Airflow, Temperature Rise, and Heat Exchanger Integrity
What the exam tests
Airflow measurement, temperature rise, blower performance, static pressure, and heat-exchanger inspection.
Core rules
- Temperature rise = supply-air temperature minus return-air temperature, measured in steady state. Nameplate typically calls for 40–70°F.
- CFM = output Btu/hr ÷ (1.08 × ΔT). Use output, not input. High rise means low airflow; low rise means high airflow or low gas input.
- Total external static pressure is measured across the blower; excessive static pressure reduces airflow and can cause limit trips, frozen coils, or high CO.
- Heat exchangers can fail by cracks, corrosion, or gasket leaks. Inspect visually, with a combustion analyzer, and with a camera where accessible. Flame distortion when the blower starts is a strong clue.
- A cracked heat exchanger can introduce CO into supply air; confirm with CO testing and do not return the unit to service if unsafe.
Worked example
An 80,000 Btu/hr, 80% furnace has 64,000 Btu/hr output. Return air is 70°F and supply air is 120°F, so ΔT is 50°F. CFM = 64,000 ÷ (1.08 × 50) = 1,185 CFM. If the blower is set for 1,200 CFM, airflow is correct. If the same unit shows a 90°F rise, CFM falls to about 658, indicating a dirty filter or wrong blower speed.
Traps
Do not use input Btu in the CFM formula. Do not measure rise before the system stabilizes. Do not condemn a heat exchanger on visual rust alone; use multiple indicators. Do not ignore a high CO reading that changes when the blower starts.