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Achieving Duct Tightness Recommendations

Many of the manufacturers working with BAIHP are striving to meet the quantitative goals set by the EPA Energy Star Program for Manufactured Homes. The program provides compliance paths for homes with three levels of duct leakage to the outside Qn_out = 3%, 5%, or 7% (MHRA, 2001.)

Among the earliest BAIHP data, four houses built by the same manufacture in 1997 exemplify the achievability of duct tightness in the manufactured housing setting. Two were standard homes used as control homes for comparison to an “energy improved” model and a “health improved” model (Chandra, et. al., 1998.) Standard manufacturing methods were changed to mastic and the 3% Qn_out leakage target was easily met (Table 4).

Repair Improvements to Taped and Mastic
Of the 190 floors in this data set, 9 test results show improvements from repairs made to initially leaky systems (Table 5). This is a very persuasive example for factory personnel when conducted on a freshly produced duct system on the factory floor. This also shows the iterative problem solving process that BAIHP researchers use with factory staff to foster improvement in performance and production procedures.

For a description of improvements implemented at a single manufactured housing plant, see Appendix A.

Tightness of all 9 systems was improved by BAIHP repair (Table 5). Maximum improvement was a reduction of Qn_total from 11.7% to 5.5%. The least improved system was tightened from Qn_total of 8.1% to 7.1%. Qn_total was reduced an average of 3.5, representing a 43% improvement in duct tightness.

The duct systems in Houses 14, 47, 51A, and 52A were assembled with mastic. However, the production was marred by other problems such as inaccurate cutting, inaccessible joints, and misalignment of components. These problems occurred in other mastic sealed systems which also failed to make the target Qn_total (see Duct Tightness Data).

Unit 14 illustrates BAIHP’s iterative approach to working with factories. Unit 14 was a section of a double wide manufactured house containing an air handler for a split heating and cooling system. Initially, it showed duct leakage of 10.6% (Qn_total) on the air handler side. Researchers removed the air handler fan and connected the duct testing equipment directly to the duct system to quantify how much of the leak was associated with the unit itself. This dropped the CFM25_total to 33cfm bringing the Qn_total to 6.4%, still too high to meet the tightness target of less than 6%.

Upon investigation, researchers and factory staff found holes in the main trunk line that were cut much larger than the connecting floor boots, too large to seal with standard mastic practice in use on the production line. The holes had been left open with no attempt to seal them. Researchers used fiberglass mesh and mastic to build up patches over the holes. CFM25_total dropped another 29cfm bringing the Qn_total to 2.6%, well within the target zone.

Similar holes were found on the non-air handler side of the double wide suggesting that this was a systemic production problem. Improved cutting using a template to match supply boot dimensions was recommended.

A subsequent factory visit a month later found only one out of nine tested floors did not meet the Qn_total = 6%. Qn_total measurements fell from the original 10.6% to: 2.3%, 2.7%, 3.9%, 3.6%, 4.4%, 4.5%, 4.5%, 5.8%, and 6.4%.

This result is exactly the desired effect of the BAIHP approach:

• Introduce Building America Industrialized Housing Partnership and systems engineering
• Build collaborative relationship with industry partners to solve problems
• Set goals with factory managers
• Quantify duct tightness resulting from existing practices
• Identify opportunities for improvement
• Report findings and make recommendations
• Assist with implementation when needed (e.g. training, problem solving for production process)
• Reevaluate to assess progress until goals are reached
• Implement factory quality control procedures, including pressure testing, to sustain success

Commonly Implemented Improvements
BAIHP researchers have observed that certain aspects of duct system fabrication must be addressed, in addition to the change from tape to mastic, in order to consistently deliver a substantially leak free duct system. Data compiled here show that manufacturers have consistently achieved duct tightness goals by implementing correct mastic application, improved cutting precision, better dimensional coordination between duct system components as well as between ducts and the house. The following list represents steps commonly taken by manufacturers to achieve their duct tightness goals:

All systems

• Train supervisors and line workers on air flow concepts
• Systemize the duct assembly process
• Use circle cutters and templates for standard duct cutouts
• Use mastic in a form that fits with the production process (tubes/buckets)
• Seal joints with a “pinky” size bead of mastic
• Seal the duct to the house air barrier (ceiling drywall or floor deck)
• Seal joints in return and supply plenums
• Institute quality control measures, such as pressure testing ducts during production, in addition to visual inspection
  
  Floor System (Typically Sheet Metal) Improvements:
• Replace tape with mastic and, when needed, fiberglass mesh
• Ensure the supply plenum is well attached and thoroughly sealed to the trunk duct with mastic
• Select trunk duct dimensions that allow for slight misalignment of floor register cutouts without creating a failed joint.
• Improve alignment of floor register cutouts with trunk and branch ducts
• Provide adequate attachment area (tabs on all components should be fully bent)
• Use templates to cut holes in trunk ducts for floor risers
• Fully bend tabs on floor risers
  
  Ceiling System (Typically Duct Board and Flex Duct Improvements)
• Use circle cutter to cut hole in duct board for flex duct collar
• Provide adequate attachment area around collar cut outs to allow all collar tabs to be fully bent.
• Lay generous bead of mastic (about the size of smallest finger) into corner where the flange meets the main body of the collar.
• Press collar into duct board hole. Mastic should spread.
• Spread mastic to completely cover collar flange and beyond edge of collar flange about ½”.
• Slide inner line of flex duct onto collar past the positioning ridge and secure with a plastic zip tie trimmed with a tensioning tool.
• Slide outer lining up as close to the collar flange as possible and secure with a plastic zip tie trimmed with a tensioning tool.
• Remove excess flex duct, this can result in kinked supply runs.

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