ECM motor, DC motor, variable speed motor - all of them are different names for the same type of the furnace motor. The main feature of this furnace motor is constant air flow. High energy efficiency is the other most important feature.
Every house is different. Different ductwork, different furnace filters. Even in the same house the conditions are changing as the furnace filter get dirty, but a gas furnace burns the same amount of gas. It needs the same amount of air to remove heat from the furnace and to deliver heat to the rooms.
As you can see from the chart, air flow provided by a conventional blower motor drops sharply as the ductwork resistance increases. It affects air circulation in the house. It causes furnace overheating. By contrast ECM motor maintains constant air flow through a wide range of ductwork conditions.
Manufacturers love ECM furnace motors. They even provide better warranties for the furnaces with this motors since furnace will not overheat and will live longer. Home owners love this constant air flow motor since it quiet, efficient and provides good air circulation through the house.
One thing to remember though. This is not a free ride. To compensate for the tight ductwork (dirty furnace filter), the ECM motor has to bring speed up. It results in motor's wear and tear, extra electricity use and
noise. Constant air flow feature of the ECM motor can mask a problem for some time, but it can be costly.
Speaking of energy efficiency of a furnace with ECM motor we should remember that it is efficient in both gas consumption and electricity consumption by ECM motor. Previously, wasted hydro would contribute to house heating at winter time. The new furnace has to burn extra gas to compensate for saved electric heat which is cheaper anyway, but confuses the entire picture of saving.
Basically, the new furnace gas efficiency even better than it appears from the gas bill. To get the full story of energy savings you'd need to look into the hydro bill as well.