Probably heatpumps, depending on the climate. Heatpump effectiveness drops off rapidly as you head below 0C. Considering the temperature extremes that southern

Probably heatpumps, depending on the climate. Heatpump effectiveness drops off rapidly as you head below 0C. Considering the temperature extremes that southern US versus northern US housing experiences over the entire year, it's probably easier to keep the southern house around 21C than the northern one.