I've had a Carrier dealer discourage use of a variable speed motor
with my ductwork in a 15 year old home. As he explained, the
transitions from square duct to round can confuse the sensors in the
variable speed motor to cause it to ramp up to higher speeds
unnecessarily. Does this make sense? I ask because other dealers of
different equipment have not expressed this concern.
This led me to wonder how specifically Carrier variable speed motors
determine what speed to run at. What is the algorithm for speed
control--are there sensors put in the plenum, on-board sensors, are
they looking at temperature and inferring airflow from that, directly
measuring airflow, or. Are there fixed intervals involved like run at
X% for Y minutes, if tstat isn't satisfied ramp up to blah% for Z more
minutes, if not tstate, ramp up further? Or is airflow
Other dealers who've quoted have not expressed any concern with the
duct situation, even when directly asked. It's always possible they'd
love to sell a variable speed motor setup.
Of course, one of those dealers (a Frigidaire dealer) had no clue how
their variable speed motors worked and couldn't explain it other than
mentioning DIP switches for low and high speeds. Based on their
knowledge, I don't think I'll be using them I'm afraid.
A Ruud dealer explained that their variable speed sensed airflow
somehow (he said he wasn't sure precisely what was being mesured) but
did offer the anecdotal evidence that he had forgotten to change his
filter at his house once and the clue he had was that his variable
speed motor was running faster than normal due to the airflow
Thanks for sharing and info you know on various manufacturer's
variable speed motor operation.