How can I determine what size and type of filter is required in my air handler?

Question

I just moved into a new house with a Trane Air handler (model TWE065E13FB) in the crawlsapce. I went to replace the filter as this was suggested maintenance and I found that there isn't a filter in the slot. Should I install a filter? If so what kind of filter? Looking online it seems that a 20x25x1 filter is the correct size.

http://www.jonesairconditioning.com/trane-filter-size-guide.html

The manual for the unit indicates the filter surface area (sq.ft.) should equal to CFM/300 for low flow and CFM/500 for high flow. I have 1600 CFM with a 4 ton heat pump (400 CFM/ton). If I use the high flow this gives 3.2 sq.ft., about a 20x25" filter. Is this the correct way to do this calculation. I can't see the filter slot accepting different sizes of filters without modification.

I'm not particularly sensitive to dust or allergens and on my return air vents I just use the basic Rheem/home depot pleated filter (20x20x1). Would a similiar filter be suitable for the air handler?

Answer 1

You should only have one filter installed for the system, so if the return with the filter leads to the air handler, then you're already covered, and don't need a filter in the air handler too. In fact putting a second filter in could be bad for your system, and could even cause your blower motor to fail because restricting the air flow even a little will slightly increase the load on the motor. Is that likely? No. But there's no extra benefit from having an extra filter, so why risk it?