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Building Science

The Hidden Flaw in Some High-Efficiency Furnaces

Hint: Furnaces use two forms of energy

Some high-efficiency furnaces aren't as efficient as you might think.
Image Credit: Energy Vanguard

You can tell how energy-efficient a furnace is by its official efficiency rating, the Annual Fuel Utilization Efficiency (AFUE). It’s a measure of how much of the heat originally in the fuel that’s being burned is available for delivery to the home. The more heat that gets lost up the flue or through the cabinet, the lower the AFUE.

But that rating doesn’t capture all the ways a furnace can lose efficiency. Some, like how well the heat gets distributed to the house, aren’t related to the furnace itself. But there’s one big one that is related to the furnace.

Unlike air conditioners and heat pumps, furnaces use two sources of energy. The fuel that it burns is the obvious one. (It’s usually natural gas or propane.) The other is the electricity that runs the blower. And that’s the key to finding a high-efficiency furnace that truly operates at high efficiency.

A peek at manufacturer’s specs

I was working on a mechanical design for one of our clients recently and was going through the equipment selection process. The software we use (RightSuite Universal) has a lot of equipment data built in, so I can just find a manufacturer, set the limits for sizing and efficiency, and look at the options.

Once I get my list of options reduced to a reasonable number, I can choose a model that meets my requirements. But I have to go in and make sure to look at the blower power rating for each one or I could end up saddling the client with a furnace that looks like it should be efficient but really isn’t.

I took a look at the RightSuite listed blower power ratings for five different manufacturers. They were all in…

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One Comment

  1. Bennett Sandler | | #1

    Downside to ECM blower efficiency
    As you point out, duct design is an important factor in blower performance. There's even an added wrinkle: as I understand it, newer ECM blowers respond to friction head in ducted systems differently than previous generation blower motors did. In the old days a poorly designed duct system with high friction head, for example one with inadequate supply and return, would result in lower cfm (and perhaps decreased comfort), but the power consumption of the blower would not be affected. But ECM blowers will actually ramp up in response to higher head pressure. So in the worst case you might end up paying significantly more to operate a "more efficient" blower if your ducts aren't properly sized.

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