AFUE is the big number on every furnace brochure: 80 percent, 95 percent, 97 percent. It is a real, standardized rating, and it matters. But it only measures what happens inside the furnace cabinet. In Tacoma and Pierce County homes, a lot of heat is lost after the air leaves the furnace, and a lot of electricity goes into moving it. This guide explains what AFUE does and does not tell you, with the math to compare options for your own home.
What AFUE actually measures
AFUE stands for Annual Fuel Utilization Efficiency. A 95 percent AFUE furnace turns 95 of every 100 units of gas energy into heat leaving the cabinet over a standardized heating season. The other 5 go up the vent. The rating includes cycling and standby losses inside the furnace.
What it does not include:
- Heat lost from ducts running through crawlspaces, garages, or attics
- Air leaking out of duct joints before it reaches a room
- Electricity used by the blower motor
- Extra cycling caused by an oversized furnace in a real house
The upgrade math most people get wrong
Going from 80 to 95 percent AFUE sounds like a 15 point improvement. In fuel, the savings for the same heat delivered is:
Fuel savings = 1 minus (old AFUE divided by new AFUE)
1 minus (80 divided by 95) = about 15.8 percent less gas
That is a solid improvement. If your house used 600 therms of gas for heat last winter on an 80 percent furnace, a 95 percent furnace would need about 505 therms for the same heat, assuming nothing else changes. You can find your own therms on your PSE gas bill history.
Where Tacoma homes lose the difference
Crawlspace and garage ductwork
A large share of Tacoma, Lakewood, and Puyallup homes from the 1950s through the 1990s run supply ducts through a vented crawlspace. Those spaces sit near outdoor temperature and humidity all winter. Uninsulated or leaky ducts there can lose a big share of the furnace’s output before it reaches a room. Energy.gov estimates duct losses can reach about 35 percent when ducts run through unconditioned space.
Blower electricity
Older furnaces often use a PSC blower motor that draws far more power than a modern variable speed ECM motor at the same airflow. In a mild, long heating season like ours, the blower runs many hours, so this shows up on the electric bill even though AFUE ignores it.
Oversizing
Many original furnaces were oversized. An oversized furnace short cycles, heats in bursts, and spends more of its life in the inefficient warm up part of each cycle.
Delivered efficiency: the number that matters
A simple way to compare is to multiply the AFUE by the share of heat your ducts actually deliver.
| Setup | AFUE | Share delivered by ducts | Delivered efficiency |
|---|---|---|---|
| New 95 percent furnace, leaky uninsulated crawlspace ducts | 95 percent | About 75 percent | About 71 percent |
| Older 80 percent furnace, ducts sealed and insulated | 80 percent | About 90 percent | About 72 percent |
| New 95 percent furnace, ducts sealed and insulated | 95 percent | About 90 percent | About 86 percent |
The duct percentages are illustrative, not measured values for your home. The point is that a new high efficiency furnace on bad ducts can deliver about the same heat per therm as an old furnace on good ducts. The best result comes from fixing both.
How to decide for your home
- Look at where your ducts run. If you can see them in the crawlspace, note whether they are insulated and whether joints are sealed with mastic or just old tape.
- Ask for a duct leakage or static pressure test. These turn a guess into a number.
- Check the blower type. If you are replacing the furnace, a variable speed blower is worth more here than in a short, cold climate.
- Size from a load calculation. Do not copy the old nameplate.
- Consider a heat pump. Heat pumps run lower supply air temperatures, but the same duct rules apply. See our guide on SEER2 and HSPF2 for how heat pump ratings work here.
AFUE in the PNW: FAQ
Is a 95 percent furnace worth it over 80 percent?
Usually, for a gas home with a long heating season. It cuts gas use by about 16 percent for the same heat. But sealing and insulating crawlspace ducts can matter as much or more.
Does AFUE include the blower motor?
No. AFUE only measures fuel converted to heat. Blower electricity is separate, and older PSC motors can use much more power than modern variable speed motors.
How do I know if my ducts are losing heat?
Rooms far from the furnace run cold, the crawlspace feels warm in winter, or duct joints are sealed with old cloth tape. A duct leakage test gives a real number.
Why does my new furnace not seem to save much?
Common causes are leaky or uninsulated ducts, an oversized unit that short cycles, or a thermostat setup that runs the blower constantly. A tech can measure each.
Want to know your home’s real efficiency?
We measure static pressure, temperature rise, and duct condition so you know where the heat is going before you spend money on equipment. Call (253) 370-8641 or send us a message.