Two furnaces, same brand, same size, sitting on the same shop floor. One is rated 80% AFUE. One is rated 96%. The 96% will burn about 16% less gas for the same heat. That sounds like an easy call until you find out why it costs more to install, and how little gas a typical Puget Sound home actually burns in a winter that rarely drops below 25°F.
Here's the honest breakdown we give homeowners in Federal Way, Kent, Puyallup and Olympia when they're standing in the basement deciding.
What AFUE actually measures
AFUE stands for Annual Fuel Utilization Efficiency. It's the percentage of the fuel you pay for that ends up as heat inside your house over a full heating season.
- 80% AFUE: for every $100 of gas, about $80 becomes heat. The other $20 goes up the flue as hot exhaust.
- 96% AFUE: for every $100 of gas, about $96 becomes heat. Only about $4 leaves the building.
The difference isn't marketing. A condensing furnace has a second heat exchanger that pulls so much energy out of the exhaust that water vapor condenses out of it. That's where the name comes from, and it's also where every complication in the install comes from.
The cost gap is about venting, not the equipment
The cabinet itself is only part of the price difference. The install scope is the rest.
In standard Washington markets, furnace replacement runs $4,000 to $8,000 installed. An 80% unit generally lands in the lower and middle of that. A 96% condensing unit typically lands at the top of it, or into the $5,000 to $10,000 range in affluent markets and larger homes.
What a 96% furnace needs that an 80% doesn't
- New PVC venting. Condensing exhaust is cool and acidic. It cannot go up a masonry chimney or a B-vent flue — it would destroy them. It vents through PVC pipe, usually out a sidewall, sometimes through the roof. That means a new penetration in your house, routed to a legal location away from windows, doors and property lines.
- A condensate drain. The furnace produces several gallons of acidic water per day of hard running. It needs a drain, and if the furnace sits below the nearest drain — common in Puget Sound basements and crawl spaces — it needs a condensate pump.
- A combustion air path. Most condensing installs are sealed combustion, pulling outside air in through a second PVC pipe. Better for tight houses, but that's a second penetration.
- Chimney orphaning. This is the one that surprises people. If your gas water heater shares the chimney with the old 80% furnace, removing the furnace can leave that chimney too big and too cool to draft properly for the water heater alone. Fixing that is real work, and it belongs on the estimate, not on a surprise change order.
When 80% is the cleaner install
If the existing metal flue is in good condition and correctly sized, an 80% swap is straightforward: pull the old cabinet, set the new one, adapt the plenum, reconnect the flue, gas and electrical, pull the permit, test, done. Most of these are finished in one day.
The payback math, done honestly
Do this with your own numbers instead of a national average, because Western Washington is not a national average.
Pull twelve months of gas bills. Subtract your summer baseline — that's cooking, water heating and the dryer — to isolate what you actually spend on space heating. Then a 96% furnace saves you roughly 16% of that heating number compared to an 80%.
| Annual gas spent on heat | Rough yearly savings at 96% |
|---|---|
| Annual gas spent on heat$400 | Rough yearly savings at 96%about $64 |
| Annual gas spent on heat$700 | Rough yearly savings at 96%about $112 |
| Annual gas spent on heat$1,200 | Rough yearly savings at 96%about $192 |
Now divide the install price difference by that number. For most Puget Sound homes the answer lands somewhere in the 8 to 15 year range. Gas prices move, so the real payback may be shorter or longer than today's arithmetic suggests — but nobody should be promised a five-year payback on a mild-climate furnace upgrade.
Also worth saying plainly: the federal 25C tax credit that used to help with high-efficiency equipment ended December 31, 2025. No 2026 furnace install qualifies for it. And the up-to-$4,400 in PSE rebate money is aimed at qualifying heat pump installations, not furnace replacements. Programs change — we confirm your current eligibility and handle the paperwork either way.
Where 96% clearly wins
- Bigger homes with bigger heating loads. More therms burned means more therms saved. A 3,200 sq ft two-story in Olympia (17°F design temperature) saves meaningfully more than a 1,400 sq ft rambler in Tacoma (29°F).
- You're staying put. Ten-plus years in the house turns a slow payback into a real one.
- The chimney is already shot. If the flue needs relining anyway, the venting cost argument for 80% weakens fast.
- You want the sealed-combustion comfort. Condensing furnaces don't pull heated indoor air up a flue, and they run quieter with variable-speed blowers at the higher tiers.
Where 80% is the right call
- Tight budget, dead furnace, cold house. Getting reliable heat back in one day at the lower end of the range is not a compromise to apologize for.
- A short remaining horizon in the home. Buyers rarely pay a premium for AFUE.
- No practical venting or drain path. Some crawl-space and closet installs make condensing venting genuinely expensive.
- Low heating spend already. If you're heating a small, well-insulated home, 16% of a small number is a small number.
Neither choice is the "cheap" one and neither is the "smart" one. They're different trade-offs, and an honest estimate should say which one your house is built for.
The third option a lot of people should price
If you're already spending $6,000 to $9,000 on a condensing furnace, price a heat pump before you sign. A ducted heat pump runs $12,000 to $18,000 in standard markets and replaces heating and air conditioning in one system — and unlike a furnace, it can qualify for PSE rebate money on qualifying conversions.
Western Washington is genuinely excellent heat pump territory; our winter design temperatures run 17°F to 29°F across the service area, well inside the range where modern equipment delivers full comfort. If your furnace is young and healthy, keeping it as backup in a hybrid setup is often the better play — we say so when it's true. The full comparison lives in heat pump vs. gas furnace in Western Washington and should you replace your gas furnace with a heat pump.
Not sure which lane you're in? Get a real number in about a minute with our instant estimate tool, or call (253) 785-4622.
What AFUE doesn't tell you
AFUE rates fuel efficiency. It says nothing about comfort. Two things matter more day to day:
Staging and blower type
A single-stage furnace is on at full blast or off. A two-stage or modulating furnace runs longer at lower output, which means steadier temperatures and fewer blasts of hot air. Pair that with a variable-speed blower and the system gets noticeably quieter and better at even distribution. Those features track with the higher-efficiency tiers, which is part of why 96% homeowners report they "feel" the difference.
Sizing and ductwork
An oversized furnace short-cycles no matter what its AFUE says, and undersized return ducts will make a 96% unit feel worse than the 80% it replaced. Every Varsity estimate includes a real load calculation, and we'll tell you if the ducts — not the furnace — are the actual problem.
How we handle it
On the estimate visit we measure the space, check the flue and drain options, look at the water heater venting, and price the realistic paths side by side in Good, Better, Best tiers so you're comparing apples to apples instead of guessing. Permits get pulled on every job. Every installation carries a 10-year parts and 10-year labor warranty, and most replacements finish in one day.
We're owner-operated out of Federal Way, licensed VARSIHC766RJ, serving King, Pierce, Thurston and the edge of Kitsap county. Not the cheapest bid — the one done right by people who answer the phone after the check clears.
Ready for real numbers on your house? Get an estimate or call (253) 785-4622, Monday through Friday, 8 to 6.