66F is ridiculously cold to me, and I live in Canada where it can reach -40(F or C) in the winter. I would find that very uncomfortable and elderly people would be shivering constantly and highly susceptible to respiratory illness.

I have a modern cold climate air source heat pump which essentially needs to run 24 hours a day to maintain a stable 20C when the outdoor temperatures reach -15C. Below that, the heat pump shuts off and the furnace kicks in to provide emergency heating. My thermostat is a modern one with full time-of-day and day-of-week scheduling for heating and cooling, but it doesn't matter because the heat pump by itself is not able to swing the temperature up (by even half a degree) on its own, so this causes the furnace to kick in every time the schedule calls for a higher temperature, defeating the entire purpose of time-of-day scheduling.

I will also add that where I live (Southern Ontario) the sky is overcast 90% of the time during the winter. Solar panels, even somehow free of snow and ice, are going to produce almost nothing on those dark days. Add in the need to keep the panels free of snow and ice (presumably with heating, since nobody is going to be climbing around on their roof in the winter), and you'd likely reach energy net-negative trying to make use of them.

Modern heat pumps can heat efficiently at -25 degrees Celsius or even lower. Not sure if they are available in Canada, but in Scandinavia (similar climate) they are pretty common.

I also agree that 66F (about 19 degrees Celsius) is not comfortable during day time. It is fine for sleeping temperature. During winter homes in heating dominated climates typically have higher indoor temperatures. One advantage of lower inside temperature is that relative humidity stays slightly higher when it is very cold outside.

I actually live on the same latitude as Ontario so -40F/C is not unusual. Add in windchill, and it gets even more common, given my windy location.

Yeah, I understand I'm probably an outlier at 66F. I was using the numbers more to point out how little a house temperature will drop with good windows and insulation.

People acclimatize pretty well if you let them. We keep our house at 65F all winter, and set the AC for 85F in the summer and everyone is pretty happy. The payback period on a good sweater is not very long.

“elderly people would be shivering constantly and highly susceptible to respiratory illness.”

At 66 degrees F? That sounds like put a sweater on if you’re chilly, not some near death extreme.

Any evidence that such an ‘extreme’ would cause issues?

>66F is ridiculously cold to me...I would find that very uncomfortable and elderly people would be shivering constantly and highly susceptible to respiratory illness.

I know people who live in the Mediterranean and get by with no heating during the winter with indoor and outdoor tempuratures this low or lower, so it seems that one can be conditioned into doing so.

Perhaps it's the presence of more sunlight on average rather than the temperature that makes the difference.