One other obvious point -- smaller furnaces running longer tend to make for more comfortable houses and lower fuel bills. If you size the furnace to keep your house toasty on the "coldest day in 5 years" that's probably bigger than you need... next time I would go with something sized to "keep the house comfortable on the colder days running most of the time" and put on a sweater or light a fire on the occasional freeze-ups.
Spot on, John. That's why there are multi-stage furnaces that will match their firing rate to what the house needs. Far more efficient heat delivery and comfort than a single stage unit.
I guess having the furnace burning relatively more of the time would translate into a shorter heat-exchanger life but I've never actually heard that discussed.
Actually, just the opposite. The thermal expansion & contraction from repeated short heating cycles, causes more wear and aging than fewer, longer, heating cycles. Similar to a car with mostly "highway miles" vs "city miles"
