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BillyHvac, Journeyman HVAC Tech
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Connect fresh air damper to blower unit and bypass HW EIM

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Where does the fresh air damper connect on the HVAC blower unit (Lennox Model CX34-38A-6F-3)?

I am trying to figure out how to bypass a Honeywell Equipment Interface Module THM5421C to allow me to install a different thermostat. The existing thermostat wires are connected red to 1, green to 2, white to 3 on the left side top there is a wire from a damper going to green to C on the left top and red to VNT 2 on the left bottom and there is a wire from the unit with white to W1 O/B, yellow to Y, green to G on the right top, and then blue to C and red to R. There is a jumper from R to RC and RH on the left top and a wire jumper from RH to VNT 1 on the left top to bottom.


The HVAC guy I just paid a $59 service call to look at wanted $600 to bypass!


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Customer: replied 4 years ago.

I still need assistance if someone is willing to answer. I can add take pictures if needed.



Thank you, XXXXX XXXXX continue to look for a professional to assist you. Please let me know if I can be of any further assistance while you wait.




could you please verify the furnace number? Found behind upper door in furnace

Customer: replied 4 years ago.

Lennox Model CX34-38A-6F-3 is on the outside. I will have to look tonight if there is a different number inside.

The numbr given is an ac coil number

Customer: replied 4 years ago.

Ok. What does the furnace number look like?


Depends on brand.

If the furnace is also a Lennox it will like ressemble G40, G23, ML180...

Customer: replied 4 years ago.

It is a G40UH. I found the following wiring diagrams on the Lennox website.

Thank you,

How many zones do you have? (thermostats)

Customer: replied 4 years ago.
2 but the fresh air damper and EIM (and new thermostat I'm trying to install) are all on one unit/ zone. Just really need to know where to move the wires from the damper to on the main unit to bypass the EIM. you just need to keep the dampers but move them to an auxillary set of contacts in the EIM?
Customer: replied 4 years ago.
Right now everything goes through the EIM. I have a new thermostat that needs to bypass the EIM and I have all of the wiring and that part figured out. I need to the disconnect the fresh air damper from the EIM and connect it on the main unit - that is the part I'm not sure about.
Now I the damper 24v or 120v
Customer: replied 4 years ago.
I don't know. How would I tell? It's buried under a bunch of insulation.
At the damper will be a damper motor that opens and closes the blade damper.

May I ask - Is there a reason you are utilizing a mechanical damper instead of a fixed damper that does not require operation?

Customer: replied 4 years ago.
The damper is already there. I was am trying to install a thermostat that I can automate but it needs to bypass the EIM.
I understand and there is a way to do this from the furnace.

But I need to know what your intent is with the vent damper.

You see less than 50 % use a fresh air damper, and then 99% of those use a fixed nonmotorized damper.

So when I find one I ask about how you use the system. Does your damper simply open everytime you heat/cool or do you have a logic that measures humidity and amount of air.?

We can have the furnace open this damper during heat or cool cycles...or fan on. I just need to know how you intend to use it.

Then I can instruct on how to hook up.

Customer: replied 4 years ago.

I guess it really isn't a fresh air damper. I found my EIM install manual from the builder and attached a picture of the wiring diagram. It is wired like the top right "Typical hookup of non-powered ventilation". The bottom left shows the wiring for a fresh air damper and that is not what it is. I am really not sure what it is for, I sort of doubt it is for humidity control - we live in the south, very humid, usually trying to get humidity out. There is a humidity reading on the thermostat. If I knew which letter or wire from the furnace or thermostat drive the connection between VNT1 and VNT2 that would be the answer. The other side looks like it connects to "C". The damper and everything is under insulation, only a small brown wire going into the insulation.

From what you is a 6" damper to the outside air which is closed when the unit is not heating or cooling. When it calls for heat or cool the damper rotates open and allows outdoor air to come into the system.

Most units do not have this auto feature and are just a manual damper set for open.

We can solve this easy enough you want the outdoor air feature?

Customer: replied 4 years ago.

I can't really tell if it is closed when not heating or cooling (I would hope it works that way). Why would one need this external source of air into the return air at all? I guess you say most just have a manual damper that is open - doesn't that make the unit less efficient? (FYI - I have a second system that is totally separate for the upstairs that does not have this damper or duct to the outside).


Anyway, yes - please tell me how to connect it properly to the wiring connection on the G40UH.



Here is the final fix...

If you want the damper to open in heating season = take the 2 wires from damper and put on W and the other on C.

If you want it to open during cooling = put one wire to Y and one to C

If you want it to open anytime the furnace heats and cools (which it does currently)

Then you will need this sail switch.

You would run a wire from R to the sail switch...then the wire would go to the damper, and back to C

So lets say you run a red and white 2 strand wire...

red leaves R and goes to sail switch on 1 terminal. Red leaves sail switch from the 2nd terminal....and goes to damper motor Now white leaves damper motor and returns back to C.

You can do this for less than $70.

Whenever the blower in the furnace creates air flow, it will open the damper.


BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22675
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
BillyHvac and other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

What if I did one wire to G and one to C?

Then the damper will only open in cooling and in fan on position.