Thermostat replacement

Problems related to residential installations.

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Thermostat replacement

Post by newbhvac » Fri Dec 22, 2017 1:41 pm

I have a Lennox G61 MPV with 2 stage heating and single stage cooling with non -powered humidifier and ability to run fan separately. Had a Honeywell Visionpro IAQ with TH9421C1004 thermostat and THM5421C1008 interface module. Thermostat failed, found out is discontinued and the subsequent model also discontinued. Only 3 wires (red, white blue) connect stat to module only labelled terminal 1,2,3. Manual shows terminal 1 is data to/from, 2 is power from stat (24 Vac) 3 is common from stat (24 Vac). I'm told by Honeywell the replacement that will work with the module is THX9421R5021 which has HD color screen and other features I don't want. I bought model Honeywell RTH8500D which is programmable touch screen and looks like my old thermostat. This one has terminals for up to 8 - Rc (jumpered to R), W, Y, G, C, Y2 and W2. I have full thermostat wire to the stat, and looking at the terminals and hookup diagram for the module the W1, W2 Y, G, R and C are connected to the module right side from the furnace side and R and C are connected to the left side of the module along with a jumper for non-power humidifier. I don't really need the humidifier and don't need to run the fan separately, just want to be able to heat or cool. Would I be able to disconnect the module wires, match the connections between the two thermostat wires by the module (one from stat and other to furnace) using wire nuts and hook up the appropriate wires at the stat terminals and then just operate the furnace directly from the stat? Stat can use batteries for power but the old one used the 24Vac for the clock and display and I think the common C (blue) wire would do the same. In would disconnect the wire to the humidifier terminal at the furnace board and may have to change DIP switch. Wife had contractor come over and they told her that wouldnt work, need to get the stat to match the module. Can someone tell me if the contractor is right and why what I am suggesting wouldn't work?

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- Thermostat replacement

Post by Freon » Sun Dec 31, 2017 10:13 am

Did you read the installation manual for the Lennox G61? I am sure your question can be answered there.

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- Thermostat replacement

Post by joycewhitaker » Fri Jan 19, 2018 11:31 pm

There may be some serious issue with your thermostat. If you still haven't got any solution for this, then I will suggest you to go with some professional licensed contractor who have years of experience. Last night I have read an article about hvac company NJ who provide such service with full safety and precaution. I hope my suggestion can help you out in solving your problem. Good luck!
A professional HVAC tech at *.com/air-conditioning-north-nj.html . Love to help people out!

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