My small home’s HVAC system (installed in 2019) has a 3-ton Carrier Infinity modulating variable speed furnace paired with a Carrier Performance 3-ton 17 SEER 2-stage AC condenser. Because it has a non-communicating condenser, it does not have the Carrier Infinity controller and is using an Ecobee thermostat, essentially downgrading that fancy furnace.
That said, they installed the furnace in a very hard-to-access attic within the building’s insulated envelope because it was the only place to put it during the remodel. That makes the 4-inch or whatever size filter that is on the system very difficult to access for switching out the filter (requiring removing a couple of built-in shelving units in the small access area), so the HVAC installer put in a 14x30x1 return air opening which he said was just barely adequate to cover the CFM rating, but could be swapped out regularly instead of using the built-in filter.
I have since been doing some research to establish how high of a MERV rating I can go with when purchasing replacement filters, and it looks like none of the pleated filters are recommended at all if I am reading page 18 of the manual correctly (apologies, I had links for everything, but apparently I’m not allowed to post links or images yet). It doesn’t have an example for 14×30, but the area is barely more than the 16×25 example.
That got me wondering, Is this return air inadequate for my system? I want it to function as efficiently as possible because I hope I will not have to replace it for a very long time. It will require an immense amount of work to swap it out in the future in the location they placed it. Based on the manual, it seems only a fiberglass filter can be used, not a pleated one. I’m guessing because pleated has too big of a pressure drop. I’ve been using a Filtrete MERV 11 up until now.
The follow-up question is, will using the built-in 4-inch filter prolong the life significantly? If it’s a definite yes, then I’ll find a way to do shelf removal once a year and find a way up into that attic to swap it out rather than using the much easier to access 1-inch return filter.