CLEAN AND CHECK 4YR HEATING UNIT. CLEAN AND CHECK BURNERS AND BURNER OPERATION. ADJUST BLOWER AND LUBRICATE ALL NECESSARY MOVING PARTS. CHECK MANIFOLD GAS PRESSURE AND FOR PROPER VENTING. CHECK HEAT EXCHANGER AND HIGH LIMIT CONTROL. CHECK THERMOSTAT, AIRFLOW, AIR FILTER, HUMIDIFIER, FAN CONTROL, ENERGY CONSUMPTION, AND ALL ELECTRICAL CONNECTIONS.