Two transformers with Honeywell EIM system


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Old 08-01-22, 08:05 PM
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Two transformers with Honeywell EIM system

Curious about something. If I decide to use one 24v transformer to power my EIM, and a different 24v transformer to power my furnace/air handler control board, I'll obviously want to remove the two jumpers on the EIM (R to RC and RH). What about the common connection C between the two components? Should I be connecting a jumper between the furnace/air handler C terminal to the EIM C terminal?
 
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Old 08-01-22, 08:37 PM
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Your aux 24v transformer will connect to R and C on the EIM. That is the source of the C.
The connection lines between the EIM and the furnace don't require a C connection.

Your are connecting equipment R to W for heat, R to G for blower, R to Y for A/C.
No C required.
The condenser connects to the C at the furnace.

I can give you a diagram if needed.
 
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Old 08-01-22, 09:34 PM
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Thanks, Pete.

That is confusing to me. Without a jumper between the two systems C terminals isn't the C connection on the EIM just floating as far as the furnace sees it?

Yes, please. Maybe a diagram will clear up my misconception.
 
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Old 08-02-22, 12:46 AM
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The board is having trouble right now posting images so I'll post that when the system allows.

Picture this.... you have a standard thermostat connected to the furnace.
That stat would use R, G, W, Y for a furnace with split A/C.
No C required.

If you added a wifi or smart stat.... you would need the C to only to power the wifi section.
The actual stat doesn't need C.

The A/C condenser connects to the Y wire and C at the furnace.

So when you add an EIM with two transformers.... the C is not needed from the furnace. The only need for C at the EIM is to run the electronics and supply C to any stats that need it. That C is coming from your aux transformer.
 
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Old 08-02-22, 06:19 AM
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That is a perfect way to state it, and makes perfect sense now. We're just moving around the 24v as the furnace itself sees it - duh me.

Thanks, Pete!
 
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Old 08-02-22, 09:09 PM
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Well, I could not get the two transformer configuration to work. I was finding that the EIM could not control the furnace appropriately when attempting to do so. Pretty much all functionality between the two pieces of gear acted just crazyily. As I tried to troubleshoot the problem I found the easiest way to tell if it still did not work correctly was to go into installer options and run the equipment test/check function. It simply would not control the heating, cooling and fan operations. As I tried troubleshooting the problem I used the test function to simply turn on the fan and monitored the voltage coming off the G terminal of the EIM and found it to be completely incorrect, both while the instructing the tstat to turn it on or off. The functionality of the tstat seemed to work correctly, and the LED's on the EIM seemed to also function correctly, but the voltage at the G terminal was flakey - sat at about 4 to 13 VAC. Played with it for a couple hours trying different things to no avail. I swore my EIM was faulty or damaged. I finally decided to get rid of the two transformer setup and simply go the the single transformer setup and put the jumpers back in-place on the EIM between R and RC and RH and upon doing this everything worked perfectly. I just don't get why the two transformer setup won't work. I'm just going to leave it as a single transformer setup.
 
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Old 08-02-22, 09:46 PM
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What EIM were you wiring ?
 
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Old 08-02-22, 10:11 PM
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Prestige IAQ Kit YTHX9421R7001WW that includes the THM5421R1021 Equipment Interface Module.
 
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Old 08-02-22, 11:02 PM
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For some reason I thought we were dealing with a Honeywell zoning control.
In that case two transformers is a must as the second transformer runs the dampers also.

With a wireless EIM.... you can run it on two transformers but the current draw is so low
it doesn't present a major draw.

I left the diagram if you wanted to use two transformers.
It's probably not wired like you had done it which would explain your erroneous voltages.
You'd measure from the points to furnace C.

 
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Old 08-06-22, 07:26 AM
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Pete,

For some reason I can't view your image above, or any image on the web site for that matter. They are all fuzzed out with a spinning cogwheel in the center of the image. Started happening to me about a week ago. Tried multiple systems and multiple browsers and all act the same way. It also appears that I no longer have the ability to post images and use other advanced functions - the options seem to have simply disappeared. Any thoughts why this is happening and is there anything I or you can do to correct this?
 
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Old 08-06-22, 08:31 AM
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The board is undergoing updates. Keep trying back as sometimes it's ok.
Should be back to normal soon.

To get an immediate picture... right click on it and click on copy image address.
Paste in a new tab. It will open and then will open in the thread too.
 
 

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