I live in a 2400 sf century house that has been renovated including roof, siding, windows and is fairly well insulated and sealed. We had the geothermal system installed in 2012, the unit is a Bosch ta049. We have a closed ground loop horizontal. We found that the auxiliary was coming on a lot during the coldest days (Dec.-Feb.) with hydro bills in the $800 range for each of those months where usually we average $300. I had the system inspected and they told me that it was fine although the temp of fluid coming in was 31F and 36F going back out. ( the tech acknowledged that was i bit small of a difference) The temp at the plenum was 79F. I had the tech disconnect the aux. ( he disconnected it at the low voltage lead). ( My sister and another friend have very similar systems with the aux disconnected and their homes are warm ) With my thermostat set to 65F I went to bed. I got up at 6am and the house temp had dropped to 60F. It will get up to about 65F during the day but not any more than that. I have had the worst experience with customer service imaginable and am looking for second opinion from anyone who knows this system. Unfortunately, I don't trust the people who sold/installed it.