I have installed a 3-ton comfort aire geo max 2 system.  System works fine but when I raise the thermostat to engage the aux heat strips it trips the heat pump into emergency heat mode and shuts down the compressor.  Has anyone heard of this before?  I am using a Aprilaire model 8400 thermostat with 3 stages heating two stages cooling.  Bothe stages of heat pump enegize correct.  Do you think this sounds like a programming issue with the thermostat or the heat pump?

Thanks Richard

Sounds like a wiring/thermostat issue to me. Maybe a jumper on Tstat between W and W2.
Check your configuration on Tstat.

I'm not familiar with the Aprilaire thermostat, but if the thermostat is like a Honeywell 8000, it is a thermostat programming issue. If it is like a White Rogers t-stat series 90, the T-stat may not be capable of controling 3rd stage truely as a third stage. The older models of the series 90 would not let stage 1 and 2 run when 3rd stage was called upon.

I would get a different thermostat. We use Climatouch

I'm not an expert, just a homeowner. How much are you raising the thermostat to try to trigger the aux heat? Does the thermostat actually say "Emergency Heat" when the compressor shuts off when you raise the temperature?

Just a thought regarding the thermostat being configured correctly for your system. If aux is setup like a furnace, could the thermostat be instructing the compressor to turn off and engage the aux or emergency heat only?

Aux and emergency heat are two different modes. They share a commonality in that both energize one or more stages of strips. The difference is that emergency heat mode stops the compressor, but aux keeps the compressor going and adds resistive heat to compressor's output.

engineer, I agree when we are speaking of aux/emergency heat strips. But when geo is combined with a furnace, doesn't the compressor normally shut off to protect the compressor? Could an incorrect thermostat setup for the wrong aux/emergency heat type account for this problem? In this case, would the thermostat show "aux" or "emergency" (if the thermostat has that display capability)? Just asking questions to see if we can figure out if this is a wiring problem or a thermostat setup issue or something else. Another thought, could too great of a temperature differential trigger emergency heat without a lockout condition?

3 real possibilitys
1 defective stat , no program option I have ever seen will trigger emergency heat when you raise the stat to far above room temp so programing is out
2 wireing snafu by your controll guy which is ( on a call for aux ) backfeeding to emergency which the stat interprets as its own output signal and displays as such and shuts down the compressor ,

3 same as 2 but IFC board in the unit is the source of the short and causeing the back feed

Im not 100% sure that the theroy expressed in 2 is possible ( dont do electronic power in power out ) but I would test for it before replacing stat , the test would be to remove the emergency heat wire ( most cases L or E ) from the back of the stat and see if the stat operates normally if not replace stat

Thank you all for your help.  I finally figured it out.  Just a simple programming step I over looked.  I had the dip switches adjusted for heat pump so no problem there but that still did not switch the operation of the fan to be controlled by the thermostat.  When the thermostat called for Aux heat heat pump had no fan signal and would go into emergency heat mode.  All is well.  Thank you again.