Another way of looking at this might be to figure out how many tons or BTUS those heat pumps would be. I don't know heat pump ratings, but they are going to have to be in the range to heat a house. So for those 4 small condos in DC, let's say the total capacity needed is 100,000 BTUs. A typical window AC unit is 6,000. So, when you're using the heat pump to heat the house, it's like having 17 window AC units pumping cold air into the basement. It doesn't sound like it would take very long for that to drop the temperature way down. And the lower the temp, the less efficient the heat pump is, the less heat you get out, etc. In the summer, you would have the reverse, with the equivalent of 17 window AC units pumping their heat into the basement.