You can help keep your furnace filter clean. When the filter is dirty, your furnace loses efficiency and has to work harder; it uses more fuel and costs you money. If you have a standard disposable filter, replace it about once a month during the heating season. Washable filters should be removed and cleaned as needed. After changing or cleaning the filter, make sure the furnace door or panel is completely closed. (The furnace won't run unless you do this.)