I have seen that just about everywhere I have lived. The 'dealers' will only sell the equipment they know, opr want to sell. Starts with plumbing companies that will only stock white fixtures, to AC contractors that only stock one type of compressor. They all hate Home depo which sells things the dealers don't want to work on.
It might be a profit motive, or just knowledge of equipment. Or it might be just what the warehouse they buy from has in stock.

A typical air handling motor will rotate at the same speed, thus with a fixed amount of back pressure, will consume a specific amount of energy. A variable speed will use less energy when it is run at a slower speed.
The things is, a typical compressor unit switches on and off, so to keep the inside coil cool but not too cool. At the same time most air handlers will also switch on and off. If however the air handler did not shut off, but operated at a slower speed during the compressor off time, it can still cool, by blowing air over the cool inside coils.

I found this out years ago, as I had a air handler that would freeze up. What would happen is when the fan shut off the coal would freeze the water that would normally drip off the coil. By keeping the air moving the water would not freeze, and it would not freeze up.

It was annoying to keep fan on all the time, but it kept the coils warm enough to prevent the condensation from freezing.

Unlike my traditional on with a 3 speed fan motor.
It runs on low all of the time to keep the air in the house relatively uniform.
If it needs to dehumidify it stays on low and turns on the compressor.
If it needs to Cool it runs on high and turns on the compressor.
If it needs to heat it runs on med and lights the furnace.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

At my first job out of college, I did a rotation with the Building Mechanical Department, and one of our office buildings had a variable air volume HVAC system; the roof-mounted unit had six individual refrigeration compressors that were cycled on and off automatically on a shared duty / cumulative hours basis, and iIrc the fresh air, recirc and exhaust fans were all regulated with a combination of VFDs and dampers. Since the two much-more-senior guys I worked under were very close-to-the-vest characters and I was reportedly such a smart-ass, I was never inducted into the secret VAV HVAC Society, and therefore never became privy to the presumably labyrinthine details of how everything was controlled.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

>> If it needs to heat it runs on med and lights the furnace.

Instead of running the compressor as usual... must be saving plenty of energy :-/

>> If however the air handler did not shut off, but operated at a slower speed during the compressor off time, it can still cool, by blowing air over the cool inside coils.

For how long, an additional 5 - 10 seconds ?

How about this thought, Mike was saying "and includes both an inverter and some means of measuring the motor current dynamically. ... from which it magically derives an estimate of the load on the motor. ... using which information it is able to both throttle the house airflow down to imperceptible but effective levels" - and so, I am thinking he was referring to the load on the compressor motor, which ought to be constant being an ON/OFF motor => contradiction.

What’s happening in this residential HVAC market is a trend toward using what are called an ”ECM” on the air handler. ECM stands for Electronically CommuTated Motor, essentially a 3 phase Brushless DC motor with the “electronic commutation” module (essentially a VFD) integrated into the motor housing. You can’t separate the motor and drive, so when one part fails, you have to replace the entire unit and because it will be integrated into the entire unit’s control system, you have to get the replacement from the OEM.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

"I am thinking he was referring to the load on the compressor motor, which ought to be constant being an ON/OFF motor => contradiction."

You appear to have the common misconception that a constant speed motor always draws a constant amount of power. An induction motor runs at a fairly constant speed but only uses the amount of power needed to run at that speed; fron zero to full load. A refrigeration compressor's load is not constant.

kellog, he was talking about the air (fan) motor - it can be varied in speed, which varies the load on the motor. Clogged filters also change the fan load, which the vfd circuitry detects and sets a warning flag.

btrueblood, why would the control cicuit then need to "magically derives an estimate of the load on the motor" when the very same control circuit is the one driving that fan motor (i.e. no need to magically derive anything, we can measure that draw in real time) ?

So I still think Mike was referring to the compressor motor.

Compositepro, of course, what a blunder... When indoor temps are kept the same, air blown out of the inside unit will be warmer when outdoor temps are higher than the same air blown when outdoor temps are lower. That being said, I surmise this is perhaps the main mechanism of how power is getting saved: sense the room temperature as well as warm blown air temperature, run them through an algorithm and when numbers align turn the fan motor lower speed so that the refrigerant returns at higher temperatures at the compressor thus requiring less effort to reheat. Does that sound plausible ?

LPS to Jraef for the correct explanation. Here is an article with a bit of history on ECM air handers: https://www.hpacmag.com/features/are-ecm-upgrades-...