I don't see this as logical

[Ron]

I have seen numerous older furnaces and now more and more newer installs with the following filter set up. Filter rack on the old ones, media cabinet on the new ones screwed to the side of furnace where the side is cut at roughly 14x23 but the filter size is 20x25. I see this as being a waste of some filter. Why not just use a 16x25 ? Also with a lot of these the return drop is 8x24 straight down to an end cap and most times that drop needs to be bigger.

[mechanical'al]

well there are two comon media cleaner sizes 1400 & 2000 the smaller 1400 has a opening of arround 14x22 the larger 2000 arround 18x22 so thats whats used
as far as installers not suppling adaquite return air ?????

[syndicated]

14 is the max height you can cut open the blower compartment on most newer units.
23 is the standard length of a 25" filter rack opening if S-cleat is used to attach it to the furnace.
Most jobs are price driven, if the customer doesn't understand why the return drop needs to be bigger, because the old system worked "fine" and the estimator doesn't explain it. Why would the homeowner pay for the change?

[mechanical'al]

as far as the filter being to large its simply to use a comon filter size (16x25 20x25) as so a customer can pick 1 up any where instead of some bastard size thats hard to find

[SBKold]

Yes most would benefit from having the bottom opening of furnace returning some as well.

Especially when the motor is orientaed on the same side as inlet.

[gravity]

installers should be using a transition to go from 16x24 to 20x25.

i believe a 16x25 filter is good for 1000cfm at 300fpm. correct me if im wrong.

there are some old threads about filter capacity.

i believe 20x20 is only good for 800cfm

[pcp]

The filter should be angled to pull across the entire filter instead of flat against the side of the furnace where 5 inches of filter isn't even used.

[beenthere]

I prefer to transition down to the furnace opening. Less turbulence.

[t527ed]

sometimes there is just no room for a transition and the filter must be tight up against the furnace cabinet. with the Lennox media cabinets we use there is a gap of about an 1 1/2" from filter to edge of housing that will allow the whole filter area to be used. pulling out a dirty filter and looking at it shows dust spread evenly across the filter.