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I looked into this, What is full model number THM5421C1008/U ??? is that it?
IF it is, The thermostat you purchased will not work and you will need to get the thermostat that matches that equipment which is TH9421 VisionPRO IAQ Thermostat
How many wires do you have at that the thermostat now?
would you like to work on this?
Alright ill hold tight if you have any questions ill help ya.
If you wanna try and fix it we can ?
Just wanna touch base with you and see if Honeywell got ya squared away or if you have any questions.
IF you completely changed thermostats i would recommend Sensi if its not too late. Great easy thermostat!
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