Ultimate Heating Tune-Up Gas Heat: serviced and inspected gas heating system checking air flow, temperature rise, electrical, gas pressure, safeties, controls and limits, heat exchanger and venting. cleaned and checked system components to manufacturer specifications and performed combustion inspection. Crawl space furnace should be replaced as it is old. Heat exchanger is rusty with possible crack or leak. CO was well over 100 ppm was able to adjust it to 60 however I did the same thing last year. Not sure how much longer this furnace will run safely