Airflow, Heat Exchangers, and Temperature Rise
Domain 5, Airflow, Heat Exchangers, and Temperature Rise, tests temperature-rise measurement, blower performance, static pressure, and heat-exchanger integrity.
Temperature rise
Temperature rise is supply-air temperature minus return-air temperature after steady operation. The nameplate range is commonly 35–65°F. Calculate airflow with CFM = output BTU/hr ÷ (1.08 × ΔT). Use output, not input: output equals input times steady-state efficiency.
Static pressure and blower
Total external static pressure is supply static plus return static, measured with a manometer; about 0.5 in. w.c. is a common target. High static pressure points to a restrictive filter, coil, or duct system. Blower speed taps change airflow. A dirty filter lowers airflow, raises temperature rise, and can trip the high limit.
Heat exchanger
Inspect for cracks, soot, discoloration, flame rollout, and CO. Condensing furnaces add a secondary heat exchanger; a blocked condensate drain can trip a pressure switch. A cracked heat exchanger can let flue gases into supply air, so confirm with visual inspection, combustion analysis, and flame behavior.
Worked example
An 80,000 BTU/hr furnace at 80% efficiency has a 60°F temperature rise. Output = 64,000 BTU/hr. CFM = 64,000 ÷ (1.08 × 60) = 988 CFM. If the blower is rated for 1,200 CFM, airflow is low. Check the filter, coil, ducts, and blower speed. Increasing blower speed lowers temperature rise.
Traps
Using input BTU instead of output overstates CFM. Measuring temperature rise before the furnace stabilizes gives a false reading. Lowering blower speed to 'fix' a high temperature rise is backwards - increase airflow. Do not overlook return-air restrictions and closed registers.