I’d been planning for a new HVAC system for a while when that video came out, and it gave me the idea to cross-check the thermostat data with the Manual J calc I’d already done. They were in general agreement, though the Manual J block load was more conservative than empirical data for a design day.

In your case, since you don’t have data from a healthy system on a representative heating design day, I’d suggest using a web tool like CoolCalc to simply calculate an approximate Manual J total heating and cooling load, and use that to guide your choices.

A little headroom ain’t bad, but it had three times the required heating capacity for my area’s “design day” low, which meant that for most of the winter it was kicking on for maybe 5-10 minutes per hour and then leaving massive cold spots in the house, because the thermostat was smack in the middle and all the walls were bleeding heat.

My new heat pump is just about 2x the design day heat requirement, but that also means it’s got capacity to handle extreme lows without resorting to resistance heat, and in any case it’s fully modulating so the house has stayed quite comfortable so far.

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.