Diagnostic Testing and Measurement
Core concepts
Diagnostic testing isolates a variable. Pressure diagnostics (blower door, zonal pressure, duct leakage) rely on controlled pressure differences and consistent reference points. Understand CFM50, ACH50, Effective Leakage Area, and how building size normalizes results. Know duct leakage to outside versus total duct leakage and why the distinction matters. Grasp infrared thermography: emissivity, reflectivity, and the need for steady-state conditions and adequate delta-T. Understand moisture measurement methods (pin, pinless, RH sensors) and their limitations. Recognize test sequencing: establish baseline, then perturb, then measure response.
Traps
Classic traps: reporting ACH50 without volume, or CFM50 without pressure reference. Another: assuming a blower door test alone locates leaks - it quantifies, not locates. Infrared on a low-delta-T day yields false negatives. Duct leakage tests can be confounded by house pressure changes; control them. Also watch instrument error propagation - combining uncertain readings compounds error.
Worked example / rule of thumb
A 2,000 ft² home with 8 ft ceilings has 16,000 ft³ volume. If CFM50 = 3,200, then ACH50 = (3,200 × 60) / 16,000 = 12. Rule of thumb: multiply CFM by 60, divide by volume for ACH. Sanity-check: ACH50 of 3–5 is tight; 10+ is leaky.
Study approach
For each test, know the setup, the controlled variable, the output units, and the confounders. Practice unit conversions until automatic.