I'm the new owner of a residential geothermal HVAC system and have encountered some troubling surprises concerning the way the system uses the aux (gas) heat. First, I learned that using the programmable setback feature of the t-stat is not advised, because: "If the geo is called upon to raise the temperature more than 2 degrees, it will trigger the aux heat, which is much more expensive to run." What's more, "when the aux heat is triggered, the geo shuts down," thus deriving zero benefit from it.

Is this correct, and is it necessary? I can understand that an A-coil mounted above the furnace (that is, in the path of the gas-heated air) won't be able to transfer much, if any, heat to air that has been preheated, perhaps even to a temperature hotter than the geo fluid. If that is the reason behind shutting off the geo, then couldn't the A-coil be placed ahead of the furnace??

And if there is no getting around it, is there a way to make the system run only on geo? Could it lock out the aux heat as long as the indoor temp remains above a programmed threshold? There are many times (overnight, for one, or when the house is empty during the day) when I would choose economy over comfort.

(The system consists of a Bryant Envision 120,000 BTU gas furnace and Waterfurnace NDZ064 with de-superheater and 5-ton loops; NRAC 060 coil; Honeywell VisionPro IAQ thermostat.)

Thanks for any insight, options, suggestions, or other help you can provide.

- MM / montgomd