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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
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Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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I am not at the house, so I cannot check the heater but, I

Customer Question

I am not at the house, so I cannot check the heater but, I had a Honeywell VisionPRO® IAQ Total Home Comfort System that was digital and does not require batteries. The T\Stat display died (would not light up at all) and the heater light was indicating "no call for heat" . My friend, who's house it is bought a New Honeywell Rth2300/Rth221 digital T/stat that does take batteries. The old unit has a wire harness with 3 wires labeled 1 2 and 3. The new unit has terminals labeled B,O,G,Y,W,R,Rc and one for not used. Is this unit compatible? I put the 1 2 and 3 wires in the B, W and R terminals to basically match colors.
Submitted: 1 year ago.
Category: HVAC
Expert:  Brian HVAC Guy replied 1 year ago.

Hi I'm Brian and I'm here to help!

The new thermostat is NOT compatible with the Honeywell IAQ system board (EIM board). That system uses a proprietary thermostat that just receives power from the EIM (or another source) and the thermostat is wireless to the EIM. In order to go to a standard thermostat like the new one, you would have to eliminate the EIM panel entirely and also have enough wires in the thermostat cable between the thermostat location, the EIM location and the unit location. If you do, then the rewiring can begin. It may not be a simple job if you want to revert back to a standard thermostat.

Is it possible that the EIM has just lost power? That would give you the symptoms you have now. If you have a volt meter and knowledge of its use, I can help trouble shoot this. It may require you to upload pictures here.

Let me know how you want to proceed.



Customer: replied 1 year ago.
The old unit was digital as well and I know that there is an IEM between the T\stat and the heater. Unfortunately, I am not currently at the house where the T\stat in question is but I have the IEM cover Model# ***** When I was there, the new T\stat never powered on so I put the old T\stat back and it powered on for a little while before dying again. I am pretty sure the contacts are tightly secured.
Customer: replied 1 year ago.
I do not have a volt meter, I may be able to borrow one but don't really know how to use one.
Expert:  Brian HVAC Guy replied 1 year ago.

Yeah, so that could either be a wiring issue between the EIM and the thermostat or the thermostat may have indeed failed. It would be very unusual for an EIM to show those symptoms. If you can verify that there is ~24 volts ac at the EIM R and C thermostat terminals and the thermostat is not powering up and the wiring between the EIM and the thermostat wire is ok then you do indeed have a failed thermostat. The owner would have to make a choice (based on what option is available as stated above) on whether to replace the 9421 thermostat or to revert to a conventional thermostat. Bear in mind if a new IAQ stat is chosen, it will have to be set up with the installer set up since the programming actually resides in the stat and not the EIM.

Let me know if you have further questions on this subject. Otherwise, please remember to rate before leaving (3 stars or higher please!). Bonuses are always appreciated! Thanks for allowing me to help and thanks for using Just Answer!

Thank you,

Customer: replied 1 year ago.
Thank you. So, if the EIM is not getting the correct voltage, then I would need to bypass the EIM and possible re-run the wires. If I remember correctly there are only 3 wires on the T-Stat side. Does the EIM only work with the IAQ T/stat? I would like a cheaper option, and would rather not fish new wires through the walls.
Expert:  Brian HVAC Guy replied 1 year ago.

Sorry for the poor grammar, sometimes my fingers get ahead of my brain :P

Expert:  Brian HVAC Guy replied 1 year ago.

Wow that was strange... I had a long response to you last post and it just disappeared! I'll try that again but it may take me a few to remember what I wrote and re-write it! Sorry about that!

Expert:  Brian HVAC Guy replied 1 year ago.

The short answer is yes, the EIM only works with a particular thermostat. I think there may be two or three available now that will connect to the EIM. The stat and EIM communicate wirelessly with the Honeywell Redlink wireless protocol so you have to choose a Redlink wireless thermostat. Confirming the unit is supplying the EIM proper 24 volts ac then the EIM is sending 24 volts ac over the wires to the thermostat would be the first step to proper diagnosis. If the stat is receiving proper 24 volts and not staying powered up, it is shot and will need replaced. I think the most current model is a TH8321 thermostat but I'll check that and post back.

Customer: replied 1 year ago.
Thank's I would rather not spend a lot of money, my friend is renting the house. Can I get in touch with you in a day or so, if I can get my hands on a volt meter?
Expert:  Brian HVAC Guy replied 1 year ago.

Sure! This question will be open to you for follow ups, even after positively rating, for 30 days. FYI Click Here for the current cheapest Honeywell stat that will connect to an EIM. It can be FULLY wired and stand alone (without EIM) or just power wired and wirelessly connected to an EIM.

Customer: replied 1 year ago.
Thank you, ***** ***** get my hands on a multimeter and reach back out to you in 1-3 days.
Expert:  Brian HVAC Guy replied 1 year ago.

Very good then, Ill keep and eye out for your next post.


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