Over time, you can cut down on your home maintenance and heating costs by using washable metal air filters in your furnace. As with disposable air filters, there is a wide range of efficiency with washable metal air filters. You'll probably have to spend more for a very efficient filter. The fact that you can wash your filter instead of just throwing it away, however, can help you justify the cost of a quality filter. Instead of replacing your filter every three to four months, you may not have to buy a new one for four to five years if you clean it regularly.
Use a wet/dry vacuum or the hose attachment to your upright vacuum to vacuum around the furnace.
Remove the air filter from the furnace, and use your vacuum to remove as much dust and dirt as possible. Vacuum before you get the filter wet; otherwise, the dust and dirt will turn to mud and may be difficult to remove.
Run water over the air filter. A kitchen sprayer works well for this; if the weather is warm, you can use a garden hose to spray out any remaining particles.
Use a mild detergent such as dish soap to wash both sides of the filter. Rinse thoroughly.
Use a dry cloth or paper towel to dry the filter as thoroughly as possible. Then let the filter air-dry until it is completely dry. Do not put the filter back in the furnace until it is completely dry.