A gas furnace should be serviced at least once every two years by a professional serviceman. Oil furnaces should receive the same treatment once a year. For the oil system, this includes everything from checking the furnace completely to looking up the chimney. In terms of what maintenance you will have to do yourself, you should check the filters in either oil or gas furnaces at least once a month, and while you are doing that you should make sure that the fan belt in the air-blowing mechanism is not loose. Beyond that, there is not much else that you can (or should) do yourself. If the flame inside the furnace is not a sharp, clean flame, but looks dirty and wild (see illustration on page 98), you should call your serviceman.