I got a quote from a well qualified dealer for a Rheem RGFD modulating furnace. It is replacing a Bryant #320AAZ042080AAKA 2-stage (79K/55K BTU out). He calculated (rule-of-thumb) that we need about 77K for our 2200 sq ft house. As such, he wasn't comfortable with the 75K Rheem, so bumped it up to the 90K BTU (85K out?). With the modulating (3-stage for our zoned system), I'm not too worried about the extra heating as it will likely run at 40-65% of capacity most of the time, but am concerned about the increase in blower size.
My Bryant lists a 1/2 HP blower. I don't have airflow specs for that motor, but if anyone does, I'd appreciate the numbers. The 75K Rheem also has a 1/2HP blower and lists that at 1200 CFM max, so I'm guessing that's the ballpark I'm in with the Bryant. The 90K Rheem jumps to a 1HP blower, which lists a max of 2000 CFM and min of 1200 CFM. That's considerably more than I have now, which I know has benefits, but I'm concerned about the additional noise due to increaseed air velocity.
In my system, the 2 main zones each have about 225 sq. in (total area) of round ducts running to various registers from main trunk lines. At 800 CFM (guess at low speed of current blower), that gives an outlet velocity of about 500 ft/min if only one zone is open and 250 ft/min if both are open. At 1200 CFM (guess at high speed of current blower), 770 ft/min for 1 zone or 385 ft/min for both. At 2000 CFM, I get 1280 ft/min for 1 zone and 640 ft/min for both.
So, even at my minimum speeds (1200 vs 800 CFM), my velocity increases 50%. Will this be noticable and are these speeds considered acceptable?
I raised this question with the dealer, and he wasn't concerned. We like how quiet the air delivery is with our current system and I want to make sure we don't do something we'll regret.