1.   A dirty filter can restrict the flow of hot air from the furnace to the delivery ducts.

2.   But remember, a dirty filter is more efficient than a clean one because it will stop more dust and other particles.

3.   If it is from a bacterial buildup, it may be due to a dirty filter, which could be replaced.

4.   So replace all dirty filters.

5.   NASDA determined that the most recent glitch was due to a dirty filter in the problem valve, Yamada said.