Featured Article

Ductology Part 2

Does Your Ventilation System Make the Grade?

Respiratory System; National Cancer Institute

The human respiration system is an exquisite ventilation system that efficiently moves air through the nasal cavity, larynx, pharynx, trachea, bronchial tubes and into the lungs. We go into distress when any part of the respiratory system is not functioning properly. Our health is similarly stressed when any section of our home’s ventilation system is improperly designed or poorly installed.

Part 2 of Ductology focuses on the performance of ventilation ductwork. We describe two “rough-in” tests that should be performed on installed duct systems before ducts are hidden behind wallboard and ceilings. The cost and heartache associated with inadequate ducting after finishing touches have been applied to a home are immense. And the cost of living with a poor ventilation system is even greater in terms of impact on home occupant health and lifetime operational cost for excessive blower power.

The ventilation system test we present is straightforward to implement and interpret. Ventilation ductwork that scores above 1000 are good. Duct systems scoring below 500 are not so great.

A second test assesses duct system leakage. The duct leakage test uses the same duct leakage test procedure currently used in homes. We re-interpret test results differently, however, by relating duct leakage to the duct air flow performance test. Duct leakage should be less than 5% of the design air flow, consistent with ASHRAE’s view of duct leakage. Using the same scale used for duct performance, a duct leakage score of 50 or less is desired for a duct network with a performance score of 1000.

Our detailed report, Ductology – Part 2, includes analytical and experimental validation of the duct system performance and duct system leakage tests. Example duct system performances are included. Also included in the report is a section that develops a model for relating house infiltration to blower door performance. The infiltration model uses the same analyses used to characterize duct performance and duct leakage. In this manner, one can understand the interrelation of building infiltration, duct performance and duct leakage.

Blower door testing of homes is now commonplace and is often required by local building code authorities. The duct testing procedures we describe are ones that should similarly be conducted on every home. Select Test 1 (Duct Performance) and Test 2 (Duct leakage) for PDF copies of the test procedures. Appendix C in the Ductology – Part 2 report shows a test setup for determining duct performance.

Stay tuned for future articles and reports that explore these effects, and how the CERV smart ventilation system is designed to manage air quality in all homes, regardless of whether they are today’s high performance houses or homes built years ago.

1919 Farmers & Merchants Bank in Columbus Wisconsin is the last of Louis Sullivan’s “Jewel Box” banks. A beautiful example of form following function. Walking into the bank today is no different than walking into it in 1919. The interior is gorgeously lit with daylight diffused through beautiful stain glass windows. The bank welcomes guests and maintains a historical collection including original blueprints and samples of Sullivan’s famed terracotta decorations.
Louis Sullivan did not leave fresh air to chance as shown in these 1919 blueprints for the Farmers & Merchants Bank. Why has it taken us so long to realize that fresh air in our homes is just as important?

Test 1: Duct System Performance

Build Equinox recommends performing the following “rough-in” duct test on supply and return ventilation ducts prior to installing air handlers or fresh air ventilation units.

The performance test consists of the following steps:

1) Connect fan with air flow measurement and pressure measurement sensors to the duct network (supply or return) to be tested
2) Switch the fan “on”, and adjust fan speed (if speed adjustment is available) to desired level
3) Balance registers to desired air flow levels
a. “Relative” air flow balancing can be used when the performance test air flow is different than design air flow. For example, if test air flow is 400cfm and design air flow is 200cfm, an outlet with a design air flow of 50cfm (25% of 200cfm) should have 100cfm (25% of 400cfm) for the performance test.
b. Air flow direction for the test should be the same as operational air flow direction. Supply duct systems will have air blown into the supply trunk while return duct systems pull air through the duct network
4) Record fan air flow and duct pressure
a. If fan speed variation is available, record 2 or 3 air flow and pressure levels
Calculate the “C” value as: C = Q / DP0.57
Where DP = static pressure drop across duct length (“H2O)
Q = air flow (cfm, cubic feet per minute)
C = duct system coefficient
b. Note that pressure will be positive (fan discharge pressure) for supply ducts and pressure will be negative (fan inlet pressure) for return ducts. Use the absolute pressure reading (ignore the negative sign for return ducts)
c. Figure 1 can be used to determine C value directly from air flow (cfm) and duct pressure (“H2O) measurements
d. If multiple fan air flow rate tests are conducted, average the C values
5) Figure 2 describes the performance of the duct system.
a. C values greater than 1000 indicate good air flow performance for systems with ventilation air flow rates up to 300cfm.
b. C values above 500 are reasonable for ventilation air flows of 150cfm or less.
c. C values below 500 indicate restricted duct air flow capability with high fan power requirements for residential fresh air ventilation.
Figure 1 Duct system performance test C value plot based on air flow (cfm) and pressure (“H2O).
Figure 2 Duct performance test scale.

Test 2: Duct Leakage

Build Equinox recommends the following rough-in duct leakage performance test for supply and return ventilation ducts. The leakage test procedure is the same as the performance test except that all duct outlets/inlets are sealed.

The leakage test consists of the following steps:

1) Tightly seal all duct inlets and outlets.
2) Connect fan with air flow measurement and pressure measurement sensors to the section of duct to be tested
3) Switch fan “on”, and adjust fan speed (if speed adjustment is available) to desired level
4) Record fan air flow and duct pressure
a. If fan speed variation is available, record 2 or 3 air flow and pressure levels
Calculate the “C” value as: C = Q / DP0.57
Where DP = static pressure drop across duct length (“H2O)
Q = air flow (cfm, cubic feet per minute)
C = duct system coefficient
b. Note that pressure will be positive (fan discharge pressure) for supply ducts and pressure will be negative (fan inlet pressure) for return ducts. Use the absolute pressure reading (ignore the negative sign for return ducts)
c. Figure 1 can be used to determine the leakage C value directly from air flow (cfm) and duct pressure (“H2O) measurements
d. If multiple fan air flow rate tests are conducted, average the C values
5) Figure 2 describes the leakage of the duct system. C values less than 40 indicate well sealed ducts while C values greater than 60 indicate excessive air leakage.
a. The ratio of the duct leakage C value to the duct performance C value provides an estimate of the fraction of air leakage to desired duct air flow.
i. A duct performance C value of 1000 with a duct leakage C value of 50 has an air flow leakage fraction of 50/1000 = 0.05, indicating 5% of the duct air flow is leaked (leakage air into return ducts, or leakage air loss from supply ducts)
ii. Note that leakage for ducts kept within the thermal envelope of a home are less serious (but still important) than ducts in unconditioned attics, crawl spaces or other spaces
Figure 1 Duct leakage test C value plot based on air flow (cfm) and pressure (“H2O).
Figure 2 Duct leakage test scale.

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