Performance Testing, Measurement, and Diagnostics
Measure, then diagnose
Combustion analysis gives oxygen or carbon dioxide, carbon monoxide, stack temperature, and net stack temperature, from which steady-state efficiency is derived. Temperature rise is supply air minus return air at steady state, compared to the nameplate range. Static pressure is measured with a manometer: supply and return plenum readings combine into total external static pressure (TESP), which is compared to the blower table to estimate airflow.
Airflow can also be derived from sensible heat: CFM = output Btu/h ÷ (1.08 × ΔT). Duct leakage and envelope leakage are separate tests with separate tools; a blower door does not measure duct leakage, and a duct test does not quantify envelope infiltration.
Traps
- Taking temperature rise before the system reaches steady state, or measuring return air in a closet rather than at the unit.
- Sampling flue gas at the wrong location or before the analyzer stabilizes.
- Confusing velocity pressure with static pressure, or placing probes in a poor traverse location.
- Using nameplate input instead of measured output when calculating airflow.
- Ignoring altitude or derate tables for gas input.
Worked example
An 80,000 Btu/h input, 80% AFUE furnace delivers about 64,000 Btu/h. With a measured 55°F temperature rise: CFM = 64,000 ÷ (1.08 × 55) ≈ 1,078 CFM. If the nameplate rise is 40–70°F, this is acceptable; if static pressure is high, look to the duct system. Rule of thumb: roughly 400 CFM per ton is a cooling rule, not a heating target - always use the nameplate rise for heating airflow.