Why are furnace/central AC blowers so inefficient?

Question

I can buy a ceiling fan from Home Depot capable of moving 5,000 CFM of air for about $50, and it uses a mere 50-60 watts while doing it. By contrast, most furnaces seem to have blowers that operate in a much lower CFM range but use humongous 1/2 or 3/4 horsepower motors that use 600 watts or more when energized.

How does this make any sense? Why is a 1/2 horsepower motor or larger were required for a furnace or central AC but a ceiling fan can get away with such a smaller, more efficient motor and move more air? Or is a ceiling fan CFM not the same as a furnace blower CFM? Is it that the blower requires a much higher RPM to push the air through the ductwork, so the blower CFM is more like a "delivered" value rather than a "supplied" one?

Is it possible to replace these beasts with more efficient versions? Do such things exist?

Answer 1

Okay, so I think I figured out the reason, and I learned a lot about HVAC in the process.

The answer is that a ceiling fan is moving air at basically zero "static pressure." Static pressure in an HVAC context means the amount of resistance that the air has to moving. In a free environment, that's zero, or close to it, but in a tightly restricted system of ductwork, it will be much higher. A ceiling fan is rated at such-and-such CFMs... but at zero static pressure. They don't tell you this because it's obvious; you're probably not mounting your ceiling fan in the opening of a four-foot-diameter cylindrical duct. If you found a curve of CFM versus static pressure for any given fan, it would probably drop off steeply. But of course there would be no reason to because the whole point of a ceiling fan is not to push air through a duct, but to circulate it in an open room, so the motor is optimized for that use case.

This is compounded by the fact that most ductwork is terribly done, so furnace manufacturers err on the side of oversizing the blower to make sure the unit isn't starved of air (causing it to operate poorly or even die early), or incapable of supplying the living environment with the required heat. Either of these things would make people blame the furnace and make the furnace company look bad.

So the short answer is that you have to use a powerful blower in an HVAC system because ducts suck. A ductless unit/system could probably get away with a much less powerful blower because it wouldn't have to work nearly so hard.

Answer 2

Just because the furnace fan is rated at 3/4 or more horsepower, does not mean that it is actually running at that power level.

To clarify this, consider the extreme with an electric motor with nothing connected (just a shaft sticking out) spinning at full speed: it uses almost no power: perhaps as little as 3 watts, even though its ratings plate says 700 VA, for example.

Also, if running the fan did really need 3/4 horsepower (like in a really big system), the motor would fail from running at full power sooner than we expect—perhaps in less than a year. The fan motor is overrated for reliability, quieter operation, and to accommodate variability (like when the wind blows through a window into a ducted room and creates more back-pressure).

Answer 3

Basic answer: A ceiling fan is basically an axial pump in which the fluid moved is air. A blower is a centrifugal air pump. Axial is extremely efficient when there is no resistance to flow. When there is resistance to flow, an axial will move little to no air and a centrifugal is needed.