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thm5421c with jumper on Aux1 and Aux2

771 Views 8 Replies 2 Participants Last post by  jmreynolds
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
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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?
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?

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.
We have two systems so I'll clone the thermostat settings from the other system. Hopefully, I won't have any problems.
Is the other system duel fuel?
Both systems are dual. Heat pump for AC and Heat pump for heat until outside temp drops below 35F then gas forced air kicks in.

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.
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.

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