Q: What is the best type of air filter to use in the return air ? I’ve used the Filtrete 1000 (red) filter but it seems like it gets dirty fast, which I’m assuming makes the a/c work harder. I change them once a month, but they look pretty dirty after two or three weeks.

When space permits, we recommend an inline 5″ pleated media filtration system within 1 foot of the furnace in the attic for greater filtration. If a filter gets dirty quickly, we recommend looking at sources of infiltration, such as leaking light fixtures, doors and windows. Also, carpeting and indoor clothes driers compound the issue.

Uncontrolled fresh air enters your house through cracks and crevices in your home, through the air conditioning system, and through leaks in the duct system. Uncontrolled fresh air can be very undesirable.

Also, the further the filter is away from the blower and the more dense the filter, the greater the restriction on system performance. The filter is picking up material like it should. Why is it getting so dirty in 3 weeks is the bigger question.