Variable speed good/bad in existing duct?

[mower]

For a replacement unit (dead Carrier Tech 2000) in a 15 year old house, is there a risk that one of these new variable speed furnace fans with 2 stages of both heat/air may not produce enough air flow compared to the old single speed original fan when temperatures are moderate/mild? Looking at 2-stage Trane gas unit and 2 stage heat pump compressor to hedge bets on energy prices. Live in North Carolina.

Any help really appreciated!

[dash]

The variable speed will work as well or beter on you duct system,and say away from the guy who told you diffently.they must have very little experience and/or knowledge of variable speed.

Now if the the old duct system has problems,Variable speed may help but won't compensate for a Bad duct system.

[Black Adder]

Overall air flows should be far better with variable speed even on a system with questionable ductwork. That is the main advantage to variable speeds. Gives you far more control over airflow. Have one in my home and I love it, especially the full time circulation, does a great job of evening out the hot and cold spots in the home as well as helping with drafty situations.

[davidr]

The variable speed motor is a great feature to have on any equipment.

If the present duct system is undersized you will notice a increase in noise that was not there before with the addition of a variable speed motor.

Best bet would be to have the duct system tested to insure it is adequate for the proper amount of airflow.

[docholiday]

Its in your best interest to be sure the duct is right, no blower will make a bad duct system good. The VS will more than likely deliver the airflow at higher static, but at a cost. the cost is in..

NOISE (high static nmakes for noisy air),
COST (ECM's get very expesive to operate at higher statics),
COMFORT (excess velocities can mean uncomfortable rooms and drafty systems)
LOSS OF CAPCITY. (slight leaks in duct systems can be exagerated when high statics are realized).
POOR FILTRATION. (hig velocities can cause filters to bow and bypass otherwise filtereable dirt.