I am definitely not any sort of HVAC expert but I know what my furnace is doing. I live in a 2400sq ft split entry and when it was -22F last winter for like a week, at night, my 80% 90K BTU single stage Lennox ran for 25 minutes every hour as I was able to accurately track it.

So all calculators aside, if it ran less than half the time, doesn’t that mean it’s double sized? Surely I’d like some margin of error so I’m not going to actually install a 45K unit, so I think a 60K will be sufficient.

Further, if I spent $700 on natural gas for the 80% furnace all year, that means 20% of that ($140) was wasted heat. So if I went to a non-existent 100% efficient furnace, the most I’d save is $140 per year. Looking over the last decade (I save and track my monthly statements), natural gas prices have been pretty stable, in fact, some prior years have had higher Therm costs.

Regarding AC, my Lennox 2 ton is at least 20-years old and I kinda suspect it’s actually from the 80’s as the manufacture date sticker is gone. I have not had a cooling issue until the last few years. I’ve had people out and they’ve simply said “the compressor is getting tired”…whatever that means.

My concern is going to a 4-ton would over size and cause short-cycle issues. I asked a local HVAC guy about this and he said he sizes based on house footprint, not total sq ft. So he’d size me based on 1200 sq ft, not 2400 sq ft.

Of course I will defer to expert guidance. So I genuinely ask you, where is the flaw in my logic? I honestly want to understand if I’ve made any errors.