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What AFUE Measures

It is the number every quote will mention, and it describes less than people assume.

In brief

DOE sets minimum efficiencies of 84 to 86 percent depending on boiler type, while ENERGY STAR asks for 0.90. That number covers the appliance and not the distribution, and a large part of what a house actually loses happens after the boiler.

Source: US Department of Energy, Energy Conservation Standards for Residential Boilers; US Department of Energy FEMP, Purchasing Energy-Efficient Residential Gas Boilers.

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The federal floor

DOE’s amended standards set minimum annual fuel utilization efficiency at 84% for gas-fired hot water boilers, 82% for gas-fired steam, 86% for oil-fired hot water and 85% for oil-fired steam.

Alongside the numbers are design requirements: no constant-burning pilot on gas units, and an automatic means for adjusting water temperature on hot water boilers of either fuel.

Source: US Department of Energy, Energy Conservation Standards for Residential Boilers.

And the level above it

DOE’s federal purchasing guidance requires 0.90 AFUE for ENERGY STAR certified residential gas boilers, citing 0.97 as a higher-efficiency example and 0.84 as a lower-efficiency comparison.

Source: US Department of Energy FEMP, Purchasing Energy-Efficient Residential Gas Boilers.

What the number does not include

AFUE describes how much of the fuel becomes useful heat at the appliance. It says nothing about what happens after that: an oversized boiler short-cycling, a system circulating water hotter than the house needs, or heat leaving through pipework that runs somewhere nobody lives.

Which is why a high-AFUE replacement in an unaltered system sometimes disappoints. The appliance improved; the rest of the loop did not.

The control DOE now requires, and why it is there

An automatic means for adjusting water temperature exists because a boiler does not need to send the same temperature to the house in November as in January. Running cooler when it can is where a good part of the real saving lives, and it is a control that has to be set up rather than merely fitted.

Ask whether it was commissioned, not whether it is present.

A boiler is not a furnace, and the person you want knows the difference.Tell us it is a boiler, tell us what the gauge reads if you can see it, and we will route you to somebody in your area who works on them. You are never charged. Boilers give this site its name and not its limits.

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