I built a DIY air filter by fitting a Merv-13 air filter inside a cardboard box. It's held in place by wood strips cut and glued to the box and I cut a hole for a 200mm case fan with a 12v power supply. The result is an air filter that operates continuously in the room I am in, but unlike other solutions is completely silent.
My question is: When should I change the Merv-13 filter? The recommendation on the box is every 3 months, but that's for a filter working over an entire house, not with a single 200mm fan pulling air slowly. If I can still feel air coming off the fan, does that mean mold spores and other allergens are being trapped?