90 Plus Gas Furnace - Induction Blower (-) Pressure Requirement
I have a Rheem 90Plus Upflow forced air gas furnace - about 12 years old.
After making a loud, mechanical noise, it stopped working. I figured out it was the induction blower (gee, those trouble LED lights are really useful), and I took it apart to find the blower wheel had shed a couple of fins. So, I cleaned it up and determined if I could get a new wheel, everything else appears to be working correctly.
I have installed a similar, but not identical blower wheel - it is just a little bit smaller, and I carefully re-assembled the system. (If you ask why I didn't buy the same part, I couldn't get anyone to sell me one, I guess they want you to be a contractor. The place that sells the Fasco motors didn't have the exact blower wheel, so I bought a slightly smaller one...) The induction blower now works just fine, and appears to move plenty of air, and the furnace burns great and exhausts fine. Problem is, the pressure switch can only barely activate, based on a slightly lower negative pressure from the induction blower with the slightly smaller wheel.
There are two switches: the induction blower pressure switch - set at -1.3" wc and the burn chamber exhuast switch, set at -0.4" wc. The new blower wheel has no problem activating the burn chamber vent switch but can't seem to hold open the induction blower switch, so the furnace will cut out. I'm looking at a $250 new blower assembly, or a $60 new pressure switch. I'm not sure which one to buy, I can buy a pressure switch with a lower threshold (say -1.0" w.c.), but is this really safe to operate with a slightly lowered induction blower flow rate?