Technology Connections just released this video explaining that furnaces are massively oversized, and several methods to see how oversized your furnace is.
My furnace is a single-stage, meaning it's either on, or off. This makes calculating how oversized it is easy. On the coldest day this year (Jan 13), we hit a low of 16F and a high of 24F (about the coldest it gets here). I reviewed my furnace fan power consumption for that day, which came in at 5.56 kWh. My fan runs at almost exactly 700 watts, and only runs when the furnace is on.
My weather station
https://ambientweather.net/dashboard...b5b0a74/graphs
If my furnace ran at 100% capacity (on for 24 hours in the day), the fan would consume 16.8 kWh (24hrs x 700 watts).
So, my duty cycle on the coldest day is 5.56 kWh / 16.8 kWh = 30%
The furnace is rated at 100,000 BTU/hr, so 30% of that is 30,000 BTU/hr. That means that a 2.5 ton (12,000 BTU = 1 ton) heat pump would have just maintained heat running at 100% capacity.
The real problem I'm trying to solve is to even the temperature differential between upstairs and downstairs. It's typically 6-10 degrees cooler downstairs. With my furnace blower only running 10% of the time on average during the coldest month, there's not much mixing occurring.
My thought is to replace my 5-ton, 10 SEER AC unit and 80% efficient gas furnace/blower with an inverter heat pump. Inverter heat pumps can run at any output from about 35% to 100%. They are designed to run continuously instead of turning on and off. My assumption is that running continuously will keep the air mixing between upstairs and down, somewhat evening out the temperature differential. My other assumption is that I can close registers to force more heat downstairs without risk of stressing the ductwork since this system would be running substantially less pressure.
Other minor benefits include substantially less noise from the blower and furnace.
Other thoughts are to install a door between upstairs and down, and to increase soffit ventilation since mine is such a joke, leading the attic to get very hot in the summer.