Whether they are installed in your heater or in your air conditioner, electronic air filters will always filter dust in the return air duct. By filtering the air before it enters the appliance, an electronic air filter serves two functions. First, it protects the inside of the air conditioner or heater from dust. Second, it ensures that the treated air leaving the air conditioner or heater is clean. Electronic air filters are excellent and work very well provided they are monitored and cleaned regularly. If they are cleaned regularly, they will work at about 95 percent efficiency. If they are not cleaned regularly, their air-cleaning capacity is reduced rapidly. HEPA filters are the best nonelectronic disposable filters. They are expensive, but good enough to use in hospitals. Electrostatic filters are also good. Last, there are ordinary pleated filters and the standard furnace filter that protects the furnace but does not clean the air.