The basic difference
A boiler heats water and circulates it through radiators, panel radiators, or infloor PEX tubing. A furnace heats air and blows it through ductwork. Both can reach 95%+ efficiency in modern condensing form.
Comfort: boilers win
Hydronic systems deliver heat at a much lower temperature differential than forced air — meaning no blasts of hot air followed by long cold periods. Walls and floors stay warm to the touch. There are no drafts, no whooshing sound from registers, and indoor humidity stays naturally higher in winter (a big quality-of-life win in our dry Duluth winters).
Cost: furnaces usually win up-front
A like-for-like 96% AFUE furnace replacement in a home with existing ductwork runs $4,500–$7,500. A new condensing boiler with a basic distribution system runs $8,000–$14,000. If you already have ducts, the furnace path is cheaper. If you don't, the gap is much smaller because installing ductwork in a finished house is expensive.
Lifespan: boilers win
A well-maintained condensing boiler will run 20–25 years; a furnace typically 15–18 years. Cast-iron sectional boilers we service in Duluth homes routinely run 40+ years.
Central AC: furnaces win
If you want central air conditioning, a furnace makes it trivial — the same ductwork handles cooling. With a boiler-heated home, you'd add a ductless mini-split system or a dedicated AC duct system. Both are perfectly viable; just adds cost.
What we recommend
For new construction, we usually recommend a boiler with infloor radiant heat for the main level and panel radiators upstairs — best comfort, lowest energy use, longest lifespan. For retrofits, the existing distribution system usually wins: if you have ducts, replace the furnace; if you have radiators or baseboard, upgrade the boiler. We will model both for any home where it could go either way.