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Old 11-20-2014, 11:17 AM   #1
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Standalone transformer to power Honeywell EIM?


Hi all,

I may be getting way too creative here, but I have a question for those of you smart on the Honeywell EIM/IAQ systems...

I'm interested in powering my EIM using a standalone 24vac transformer. In a scenario where the Furnace Transformer loses power (i.e. its R connection "breaks" for whatever reason), I would like to still get alerts from the EIM and IAQ.

Referring to my diagram here (click image for larger version), I think(!) I have figured out how the R-wire connection could work... If correct, then the Furnace Transformer would still power both RH and RC at the EIM, but the Honeywell Transformer would power the EIM itself via the R terminal (and thusly would also then power the IAQ downstream).

Does that pass the sanity check so far?

If so, then here's where I'm stuck... What do I do with the other wire (C) from the Honeywell Transformer's other secondary terminal?

I assume it needs to be connected to something, correct? I'm not smart on transformer basics, but I'm guessing I have to somehow use both of the secondary terminals on the transformer. But maybe not?

Assuming it does need to be connected somewhere on the EIM, then where?

My best guess would be to put it in the C termimal at the EIM, but that would then put two C-wires in that terminal (one from Furnace Transformer, other from Honeywell Transformer), and I've been unable to confirm if it's kosher to double up wires in an EIM terminal.

If two wires are connected at that C terminal, would that cause bad power or interference issues when everything is operating normally (since both the Furnace Transformer and the Honeywell Transformer would be energized)? <-- That's what I'm concerned about...



Many thanks for your time and help!

Cheers, Chris

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Old 11-20-2014, 11:32 AM   #2
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Standalone transformer to power Honeywell EIM?


Is there power between R and C on the EIM with only the single transformer connected? If there is, you don't want to put another 24V on those terminals.

Is this a wireless thermostat? If so, I'd connect the wires between the thermostat directly to the new transformer and leave the original wiring configuration as it is on the EIM board.

I looked it up and see it is wireless between the stat and the EIM.
You should be find powering up the thermostat with a separate transformer, and leaving the original 24v wiring in place.
I'd just make sure the power going to the thermostat is 24VAC. It's possible they change the voltage internally on the EIM and send the stat something different.

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Last edited by 52Caddy; 11-20-2014 at 11:42 AM. Reason: more info
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Old 11-20-2014, 11:59 AM   #3
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Standalone transformer to power Honeywell EIM?


Quote:
Originally Posted by 52Caddy View Post
Is there power between R and C on the EIM with only the single transformer connected? If there is, you don't want to put another 24V on those terminals.
Right on... Currently the R and C on EIM are powered via solely by the Furnace Transformer's R and C (on the EIM, R is jumpered to both RC & RH). I hear ya 100% on being concerned about tossing yet another 24v somehere into the mix on the EIM - I don't want to fry anything!

Quote:
Originally Posted by 52Caddy View Post
Is this a wireless thermostat? If so, I'd connect the wires between the thermostat directly to the new transformer and leave the original wiring configuration as it is on the EIM board.
It is indeed a wireless stat, and I completely agree that that a simple straighforward option is to just wire the Honeywell Transformer directly to the stat (and leave EIM's R and C wiring as-is).

That was the first thing I considered, but then I started to wonder if there was any advantage to having independent power to the EIM as well. And that answer may very well be "None at all"! That's what I'm hoping someone here can confirm for me... I'm not clear if I will be able to get alerts (such as temp out-of-limits advisories, delta-t alerts, etc) if the EIM is dead.

Bottomline here -- My biggest concern is being able to get an alert for temp out of limits (temp too high indicating AC shutdown, or temp to low indicating furnace shutdown) - is that alert generated by just the IAQ stat itself, or does it require that that EIM also have power too?

Thanks for your help!
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Old 11-20-2014, 12:22 PM   #4
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Standalone transformer to power Honeywell EIM?


Quote:
Originally Posted by Chris130 View Post
Bottomline here -- My biggest concern is being able to get an alert for temp out of limits (temp too high indicating AC shutdown, or temp to low indicating furnace shutdown) - is that alert generated by just the IAQ stat itself, or does it require that that EIM also have power too?
Going back to the big ol' Honeywell manual for the umpteenth time , I think I've answered this... No EIM seems to be required to get an temp-out-of-limit alert - from what I found, it sounds like the IAQ (i.e. stat itself) can handle that even if the EIM is dead.

So... I'm leaning toward the KISS solution - just use the Honeywell Transformer to power the only the IAQ stat (and leave the EIM alone).

That being said, just for giggles, I'd be curious if any of the pro's out there have any thoughts on wiring a standalone transformer for the EIM.

Thanks again!
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Old 11-20-2014, 12:38 PM   #5
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Standalone transformer to power Honeywell EIM?


From a commercial controls standpoint, we do that all the time. One power supply for the controls and another for the devices on the units. I can't imaging this would be much different, but it's always good to get a second opinion, especially when it's your house!
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Old 11-20-2014, 06:42 PM   #6
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Standalone transformer to power Honeywell EIM?


Don't connect the common from the furnace to the EIM's C. It not needed.

Just remove the jumper from RC to R as you said. And connect your separate transformer to R and C.
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Old 11-20-2014, 06:55 PM   #7
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Standalone transformer to power Honeywell EIM?


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Don't connect the common from the furnace to the EIM's C. It not needed.
Eureka! I can't believe that never even occurred to me, but that makes complete sense now that I see it - thank you!

Quote:
Originally Posted by beenthere View Post
Just remove the jumper from RC to R as you said. And connect your separate transformer to R and C.
Thank you for confirming that I wasn't missing something crazy with my R-jumper-switcheroo plan -- I appreciate that sanity check.

THANK YOU, beenthere!

I'll be working on this rewiring tomorrow night, so I'll post up an after-action report ASAP.

This site is so full of win: I'm very grateful for everyone's help

Cheers, Chris
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Old 11-20-2014, 08:08 PM   #8
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Standalone transformer to power Honeywell EIM?


Its what the site is for.

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