topic Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module in Nest Thermostats

Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

bobalou — Mon, 20 Jun 2022 22:01:57 GMT

I have a 3 wire Honeywell thermostat that connects to a equipment interface module which connects to the circuit board on my AC/Furnace. There is no humidifier or dehumidifier. Looks like I’ll have to run a new 7 wire to make this work. Any help with connections is appreciated.

Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

Thaler — Tue, 21 Jun 2022 16:23:08 GMT

The same question as I have. Unfortunately still no solution. Do you mind sharing your HVAC equipment setup? What exactly do you have as your AC/Furnace?

Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

bobalou — Tue, 21 Jun 2022 22:12:02 GMT

Hi Thaler, my HVAC is a Trane XV 80. No humidifier or dehumidifier. Wish nest would put out diagrams for install. It’s not like Honeywell 3 wire thermostats routed to an equipment interface module thermostats are the newbies on the block!

Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

Thaler — Wed, 22 Jun 2022 01:17:03 GMT

You still have all wires marked on the control unit installed on your HVAC system, right? Did you try to use it with the online checker tool? If there are no complications like multistage heat-pump along with your gas furnace, it should work.

Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

bobalou — Wed, 22 Jun 2022 15:48:14 GMT

Wires are still attached to the interface and functional thermostat. I live in Georgia where temps have been very hot and my attic is around 110 degrees. So I'm delaying this until fall when it's cooler, plus I don't want to have A/C problems with the heat. System is a furnace and standard A/C. I did hook up a nest at my daughters (heat pump) and it was frustrating. Doing a pro install would have helped. There is a pro install PDF which has all the possible installations here's the link: http://support-assets.nest.com/images/pro-faq/Nest-Pro-Installer-Guide.pdf I'm not familiar with the online checker tool, I'll search for it. Good luck and let me know how it goes.

Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

Jeran — Wed, 17 Aug 2022 17:17:08 GMT

Hey there,

I apologize about the late response on this thread, I appreciate your patience. Just wanted to double-check to see if anyone still had any questions. If so, I'd recommend reaching out to Honeywell Support, as they would know a bit more about their systems than we would on the Nest Forum.

I look forward to your response, and if you have any other questions or concerns for me, please let me know!

Best regards,
Jeran

Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

Jake — Mon, 22 Aug 2022 18:01:20 GMT

Hey there,

I wanted to check in and see if you managed to see Jeran's post. Please let me know if you have any questions from here. I would be happy to assist, and make sure you are good to go.

Best regards,
Jake

Re: Honeywell TH9421C1004 Thermostat wired to Honeywell THM5421C Equip Interface Module

Jake — Tue, 23 Aug 2022 17:38:44 GMT

Hey there,

I want to ensure you are good to go, and everything is working properly. Please let me know if you are still having trouble, as I will be locking the thread in 24 hours due to inactivity.

Best regards,
Jake