #47  
Old 09-04-14, 02:52 PM
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Yea with the jumper still attached. Thats where i got worried. Thus putting 24 from Rc, and 24 from R together. Wouldnt that double the voltage.
 
  #48  
Old 09-04-14, 03:04 PM
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It would depend on how the thermostat in the video has the power coming in and how the thermostat connects internally.
 
  #49  
Old 09-04-14, 03:14 PM
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But the jumper between 24V 1 and 24V 2 would automatically be connected due to the jumper between terminals.
 
  #50  
Old 09-04-14, 03:28 PM
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I have been doing some research and I can't find a definitive answer. One person claims he connected power in wire to RC and external transformer wires to R and C. Jumpered R to RC and it worked.
 
  #51  
Old 09-04-14, 03:45 PM
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Could i test by bypassing thermostat. and jump External transformer R wire to G wire to turn on fan or is that going to break something. Example if i connnected R to G shouldnt fan run?
 
  #52  
Old 09-04-14, 03:49 PM
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What model thermostat are you installing?
 
  #53  
Old 09-04-14, 03:53 PM
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Tomorrow i should be receiving a Honeywell RTH8580WF.
 
  #54  
Old 09-04-14, 04:06 PM
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Worst comes to worst if Cool mode works with Red to Rc, and Ext to R (Rh) and C. I can live without heat mode on the unit. Its florida doesnt really get cold and i never use heat.
 
  #55  
Old 09-04-14, 04:18 PM
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In that case you could disconnect the wire going to W (heat) on the thermostat and connected in the air handler to power the C terminal.
 
  #56  
Old 09-04-14, 04:59 PM
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Somebody elsewhere suggested maybe 300mA external transformer not powerful enough to trip relay.
 
  #57  
Old 09-04-14, 06:18 PM
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I see this topic seems to have gathered a lot of posts and I'm not quite sure why

I don't know where a 300ma transformer entered in the discussion. You should be using at least a 500 ma transformer to run the Wifi and electronics part of the thermostat.

Since the two 24vac supplies do not share any ground connection you can leave the Rc/R/Rh jumper in place..... and since the two 24vac supplies don't share a common connection the voltage doesn't double.
 
  #58  
Old 09-04-14, 06:34 PM
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I guess the reason there are so many posts is because the OP is apprehensive (understandably so) about connecting up an expensive thermostat with no definitive instruction on how to do so using an external transformer. Personally, I would have connected it up exactly as you stated way back in post #2.

Based on the additional research I did on the subject, this seems to be a very popular and widely sought out quest to find the solution but there have been very few solid answers forthcoming.
 
  #59  
Old 09-04-14, 06:55 PM
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Ok so your saying its ok to connect Transformer 1 to Rc and Transformer 2 to Rh and leave them jumped with no issue? If thats fine i will go that route I was just worried. I hooked it up the way it was stated in #2, and the fan didnt turn on or condensor, thats what sparked this debate.
 
  #60  
Old 09-04-14, 06:58 PM
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Many people like the idea of a second transformer but I end up running a new cable for most of my customers. Yes.... sometimes it can be tricky if access is blocked by sheetrock.

A 1A 24vac (24va) transformer will give you all you need.

Try the connection first not using the Rc/Rh jumper.
 
  #61  
Old 09-04-14, 07:01 PM
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Ok. Sounds like a plan. So your professional opinion on why it doesn't start is not enough amperage to the relay. Any specific transformer you recommend? Only thing close to 1A i could find was a 1.2 Amp is that too much?
 
  #62  
Old 09-04-14, 07:20 PM
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Agreed; running an additional cable is the best method. I still believe that the wiring connections as outlined by PJmax in post #2 will work. Perhaps it is the fact that the external transformer is not the right size.

I say it should work based on the following:

When i got home from the office and the distractions there I had a chance to study the page out of the manual you posted earlier. There are two separate charts on the page that outline connections if using two separate transformers. That seems to be the answer; connect wires according to instructions for a 2-transformer system.

As shown on chart and notes at bottom:

(1) RC is power supply for one transformer (original setup)
(1) R is power supply for second transformer (added external transformer)
(2) Remove jumper for 2-transformer system
(3) Optional 24VAC common connection

It states in note 4 that the common connection must come from cooling transformer.

All contacts in a thermostat are dry contacts, but Rh (if present), R and Rc contacts are the power source to each of the other contacts, via dry contacts.
For your thermostat, Rc is the power, and since there is not an Rh terminal, the R is the terminal that powers the heat related contacts. The RC terminal is for cooling and it only supplies power to the cooling related terminals such as G and Y terminals.

I have attached an excerpt from the manual page and a wiring diagram based on the information. It appears to be just how you had it wired earlier today.
 
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Last edited by firedawgsatx; 09-04-14 at 07:39 PM.
  #63  
Old 09-04-14, 07:31 PM
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Thanks. I just ordered a 24VAC 30VA transformer rated for 1.2 Amps.
 
  #64  
Old 09-04-14, 10:45 PM
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AHU is rhpl-hm3621ja figured i would post incase anybody else had this problem.
 
  #65  
Old 09-04-14, 10:50 PM
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I thought you were posting to say you had the job finished.
 
  #66  
Old 09-04-14, 11:00 PM
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LOL. Cant continue until at least tuesday
 
  #67  
Old 09-09-14, 09:03 AM
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Project completed. It appears you must use both transformers and jump Rc, and R together to make it work.

Put in external transformer to Rc, and internal transformer to R did not work until jumper was added. Issue appears to have been related to incomplete circuit.

Final Wiring:
External Transformer to Rc, and C
Internal Transformer to R(Rh)
Jumper placed between Rc and R

Hope this helps anybody who had my issue with it not running.
 
  #68  
Old 09-09-14, 09:15 AM
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Thanks for letting us know. Does the final wiring work for both your original thermostat and your new RTH8580WF?
 

Last edited by firedawgsatx; 09-09-14 at 09:42 AM.
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