[malfist]

I've always heard this, but I don't know that that's true. The thing that dies first on an HVAC system isn't the motor, it lasts a long damn time, it's the capacitor, or contactor or some other fault not really related to the motor work load.

Plus, your "motor" isn't an engine, it's not variable speed, it only has one "hardness" that it works at. It might push less air with the thicker filter, but it's not going to ramp up and consume more wattage or anything like that.

[bombcar]

It's usually a misunderstanding of how furnaces "work" - the blower motor does its thing and doesn't really "spin up or spin down" depending on the load - it just moves more or less air.

The problem is (especially with older furnaces; modern ones have safety features to prevent this) that if you have too much back-pressure you don't get enough airflow over the heat exchanger (or air conditioner coils) and it can crack - allowing dangerous exhaust gasses into the airflow (or freezing the coils for the AC which isn't as bad).

Once the heat exchanger is gone the furnace is basically trash and has to be replaced (you can replace the heat exchanger but it's rarely worth it).

[danans]

> The problem is (especially with older furnaces; modern ones have safety features to prevent this) that if you have too much back-pressure

An everyday way in which people induce this scenario is by shutting the heating vents in particular rooms because they get too hot. Even on the newer furnaces, this results in the automatic controls shutting off the burner when the pressure and temperature gets too high, and then the fan starts pushing around cold air.

A secondary negative effect of this is that it pressurizes the ducts causes them to leak more, resulting in reduced efficiency, and also quicker failure.

A well designed system which has been configured to deliver the correct amount of heat to each room doesn't experience the same issues. Unfortunately, most older homes and even newer production built homes have poorly designed HVAC systems.

Heat pumps don't experience as many of these issues because they just don't get air as hot, and instead rely on higher throughput of lower temperature air to heat spaces, but that makes them far more reliant on good duct system design.

[danans]

> Plus, your "motor" isn't an engine, it's not variable speed, it only has one "hardness" that it works at.

Many newer AC motors are variable speed or at least multi-speed.

[foobarian]

>Plus, your "motor" isn't an engine, it's not variable speed, it only has one "hardness" that it works at.

This is incorrect - electric motors draw more current when under load. I would suggest researching this or playing with some toy electric motors run from a battery through a current meter.

[bityard]

> This is incorrect - electric motors draw more current when under load.

Which is a true statement but incomplete and therefore misleading. A fan motor is under the highest load when it is doing the most work. That is, moving the MOST air. If you restrict airflow (e.g. with a dirty filter), the motor is under less load and draws LESS current. A lot of people get this wrong because it's counter-intuitive on the surface of it.

You can test this very easily with a box fan and a kill-a-watt. Turn the fan on High in the center of the room and read the power. Now move it against the wall. You will hear the fan get louder because it is spinning faster, because it's doing LESS work. The meter will also show less power being drawn.

It is true that a dirty filter restricts airflow through the whole HVAC system. This extends the system's "on" cycle, which reduces the whole system's efficiency. But the popular claim that a dirty filter will burn out the motor is bunk.

[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.

[creaturemachine]

That's true, but it's not so simple anymore. Furnace blowers are all electrically commutated brushless DC motors now, and will ramp to keep a constant torque.