[malfist]

I have a degree in electrical engineering, thank you for telling me to do my research, I spent four years doing that.

Sure, some motors have characteristics that can be tweaked to run at different work loads either with PWM, or allowing them to pull more amps to drive higher loads. Hell, you can even overvolt them and make them actually work harder.

That's not how HVACs work though. Have you ever opened one up and did a repair? The circuitry is dead simple, there's no current limiting setup or PWM to control how much the motor is pulling or spinning, there's no CFM measuring device to give the motor more volts or a higher amp limit.

The motor is simply pushing air. Air isn't something like a solid load where a motor might lift something or move a lever or gear, it's fluid. The motor is going to run at max and be done.

[devenson]

Even a motor without a fancy control system will consume more power under load. See:

https://en.m.wikipedia.org/wiki/Counter-electromotive_force

[jjeaff]

Ya, and restricting airflow reduces load. It may not be intuitive, but you can plug a vacuum cleaner into a meter and put your hand over the tube to restrict airflow. The load on the fan will go down because less air means less friction. In other words, it's easier to run a fan inside a vacuum. The louder sound you get when you put your hand over a vacuum cleaner tube is the motor speeding up because it has less load.

[malfist]

I'm not saying a motor turned off will consume the same power as a motor turned on, that's foolish.

Back pressure doesn't impact the load of a fan motor, at least not as the scale we see in HVACs, we're not making vacuum chambers.

[foobarian]

Maybe our disconnect is that we're talking about two different things, one being the rated design point of the blower system, which is presumably constant (though newer systems seem to be more sophisticated), and the other being the exact load and power consumption that depends on backpressure. A higher backpressure will certainly increase the motor power consumption on a classic induction motor - if not by 100% then by some non-zero amount.

Anyway at the end of the day I agree with the original characterization of wear and tear - motor and blower will probably outlast most other components. In the case of the one I mentioned this seemed to be true. You just may end up paying a little more for the electricity to run it.

[bombcar]

Modern furnaces have variable-speed fans - Blower Motor Variable-speed constant airflow full-featured ECM

https://www.bryant.com/en/us/products/gas-furnaces/987m/

[anarticle]

The vast VAST majority of hvac systems are still bang bang systems.

[foobarian]

> I have a degree in electrical engineering, thank you for telling me to do my research, I spent four years doing that.

Apologies for the tone!

> Have you ever opened one up and did a repair?

Well funny you should ask, but yes! Two times. One time, a power relay on the control module shorted and blew a hole in the circuit board. Had to replace the control module there. Another time, the start capacitor needed to be replaced on the giant 2KW squirrel cage motor. I removed the blower, disassembled it, lubricated moving parts, and of course tested it out on a stand. That much blowing power is quite impressive when right next to your face.