80% vs. 96% Furnace: The Real Deciding Factor Is Venting

The sticker difference is efficiency: an 80% AFUE furnace sends a fifth of the fuel up the flue, a 96% keeps nearly all of it. But in replacement work, the real decision is usually the venting path, not the rating.

An 80% breathes through existing metal B-vent — drop it in, connect, done. A 96% condenses: PVC intake and exhaust out the sidewall, plus a condensate drain with freeze protection. When the venting path is ugly — interior chases, finished basements — the install cost gap can eat a decade of fuel savings on cheap natural gas.

Propane flips it. LP costs enough per BTU that the 16-point efficiency gap pays back fast, which matters across the WNC coves running on tanks. On propane, quote the 96% first and let the fuel math defend it.

Rule of thumb from the counter: natural gas with easy venting — either works, let budget decide. Natural gas with hard venting — 80% is honest value. Propane — 96%, almost every time.