The Truth about The Furnace Efficiency Rating 
Three quotes on your counter. Numbers on them running 80, 92, 96, 98. Every guy who wrote one told you his was the smart buy, and the furnace downstairs is old enough to vote. So here’s what I’d say standing next to it: the biggest furnace efficiency rating in that stack usually isn’t the one I’d put in, and it’s rarely why your bill moves.
What the Percentage Does to Your Bill
It’s how much fuel becomes heat instead of going up the flue. Eighty percent, a fifth of your money leaves as exhaust. Ninety-six, you’re down to about four cents on the dollar.
Old two-story off Division, few winters back. Plaster walls, original storms, 80% unit that had been down there since the nineties. Their January bill kept coming in past $300 and they’d just decided that’s what winter costs. After the swap, $215. She called me in March about it. Nobody calls in March.
Why that number was so big, though: that house leaks and they keep it at 71. Same furnace in a tight rambler where everybody wears a fleece indoors you’d hardly find the savings on the bill.
Why I Land on 95 or 96 Most of the Time
Our season runs long enough that the math here isn’t the math two states south. Yours isn’t kicking on a few December evenings. It’s October into April, and some years I’m still taking calls in May.
There’s a catch going from 80 to anything in the 90s and it’s the one that blows up budgets. Right now your furnace shares a chimney with the water heater. Pull the furnace, vent the new one out the sidewall, and that water heater’s alone in a flue built for two. Draft goes bad. Now you need a liner, and nobody has a liner in their head while they’re comparing quotes.
Past 96 you’re paying real money to save a few bucks a month. I’ve quit bringing it up unless somebody asks twice.
The Part Nobody Mentions Until Install Day
High efficiency comes with hardware your old unit never had. Two PVC pipes out through the rim joist, and a drain.
The pipes have to stay above the snow. Every February I get calls that turn out to be a drift packed against the intake furnace locked out, homeowner already writing the angry email. Equipment’s fine. It couldn’t breathe.
Then the condensate. Couple gallons of acidic water a day, and it has to get somewhere warm to leave the house. Run that line across an unheated crawlspace and it freezes, backs up, quits on you at 2 a.m. on the coldest night of the year. Every time. Ask where that drain ends up before anybody cuts a hole in your wall.
Two Things That Cancel Out a Good Number
Oversizing first. Somebody sizes off the old nameplate instead of running the load calc and you get 20,000 BTU more than the house ever wanted. Fires hard, satisfies fast, shuts off, does it again. Rooms swing warm and cold all day and the furnace efficiency rating on that sticker never turns up on your bill. Ask to see the Manual J. Shrug means keep shopping.
Permits second. A furnace replacement here needs a mechanical permit and an inspection, and inspection is where bad venting and lazy gas work get caught. One bid comes in $1,500 under everybody else and nobody’s said the word permit that’s the whole explanation, usually.
Price It While You Still Have Heat
Wait till it dies and you’re taking whatever’s on the truck at whatever the after-hours rate is that night. SouthSota One Hour Heating & Air Conditioning can run the load calculation on your Northfield home while you’ve still got time to think it over and room to negotiate.
Frequently Asked Questions
Is 98% actually worth it or is that just the upsell?
Depends how long you’re staying. Twenty years in a house that burns a lot of gas, it pencils. Five years, no. Make them show you the payback on paper instead of pointing at the furnace efficiency rating.
My house is from like 1900 will high efficiency even work in it?
Works fine. Ductwork’s what holds you back, not the furnace. A lot of the older places near downtown Northfield got their ducts added decades after the house went up, and undersized returns will choke good equipment no matter what the sticker says.
Can I do this myself? I’m handy.
Gas line, venting, combustion air not where you want to find out you were wrong, and an uninspected install can void the warranty. Filters, thermostat wiring, keeping snow off the vent pipes, all yours. That last one matters more than people think.
How long am I without heat?
Straight swap on existing venting, four to six hours. Add a liner or new wall penetrations, it’s a full day. Plan on one cold workday and you won’t be mad at me.
Does higher AFUE mean it heats faster?
No. That’s capacity, BTUs, separate number entirely. AFUE only tells you how little fuel gets wasted along the way.
If yours has one more winter in it, take the time to research the benefits and get quotes in July. Cheaper, and everybody’s in a better mood.
