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Discussion Starter · #1 ·
Newbie here,

Setup: Dual Heat - Heat Pump with Gas Furnace as aux heat

I am replacing my thm5421c EIM with the thm5421r1021. It looks like a pretty straight forward swap except for one issue. The thm5421c EIM wiring has a jumper across the AUX1 and AUX2 connectors. AUX1 is hooked to the Heat Pump and AUX2 is hooked to the Gas Furnace so I am not sure why :glasses:they would also be jumpered together. When I swap out this EIM will I need to also jumper the AUX1 and AUX2 together?

Thanks in advance
Jim
 

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The wire coming from the heatpump is likely for defrost heat and the terminal was used instead of going straight to the furnace.

aux 2 is probably not active. you'll have to trace all the wiring to be sure?

What's the purpose of changing the eim?
 

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Discussion Starter · #3 ·
The Thermostat touch screen has stopped working on the left half of the screen and it is discontinued. The newer thermostats are not compatible with the thm5421c EIM. So, I have to replace both the thermostat and EIM. So back to my original question, should I wire up the new eim just like the old one and use the jumper?

thanks,
Jim
 

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I don't know enough about how your system was set up to give you an answer.

Keep in mind that stats when used on dual fuel are not plug and play - they require a lot of setup.
 

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Check the settings of what you have now - see if it's set for 2-stage aux heat or 1-stage.

If 1-stage, they just used it as a dummy terminal. If 2-stage, jumpering defeats the purpose.

Most heatpump basic defrost boards use the aux heat output just for defrost.

I haven't checked the manual of the stat you have now - don't know what it supports when it comes to dual fuel setups.
 

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Discussion Starter · #9 ·
Unfortunately, I can not access the settings because the left side of the touch screen is dead. However, I do remember having to check the hp/convenient crossover temp a few years ago. Also, the heat pumps do run in the winter time but may not be a true indicator if the installer did something wonkie.

Thanks
Jim
 
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