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2 speed vs. variable speed furmace blower

I'm about to order a replacement 40,000 BTU gas furnace for my house in Traverse City, MI, CZ6. It will have a 2 stage burner with an AFUE of 96%. It is available with a 2 speed ECM blower and one with that is continuously variable.

The house is 900 sq. ft. It has R10 basement walls, R20 walls and R50 ceiling.

What difference does a variable speed fan make, and is it worth $200?

Douglas Higden

Asked by thrifttrust
Posted Jan 9, 2018 7:46 PM ET



That's an excellent question. In theory, if you have a two-stage burner, it would make sense to optimize the delivery of the heat at the two firing rates by providing two possible blower speeds, each optimized for one particular burner output.

That said, there may be a reason why there is an advantage to investing in a variable-speed blower -- but I just can't think of the reason.

GBA readers?

Answered by Martin Holladay
Posted Jan 10, 2018 6:45 AM ET


It's interesting that it's an ECM either way--that makes it sound like the hardware capability is there in either model and you just pay $200 extra for them to enable that feature in the variable speed one.

I would think that the advantage of the variable speed would be primarily in commissioning the system: if the duct resistance is a little more than the default settings are designed for, you could boost the fan a bit, or if the ducts are short and generously sized, you could actually turn the fan down a bit, and get quieter operation with less electric consumption, while still achieving the design air flow.

Some, but not all, ECM systems automatically vary the speed to maintain the airflow at the specified value. If the 2-speed model does that, then the advantage I suggest for the variable speed model does not make sense--it's already automatically doing what I suggest.

Answered by Charlie Sullivan
Posted Jan 10, 2018 9:01 AM ET


Thank you for your insight. In studying the manufacturer's data I find only vague references to variable speed providing more "comfort". I did discover why the two speed uses an ECM. It has a feature that increase or decrease each standard speed 5 or 10 percent to tune it to the duct system. (Thanks Charlie, I hadn't considered the commissioning thing.) This should help me because the ducts are no doubt oversize in that they were installed with the furnace I'm replacing in 1960 when the house had R3 insulation and single glazed windows. I suspect that the two furnaces are identical save the control board. My experience with electronics tells me that it is cheaper to make a controller with discrete steps over one that is variable.

I think I'll save the $200.

Answered by thrifttrust
Posted Jan 11, 2018 1:59 AM ET

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