7200 sq. ft. house. The house has a modern, gas fired condensing boiler with cast iron hot water radiatiors. With the foot thick stone walls of the house, this should provide very comfortable heating in the winter. However, the house has no central air conditioning, which we are going to install this fall. Because of the size of the house, we will probably do a geothermal system, and as far as I can tell, they are universally heat pump systems which both heat and cool. (As an aside, we have had a geothermal heat pump system in our house for 35 years and love it) My question is, how do the two systems, boiler and heat pump are used most efficiently in the winter. Is there a cross over outside temperature where on is more efficient? I recognize that there is peace of mind knowing that if natural gas prices go crazy at some future date, the heat pump can be used solo.