Air Distribution, Duct and Envelope Testing
Air Distribution
Static pressure is the language of air distribution
Total external static pressure (TESP) is measured with a manometer between the supply plenum and the return plenum, with the filter, coil, and all components installed and the coil wet. Compare the reading to the blower performance table, not to a generic number. In residential work, a design target near 0.5 in. w.c. TESP and about 400 CFM per nominal ton are the usual starting points. Component drops add up: filter, wet coil, electric heat strips, registers, and grilles all consume static pressure.
Duct sizing and the fan laws
Equal-friction sizing commonly uses about 0.1 in. w.c. of friction per 100 ft of equivalent length, then checks velocity for noise. The fan laws govern everything: airflow varies with RPM, static pressure with the square of RPM, and brake horsepower with the cube. That cubic relationship is why a small airflow increase can overload a motor.
Traps
Measuring static in the wrong plane or in a turbulent spot right at the blower outlet; forgetting the filter and coil drops when comparing to the table; confusing "fan static" with external static; and assuming that more airflow cures comfort complaints when the real problem is return restriction or unbalanced branches.
Worked example
A blower is moving 1,000 CFM. The speed is increased so it moves 1,200 CFM, a factor of 1.2. Static pressure rises by 1.2² = 1.44 (44 percent higher), and brake horsepower rises by 1.2³ = 1.73 (73 percent higher). Always check motor capacity before speeding up a blower.
Duct and Envelope Testing
Two tests, two reference pressures
Envelope leakage is measured with a blower door referenced to outdoors at 50 Pa. Duct leakage is measured with a duct blaster at 25 Pa, with the air handler sealed off and all registers temporarily sealed. Report duct leakage as CFM25 and as a percentage of system airflow, distinguishing total leakage from leakage to outside - they are different numbers and programs treat them differently.
Envelope math
Convert CFM50 to air changes per hour: ACH50 = CFM50 × 60 ÷ conditioned volume. Also normalize to envelope area (CFM50 per square foot of enclosure) when comparing houses of different size. Pressure diagnostics - pressure pans, zonal pressures, and blower-door subtraction - tell you whether the dominant leak is between house and garage, house and attic, or duct and outdoors rather than simply how much air escapes.
Traps
Testing ducts without sealing the registers or without accounting for the air handler as a leak path; using indoor pressure as a reference when outdoor is required; ignoring wind and stack effect; and forgetting that a duct test measures the whole distribution system, not just the trunk. Also watch for altitude and air-density corrections when equipment is calibrated at sea level.
Worked example
A 2,000 sq ft house with 8 ft ceilings has 16,000 cu ft of conditioned volume. A blower door reads 3,200 CFM50. ACH50 = 3,200 × 60 ÷ 16,000 = 12 air changes per hour at 50 Pa - a leaky envelope by most program thresholds.