September 17, 1999

Abstract: 

Duct leakage is a concern in the HVAC field, due to energy consumption, pressure balance problems, bypassing of the filter by leakage air, and contaminant draw from unconditioned spaces. Therefore, certain energy efficiency programs set duct leakage performance requirements that must be met to enter the program. However, the overall duct system tightness is limited by leakage at the air handler.

Executive Summary

Duct leakage is a concern in the HVAC field, due to energy consumption, pressure balance problems, bypassing of the filter by leakage air, and contaminant draw from unconditioned spaces. Therefore, certain energy efficiency programs, such as Greenstone's Engineered for Life™, set duct leakage performance requirements that must be met to enter the program. However, the overall duct system tightness is limited by leakage at the air handler.

Tests were run on several air handler units with cooling coils and plenums attached, to determine the unit leakage. Measurements were made by attaching a duct blaster calibrated fan to the return plenum. The supply plenum was a selaed box with no penetrations. Multipoint pressurization and depressurization tests were run, obtaining leakage values over a range of pressures. Similar tests were then conducted with the air handler unit alone (without cooling coils and plenums). The goals of these tests were as follows:

  • Quantify the leakage of various air handlers, using a common test so units can be compared uniformly
  • Determine permissible field modifications to seal the air handler/furnace cabinet. This has been the stopgap measure in the past, but some manufacturers have considered the warranty void due to these modifications.
  • Discuss possible modifications to air handler design in order to increase airtightness, and discuss status of these changes in progress by the manufacturers.
  • Determine the approximate amount of leakage that can ve attributed to the air handler in duct system leakage tests, to re-evaluate leakage targets for the Engineered for Life™ program.

Conculsions included the following points:

  • Total air handler leakage, including the coils and plenums, ranged from 45 to 60 CFM 25.
  • Air handler unit leakage alone ranged from 13 to 45 CFM 25; in general accounting for half of the total leakage. Coil and plenum leakage was calculated by subtraction, and ranged from 20 to 45 CFM 25. Since it will be easier to improve the sealing of the cooling coil section and plenums than the air handler units, the focus should be here first, while continuing discussions with the air handler unit manufacturers.
  • Sealed combustion 90% AFUE furnaces were more leaky than the 80% power vented furnaces of the same manufacturers. The single-cabinet First Company hot water coil/DX coil air handler gave the tightest measurement (28 CFM25), due to its unitized construction.
  • Leaving the joints unsealed between the air handler unit and the cooling coils and plenums increased leakage by approximately 40%.
  • All manufacturere installation directions should be followed, such as use and placement of cabinet screws, the removal of grommet plugs when soldering, and sealing of unued condensate line openings.

Background

Duct leakage is a concern in the HVAC field, due to energy consumption, pressure balance problems, bypassing of the filter by leakage air, and contaminant draw from unconditioned spaces. Therefore, certain energy efficiency programs, sucah as Greenstone's Engineered for Life™, set duct leakage performance requirements that must be met to enter the program. However, the overall system tightness reached by current duct-sealing methods is limited by leakage at the air handler unit (AHU).

Therefore, tests were run on 80% and 90% AFUE furnace/air handlers from various manufacturers (see Appendix A for complete list). Leakage values were measured both with attached coils and plenums, and standing alone (see Appendices B and C). Measurements were made by attaching a duct blaster calibrated fan to the return plenum. The supply plenum was a sealed box with no penetrations. Multipoint pressurization and depressurization tests were run, obtaining leakage values over a range of pressures.

The results included a CFM 25 measurement, the C and n values (flow coefficient and exponent), and R2 (correlation coefficient). The C and n values provides a pressure-flow relationship for these units. However, this test does not simulate the complex pressure field that occurs in the air handler during operation (i.e., negative and positive pressures of various magnitudes in different parts of the unit simulataneously). Instead, it is meant as a way to compare various air handlers, and to determine a reference number for duct whole-system leakage test.

Representatives from International Comfort Products and York observed these tests. They were invited to determine permissible field modifications to seal air hanlder/furnace cabinet. This has been the stopgap measure in the past, but some manufacturers have considered the warranty void due to these modificaitons. They were also on hand to discuss possible modifications to air handler design in order increase airtightness, and to discuss status of these types of changes in progress. . .

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