When shopping for a new boiler, you’ll see a number on the spec sheet called AFUE. Maybe it says 82%… or maybe 95%. But what does that really mean for your home and your gas bill?
👉 Is higher always better? And how do you know what’s right for your house?
I’m Tony, and I’ve been through this exact process with my own Weil-McLain boiler. In this guide, I’ll break down what AFUE is, how it’s calculated, what it means for your wallet, and how to use it to make smart choices—not just expensive ones.
Weil-McLain CGa-7 Series 3 200,000 BTU Cast Iron Natural Gas Boiler
🏷️ What Is AFUE?
AFUE stands for Annual Fuel Utilization Efficiency.
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It’s a percentage rating that measures how efficiently your boiler converts fuel into usable heat for your home.
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Example:
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An 80% AFUE boiler turns 80¢ of every $1 of gas into heat.
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The other 20¢ is lost—usually through the chimney or venting.
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It’s similar to miles-per-gallon for your car: the higher the AFUE, the more efficient the boiler.
📖 Reference: U.S. Department of Energy – Boilers
👉 My own Weil-McLain CGA boiler is rated at about 82% AFUE. For my older home, that balance of efficiency and durability made more sense than chasing 95%.
📐 How AFUE Is Calculated
AFUE isn’t just a guess—it’s calculated in controlled testing.
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Lab tests measure how much fuel the boiler burns over a season and how much heat actually makes it into the home.
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It accounts for cycling losses, pilot light consumption, and standby losses.
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But here’s the kicker: AFUE does not include heat loss in your pipes, ducts, or through poor insulation.
That means a 95% AFUE boiler in a drafty house might perform worse than an 82% AFUE boiler in a well-insulated home.
📖 Reference: Energy.gov – Heat Distribution Systems
📊 AFUE Ratings by Boiler Type
Here’s how different boilers stack up:
| Boiler Type | Typical AFUE Range | Notes |
|---|---|---|
| Cast Iron Boilers | 80–86% | Reliable, durable, standard efficiency |
| Non-Condensing Gas Boilers | 80–89% | Common for older systems |
| Condensing Gas Boilers | 90–98% | High efficiency, more complex, costlier |
| Oil-Fired Boilers | 80–87% | Efficiency varies with burner technology |
👉 My Weil-McLain CGA is a cast iron, non-condensing model at 82% AFUE. It doesn’t hit the 90s, but it’s rock-solid reliable.
📖 Reference: ENERGY STAR – Boiler Standards
💸 What AFUE Means for Fuel Bills
Let’s put AFUE into real-world dollars.
Imagine your heating costs are $2,000/year with an 80% AFUE boiler.
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Upgrade to a 90% AFUE boiler → you save ~11% in fuel = about $220/year.
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Upgrade to a 95% AFUE boiler → you save ~16% in fuel = about $320/year.
But if that high-efficiency boiler costs $4,000 more upfront, it could take 12+ years to pay for itself.
👉 In my case, natural gas in New York is relatively cheap, so the payback for going ultra-high efficiency didn’t make sense.
📖 Reference: U.S. EIA – Natural Gas Prices
🏠 Matching AFUE to Your Home
Not every house benefits equally from higher AFUE.
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Older drafty homes: Heat loss through windows, walls, and vents often cancels out the benefit of ultra-high efficiency.
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Modern well-insulated homes: These can actually take advantage of high AFUE, since less heat is wasted.
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Climate zones matter too:
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Northern states: Higher AFUE pays back faster with heavy heating demand.
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Southern states: Heating is minimal, so standard AFUE is usually fine.
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📖 Reference: ENERGY STAR Climate Regions
👉 My house (3,800 sq. ft., built in the 1960s) is moderately insulated. For me, 82% AFUE was the sweet spot—the cost of upgrading to 95% just didn’t pay back fast enough.
✅ Federal Standards & Rebates
Here’s where government rules come in:
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DOE minimum efficiency: 80% AFUE for gas boilers. Anything below that can’t be sold.
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ENERGY STAR certification: 90%+ AFUE for gas, 87%+ for oil.
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Rebates & credits: Many utilities and states offer rebates for ENERGY STAR boilers, and in 2025, federal tax credits under the Inflation Reduction Act can cover up to 30% of the cost for high-efficiency models.
📖 References:
👉 I grabbed a $500 rebate from my gas utility just for replacing my old unit with an ENERGY STAR model.
⚖️ AFUE Isn’t Everything
One of the biggest mistakes I see homeowners make is shopping only by AFUE rating.
Here’s why that’s risky:
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Sizing matters: An oversized 95% boiler that short-cycles wastes energy.
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Installation matters: A sloppy install can cut efficiency by 10–20%.
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Maintenance matters: A dirty heat exchanger can tank performance.
👉 My 82% boiler outperforms my neighbor’s 90% unit because his contractor oversized it and didn’t set up the controls right.
📖 Reference: HVAC School – Boiler Basics
👷 Tony’s Real-World Take
Here’s how I see it after living with these systems for years:
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Don’t chase the highest AFUE just for bragging rights.
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Think about your house, your climate, and your fuel costs.
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Balance efficiency with reliability and payback.
"AFUE is a good guide, but it’s not the whole story. A well-installed 82% boiler can serve you better than a poorly installed 95%. Don’t just shop by numbers—shop for quality."
In the next topic we will know more about: Noise Levels Explained: How Quiet Is a Weil-McLain Boiler?
