Nest compatible with Honeywell

[ChrisSv]

I  have a geothermal heating and cooling system with two zones. Upstairs I have a Honeywell TH 8320U1008 VISION PRO TH8000 series. It has 7 wires going to it. Downstairs I have a Honeywell TH9421C1004 Vision PRO IAQ. I have a THM5421C interface module. 

My question is if the nest is compatible with both thermostats?

I have attached some pictures of the wiring.

ChrisDownstairsInterfaceUpstairs

[CoolingWizard]

@ChrisSv ,

The simple answer is yes. However, the IAQ thermostat will need to be rewired. On the THM5421C IAQ module, the HVAC equipment is connected on the right side. And the thermostat is the 1,2,3 wires on the top left side. You also appear to have a Dehumidifier and a D90 ventilator attached.  You would need to the wire nut the wires on the right to the ones on the too left. That is color to the same color. This will attach the thermostat location to the HVAC equipment. We need to understand the equipment attached on the lower left VNT terminals. The Nest Leaning Thermostat can deal with dehumidifier, but not the second ventilator at the same time.  That is, if it is indeed a DR90.

The concern is the two cables attached to DHUM and VNT. Those cables go from the IAQ Interface Module to the equipment.  Those cables need to be where the thermostat is.  

AC Cooling Wizard 

[ChrisSv]

Thank you for your response. On the IRQ module I have a humidifier and a air exchanger connected. They are the red and white wires on the lower left side. The wires on the right side go up to a Honeywell HZ 432 Zone 1.  The other thermostat TH8000 Series goes to the HZ432 Zone 2. Do you still think it is possible. I asked Honeywell if one of their Wi-Fi enabled thermostats would work. they jus referredHZ432Overview me to a professional company. 

[CoolingWizard]

@ChrisSv ,

That configuration makes no sense to me. You would not wire the Thermostat connection in the ST zone controller to the HZ432 Thermostat Zone inputs.  That would be an Input connect to an Input. Is this a commercial property?

AC Cooling Wizard

[ChrisSv]

Don't know what to tell you. It is a private home. Has worked for 14 years. (Except for some damper motor replacements). I think I will give up the nest idea and just buy a Wi-Fi thermometer so I can keep an eye on things when away. I have to do a lot of reverse engineering to document how this was put together when installed, and to find out if it was correctly done. The company that installed it went belly up. I was just looking at monitoring and manage temperatures remotely. I wish the installer had left a wiring diagram. Any comments?

[ChrisSv]

Looked at the manual for the interface. I believe the top righthand 5 wires going from the interface to the HZ432 is an output to an input.

[CoolingWizard]

The Nest will still work you just need to be careful how to configure the system. And, add the common wire I suggested. 

AC Cooling Wizard 

[ChrisSv]

Thank you for your insight. You comments were very helpful.

[Markjosephp]

Hello folks,

I wanted to ensure that everything was covered here.

@ChrisSv, I hope you've found the answer you're looking for. If you're still in need of assistance, feel free to let us know.

I appreciate your help here, @CoolingWizard.

Regards,

Mark

[Jenelyn_O]

Hi everyone,

I wanted to follow up if you still need help. Please let me know if you’re still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

I appreciate the help, @CoolingWizard and Mark.

Best,

Jenelyn