Static Pressure Testing

Building on the static pressure measurement and duct leakage detection techniques from the previous module, this lesson establishes the performance standards your test results must meet. Knowing how to measure static pressure and leakage is only useful when you understand the code-required limits, the ventilation standards that govern envelope tightness, and the velocity thresholds that prevent noise complaints.

Duct Leakage Limits Under the 2024 IECC

The 2024 International Energy Conservation Code (IECC) replaced the older flat-rate limit with a variable sliding scale in Table R403.3.6. Unlike the 2021 IECC which used a single threshold of 4 CFM25 per 100 sq ft regardless of home size, the 2024 IECC ties the allowable Total Duct Leakage (TDL) to two factors: conditioned floor area and number of ducted return air openings.

Key rule changes in the 2024 IECC:

• Total Duct Leakage (TDL) is now the mandatory prescriptive metric for all new construction. TDL is the total CFM25 escaping the entire duct system, measured with a duct blaster at 25 Pa.
• The flat 4 CFM25 per 100 sq ft limit is abolished. Smaller homes (under 1,000 sq ft) get proportionally more flexibility; larger homes face tighter absolute limits per the Table R403.3.6 sliding scale.
• "Leakage to outside" as the primary compliance metric is now restricted to the R405/R406 energy performance or ERI alternative compliance paths. The prescriptive path requires TDL testing.
• For ducts entirely within conditioned space, no TDL limit applies because leakage stays inside the building envelope.

The sliding scale works by looking up the allowable TDL in CFM25 from Table R403.3.6 based on your home's conditioned floor area bracket and the number of ducted return air openings. A 2,000 sq ft home with two ducted returns, for example, will have a specific TDL limit from the table. Always consult the table directly - the old rule of thumb (floor area divided by 100, multiplied by 4) no longer applies to the 2024 IECC prescriptive path.

Blower Door Testing, Envelope Tightness, and Ventilation

Static pressure testing and duct leakage testing tell you how tight the duct system is, but the blower door test tells you how tight the building envelope is. These two tests work together: a tight envelope with leaky ducts is a recipe for pressure imbalances, backdrafting, and indoor air quality problems.

Blower door results are reported in ACH50 - air changes per hour at 50 Pascals of pressure. The 2024 IECC tightened the envelope air leakage limits compared to earlier editions:

• Climate Zones 0-2: Maximum 4.0 ACH50 (reduced from 5.0 ACH50 in earlier editions)
• Climate Zones 3-5: Maximum 3.0 ACH50 (unchanged)
• Climate Zones 6-8: Maximum 2.5 ACH50 (reduced from 3.0 ACH50 in earlier editions)

A home with no mechanical ventilation that has tested at 1.5 ACH50 is considered very tight. According to ASHRAE 62.2 (the residential ventilation standard), this home needs mechanical ventilation because the tight envelope does not provide sufficient natural infiltration for acceptable indoor air quality. The 2024 IECC's tighter envelope limits in CZ 6-8 mean that mechanical ventilation - typically an ERV or HRV - is now mandatory in more climate zones because passive infiltration no longer satisfies minimum ventilation rates.

The exam will test whether you understand that a tight home does not automatically have good air quality. The tighter the home, the more deliberate the ventilation strategy must be. A technician should never assume that a home with a low ACH50 already has adequate ventilation. Simply installing larger windows for cross-ventilation is not the answer - the home needs controlled mechanical ventilation. The correct solution according to ASHRAE 62.2 is a properly sized mechanical ventilation system such as an ERV, HRV, or supply fan integrated with the air handler. In cold climate zones where the 2024 IECC dropped the ACH50 limit to 2.5, an ERV or HRV is the preferred choice because it recovers sensible and latent energy from exhaust air, avoiding the energy penalty of bringing in raw outdoor air.

Supply Register Face Velocity and Noise

After verifying duct tightness and building ventilation, the technician must confirm that supply registers deliver air at appropriate velocities. The recommended maximum face velocity for supply registers in residential applications to minimize noise is 500-700 FPM (feet per minute).

Velocities above 700 FPM in residential applications cause audible air noise that generates homeowner complaints. A face velocity of 300 FPM is too low and would require oversized registers. A velocity of 1,200 FPM or 2,000 FPM would produce unacceptable noise in any residential setting. The sweet spot of 500-700 FPM balances adequate air delivery with quiet operation.

Face velocity is calculated by dividing the airflow in CFM by the free area of the register in square feet: Velocity (FPM) = CFM / Free Area (sq ft). If a register is delivering 120 CFM and has a free area of 0.2 sq ft, the face velocity is 600 FPM - well within the recommended range.

Under the 2024 IECC, duct leakage compliance uses Total Duct Leakage (TDL) on the prescriptive path - the old flat 4 CFM25 per 100 sq ft rule is gone. Table R403.3.6 provides a sliding scale based on conditioned floor area and number of ducted returns. Envelope tightness limits tightened: CZ 0-2 drops to 4.0 ACH50 and CZ 6-8 drops to 2.5 ACH50, making mechanical ventilation (ERV/HRV) mandatory in more homes. Supply register face velocity in residential applications should stay within 500-700 FPM to minimize noise.