Furnace filters are carefully designed to fulfill a single purpose. Specifically, they are designed to reduce the amount of household dust that passes through the furnace and circulates through your house. Household dust is made up of a variety of substances that could include dried skin, lint, pet dander, bacteria, mold, and plant spores.

In theory, furnace filters are pretty simple devices. They all work essentially the same way in that they trap dust particles and stop them circulating. In practice, it is a bit more complicated however. Filters that trap particles allow air to freely flow and therefore trap only the largest particles. Filters that block the smallest particles can become quickly clogged and put a severe strain on the furnace’s blower motor which may cause unnecessary wear and tear on your furnace resulting in costly repairs. The trick is to find the right balance between filter efficiency and air flow.

Disposable fiberglass filters are both common and inexpensive. The problem is that they are not very efficient. The flat surface of the fiberglass filter provides limited surface area. Trapping all the small particles would clog the filter and reduce air flow. To compensate, fiberglass filters trap only the largest particles and allow the smaller mold and bacteria particles to continue to circulate.

On the other hand, the pleated air filter design provides more surface area on the face of the filter. This allows furnace filters to be more efficient and to trap smaller particles.

Filter efficiency is rated in MERVs. A higher rating means more efficiency. MERV measurements are in microns and are related to the size of the particles can pass through the filter. A good home furnace filter should have a MERV rating ranging from 5 to 12. A rating of less than 4 would not provide efficient filtering, and rating higher than 13 would be suitable for hospital use.