Hit minus 7 this morning at 6 am (windchill -27!!!). So much for minus 4 design day for northern Illinois (between chicago/wisconsin)
Summary of my situation....
Old furnace system was 80K BTUH Bryant Single stage unit.
Home size - first floor about 1350 SF, finished basement about 1000 SF. 1964 home. Did not heat/cool finished basement before.
New system is 80K BTUH Carrier Infinity System. Added Ducts to heat/cool finished basement.
Calcs showed a load of about 70,000 BTUH using first floor + basement + future 400 SF addition. Winter Design day temp of minus 4 used.
Watched the infinity controller for a short time. Never saw it go to high stage heat but it did stay on low heat heating nearly all the time (50 mins/ hour) this morning.
Good thing I did not listen to the other bidders who wanted to sell me a 100k BTUH system. Their Logic = you had 80K + small future addition = 100K.
It appears to me I could have gone with a 60k furnace since it never went to high heat this morning - but perhaps a 60K unit would bee a tad short when I put on the family room addition next year. Glad I went with a two-stage burner and VS fan system in any case.
Should be interesting to see summer operation with new AC unit, which is also the infinity two-stage type.