The short answer
More than most people guess. The Department of Energy puts old, low-efficiency furnaces and boilers at 56% to 70% AFUE, which means 30 to 44 cents of every dollar of gas never becomes heat in your house. A new high-efficiency furnace is rated 90% to 98.5%. Plenty of Northern Virginia houses still heat with the old kind.
What the AFUE number means
AFUE stands for annual fuel utilization efficiency: the share of the fuel that ends up as heat in the house over a whole season. An 80% furnace turns 80 cents of every gas dollar into heat, and the other 20 cents is lost, most of it out the flue. A 96% furnace loses 4 cents. The rating is set when the furnace is built, and age only takes away from it.
Swipe to see all columns →
Swipe to see all columns →
| AFUE | Lost from every gas dollar | What it usually is |
|---|---|---|
| 56% to 70% | 30 to 44 cents | Old, low-efficiency furnace or boiler |
| 80% to 83% | 17 to 20 cents | Mid-efficiency, metal flue into a chimney or vent |
| 90% to 98.5% | 1.5 to 10 cents | High-efficiency condensing, white PVC vent pipes |
Bands from the Department of Energy. The cents column is the arithmetic.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
The part most people don’t know
About 1 in 5 American homes heat with equipment 20 or more years old. That’s 23.5 million homes in the Energy Information Administration housing survey. A furnace that age still lights, still blows warm air and still passes the “it works” test, so nobody replaces it until it quits. All that time it can be burning gas a newer one wouldn’t.
Age matters for more than the bill. Our how long a furnace lasts guide covers the usual life span, and an old heat exchanger is where the safety question comes in: a cracked heat exchanger can let carbon monoxide into the air you breathe. That’s the first thing worth knowing about a furnace past 20.
Should you replace it just for efficiency?
Not always. Efficiency alone rarely pays for a new furnace fast. The math depends on your gas bill, how old the unit is and what, if anything, is wrong with it. A safe, well running furnace in its teens is usually worth keeping and maintaining. One past 20 that needs a big repair is a different conversation, and if your heating bill doubled with no change in the weather, something is wrong now.
One rule you can ignore for now: a Department of Energy standard requiring most new gas furnaces to hit 95% AFUE is dated December 18, 2028 on paper, but it’s back in court, and per ACHR News DOE has proposed delaying it while the litigation continues. 80% furnaces are still sold and installed today.
How to tell what you have
- Look at the vent. Two white PVC pipes out the side of the house usually mean a condensing furnace, 90% or higher. A metal flue into a chimney means 80% or lower.
- Read the label. The AFUE is often on the rating plate or the yellow EnergyGuide label. If not, the model number tells a tech.
- Find the age. The serial number usually has the build year in it.
- Look for a standing pilot. A small flame that burns all the time behind the furnace door is a sign of an older design.
Get a straight read on yours
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. The tech checks how it’s running and tells you how old it is and whether it’s worth keeping. Holding a quote already? Our free second opinion sends a tech to look at the system you were quoted on and tell you what it actually needs, with our price if you want it. You don’t owe us anything for looking. If it is time, the estimator shows installed prices with $1,500 off a new heating and cooling system already in them, and if you get your estimate this week, a basic single-stage gas furnace comes free with a new AC and its matching coil, installed together.
Book it
Book the heating check, call (703) 997-9757, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
How efficient is an old furnace?
The Department of Energy puts old, low-efficiency furnaces and boilers at 56% to 70% AFUE. Mid-efficiency models are 80% to 83%, and high-efficiency condensing furnaces are 90% to 98.5%.
How much gas does an old furnace waste?
At 56% to 70% AFUE, 30 to 44 cents of every dollar of gas never becomes heat in the house. An 80% furnace loses about 20 cents, and a 96% furnace about 4.
How do I know if my furnace is high efficiency?
Look at the vent. White PVC pipes out the side of the house usually mean a condensing furnace rated 90% or higher. A metal flue going into a chimney means 80% or lower. The rating plate often lists the AFUE.
Is it worth replacing a furnace just for efficiency?
Not always. A safe, well running furnace in its teens is usually worth keeping. Past 20 years, with a big repair on the table, replacing it often makes more sense, and the efficiency gain helps.
Is the 95% furnace rule in effect?
No. It’s dated December 18, 2028 on paper, but it’s back in court and the Department of Energy has proposed delaying it. 80% furnaces are still sold and installed.
