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air filters
for furnaces & air conditioners

Sizes
Lifetimes
Effectiveness

Sizes

The sizes are nominal. The thickness, for example, is usually about ¼ inch thinner than the marked size. If you measure an old filter, to get a nominal size, round up to the nearest whole inch. Especially in the “box” sizes, don't throw away the old filter until the new one has been installed, as you may need parts from it.

1 inch thick (that is, probably about ¾ inch)

8 × 16;

10 × 10, 10 × 20, 10 × 24, 10 × 25, 10 × 30;

11½ × 11½;

12 × 12, 12 × 18, 12 × 20, 12 × 24, 12 × 25, 12 × 30;

14 × 14, 14 × 20, 14 × 24, 14 × 25, 14 × 30;

15 × 20, 15 × 25;

16 × 16, 16 × 20, 16 × 24, 16 × 25, 16 × 30;

18 × 18, 18 × 20, 18 × 24, 18 × 25, 18 × 30;

20 × 20, 20 × 24, 20 × 25, 20 × 30;

22 × 22

24 × 24, 24 × 30;

25 × 25;

2 inches thick

10 × 20;

12 × 12, 12 × 20, 12 × 24;

14 × 20 , 14 × 24, 14 × 25;

15 × 20;

16 × 16, 16 × 20, 16 × 24, 16 × 25;

18 × 18, 18 × 20, 18 × 24, 18 × 25;

20 × 20, 20 × 24, 20 × 25, 20 × 30;

24 × 24, 24 × 30;

25 × 25;

3 inches thick

16 × 25;

4 inches thick

12 × 24;

12½ × 20;

16 × 20, 16 × 24, 16 × 25;

18 × 24;

20 × 20, 20 × 24, 20 × 25;

24 × 24;

5 inches thick

16 × 25, 20 × 20, 20 × 25;

6 inches thick

20 × 25;

Lifetimes

At an absolute minimum, furnace filters should be changed annually. An exhausted filter can actually make the filtered air dirtier, and a clogged filter can restrict air circulation and make fans work harder. One-inch filters are often changed monthly, a fairly inexpensive routine that may save electricity.

It isn't possible to specify filter life because, among other things, it depends on how dirty the air is.

Effectiveness

Filters are rated by the size of the smallest particles the filter will reliably trap. In the United States, a rating called MERV is used, and in Europe, Eurovent categories.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends at least MERV 6 for homes. Usually, MERV 8 is chosen for residental use; MERV 12 is better, particularly if residents are allergic to, for example, pollen. The highest MERV rating defined in the standard is 16, which is suitable for general surgery suites. Beyond that HEPA-rated filters are used, although some companies have developed their own rating system for such high-end filters.

air filters for furnaces & air conditioners