My old furnace was hilariously oversized for the house.

One of the nifty things about smart thermostats like Ecobees is that you can pull usage data from their web portal. I grabbed a CSV file covering a cold snap last year that reached a 100-year record low, and using Excel I summed up the total heat output while we were at that low.

The furnace was only running 50% of the time, even when it was with a couple degrees of as cold as it’s ever been where I live.

Needless to say, when I got a new system installed I made sure it was more properly sized, and given that I had a convenient empirical measurement of exactly how many btus I actually needed in the worst case as scenario, that was easily done.