A standard protocol applies to furnaces, makeup-air heaters, and unit heaters:
1. Keep all heat-exchange surfaces clean. Heavy fouling may indicate improper air-to-fuel ratio, which should, in any event, be checked and adjusted on a regular basis.
2. The burner couplings and linkages need to be adjusted and tightened at least once every season.
3. Older units may develop air leaks in the unit casing, which can skew the performance of the system. These leaks should be sealed.
4. Since the temperature in the firebox is much higher than ambient, insulation is more important than elsewhere. Older units may have no insulation at all or need an extra layer of insulation to reduce wasted heat. Be quite sure the insulation added is rated for the temperature of the system and is installed according to the manufacturer’s recommendations.
5. As with any air-moving system, fan and motor maintenance is a vital necessity.
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Excerpt from Energy Conservation Projects.