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 involved/measured somehow?
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 blockage.
Thanks for sharing and info you know on various manufacturer's variable speed motor operation.
Best Regards,
-- Todd H.