Here in the Great White North, I've never had AC at home. The new house has it, though -- required by the developer, to keep house values up, or something. We'll probably make use of it too, given the lack of trees on the lot. But all of our windows can be opened, and that will remain our primary means of cooling at night.

Meanwhile, it was -25C outside the other day. The furnace was set for 17C (but not running), and the inside was a balmy 22F. Passive solar heating courtesy of the South windows.

Can you heat your house with the AC running as a heat pump, or only with the furnace? Supposedly, "heat pump" heating is radically more energy efficient than burning gas or using resistive heat, but it doesn't work as well when the temperatures drop too low.

My mother's house has a heat pump in the reasonably large addition to her North Carolina house. It just can't keep up with very cold nights - in anything below 20F (not too usual in NC, but not too unusual, either); it runs incessantly. Those nights, she has to use the "emergency heat" setting, a space heater, or a couple fans to move heat from the gas side. The gas side is always comfy.

Perhaps the worst part is that in the rest of winter, the "heated" air blowing from the registers always feels downright cold. It doesn't blow warm enough air.

I can't speak to efficiency, though.

-jk

In this area, air-source heat pumps are apparently okay for all but the coldest 2-3 weeks of the winter. One of our new neighbours has one, which we can hear rattling away every day of the year. Thankfully, it's about 300ft away from our house.

The local vendors of such systems say they're a no-brainer for homes which don't have access to natural gas for heating. Otherwise, they're of similar operating cost (at present).

Cheers