Installation of my Carrier FE4BNF003 air handler was completed about 2 weeks ago. I decided to replace the cheap fiberglass filter with a pleated unit. Well, I thought it was a 20x20. Nope! I measured the old one and it's about 20x21. Is that right? Seems like an odd size. Anyone know the correct size for this unit? Any recommendations on what kind of filter to use? The system is new, so I have a fresh start on trying to keep things clean. Thanks.