Tony Hwang wrote:

Just wondering if anyone has replaced their conventional furnace
/ HVAC fan motor (PSC, single phase, etc) with one of the new
ECM motors to realize a reduction in your electricity bill.

Don't even bother. It is DC servorized motor with matching control
logic board. High efficiency furnaces are made to take advantage
of this variable speed motor.

Everything you're saying is true, but it doesn't mean that
fundamentally ECM motors are more efficient regardless in what type of
furnace they're used in.

Even if all I do is use it as a constant-speed replacement for a 50%
efficient PSC motor, it will use less electricity.

The question is - how much, and what is the over-the-counter cost of a
suitable ECM motor (not the HVAC-contractor-installed price).

Also they are not as reliable as ordinary motor.

That is probably true, and along the lines of planned obsolescence
that's designed into modern residential HVAC systems.