I work as a maintenance assistant in a Nursing and Rehab center, part of my duties require that I clean the filters on room heating and a/c units. So far it's been to take a brush and clean whatever lint has built up on them every month or so, I have noticed that some have a lot of buildup and some have very little or none, even though I have the utmost respect for my maintenance director he seems far to busy with big details of the building to properly address this, and a few of the units that we have had to change out were really dirty behind the filters inside of the units, my question is what is the proper way to maintain these units? I am assuming that most hotel, motels have the same sort of units in the rooms.