Furnace Inspection To the Editor: I consider Richard Trethewey the premier ambassador for our industry. I would add one item to the maintenance checklist in his article "Heating System Tuneups" (2/96): namely, to check for heat exchanger perforations or leaks. Generally, perforations in boiler heat exchangers show readily. Cracks or holes in furnaces are sneakier and more dangerous but should be searched for on an annual basis. Anyone can check for perforations visually with a light and a mirror. If imperfections are found, a professional should be called. Sometimes these will evidence themselves by a wavering flame, caused by air from the fan side impinging upon the flame. Whenever we suspect a bad heat exchanger, we'll do a smoke test in addition to