87
Ta ble 7
Notes
1) The Table 7 load fac tors account for the con duc tion loads and leak age loads that occur on the sup ply and return sides of the duct sys tem.
These fac tors also include an adjust ment for an increase or decrease in the enve lope infil tra tion load (per a set of power law equa tions), depend ing on the rel a tive amount of sup ply-side and return-side leak age.
2) The Table 7 data is com pat i ble with duct sys tems that have dom i nant sup plyside leak age (with the excep tion of return runs located in exte -rior walls). The load fac tors and latent load val ues are com pat i ble with duct sys tem sur face areas gen er ated by theMan ual D siz ing pro- cedure and the five default leak age ratesprovided by Table 7. Com put er ized duct load solu tions (mod els) are required for duct sys tems that have dom i nant return side leak age or a leak age rate that is sub stan tially dif fer ent than the default val ues.
3) The Table 7 heat loss fac tors depend on the tem per a ture of the sup ply air. These fac tors tend to get smaller as the dis charge tem per a ture increases because (by the sen si ble heat equa tion) the sup ply CFM val ues and air way sizes (exposed area) get smaller as the sup ply tem per -a ture incre-ases. T-able 7 uses -a 100 °F def-ault for dis ch-arge tem per -a ture bec-ause it pro duces con ser v-a tive duct lo-ad v-al ues -and bec-ause the heat loss fac tors are com pat i ble with air way sizes required for cool ing. Com put er ized duct load solu tions (mod els) are required for other sup -ply air tem per a tures.
4) Table 7 pro duces val ues duct load val ues if a duct sys tem instal la tion is rea son ably sim i lar to one of the default sce nar ios. The Table 7 data is based on assump tions per tain ing to the floor plan of the home, the loca tion of the air han dler, the num ber of sup ply runs, the num ber of the return runs and a set of leak age rate val ues. These assump tions are listed here.
• Rect an gu lar floor plan with a 2:1 aspect ratio.
• Air han dler located in the cen ter of the floor plan.
• One sup ply branch per 100 CFM of sup ply air.
• One return branch per 400 CFM of return air (the four AE tables and the eight O and P tables are for 1,000 cfm per return).
• Sup ply ducts not sealed (0.35 / .070 sce nario) have 35 CFM of leak age per 100 SqFt duct sur face area.
• Return ducts not sealed (0.35 / .070 sce nario) have 70 CFM of leak age per 100 SqFt duct sur face area.
• Sup ply ducts sealed (0.12 / 0.24 sce nario) have 12 CFM of leak age per 100 SqFt duct sur face area.
• Return ducts sealed (0.12 / 0.24 sce nario) have 24 CFM of leak age per 100 SqFt duct sur face area.
• Duct runs below slab have an aver age of 3 CFM leak age per 100 SqFt duct sys tem sur face area. (Duct runs below the slab have no leak age and are water tight. The leak age occurs along the above grade duct runs).
• Table 4E pro vides default drybulb tem per a tures for the ambi ent air in an encap su lated attic; and the default for grains of mois ture dif fer -ence (for the attic air and return air) is 20% of the Table 1A or Table 1B value.
• For heat ing, the default value for ambi ent dry-bulb tem per a ture for duct on a roof equals the Table 1A or1B value.
• For cool ing, the default value for ambi ent dry-bulb tem per a ture for duct on a roof depends on the color of the out side sur face of the duct, and weather the duct is in the sun or shade.
a) For a reflec tive sur face in the sun, the ambi ent dry-bulb equals the Table 1A or 1B dry-bulb plus 20°F.
b) For a white or light sur face in the sun, the ambi ent dry-bulb equals the Table 1A or 1B dry-bulb plus 35°F.
c) For a black or dark sur face in the sun, the ambi ent dry-bulb equals the Table 1A or 1B dry-bulb plus 65°F.
d) For any sur face in con tin u ous shade, the ambi ent dry-bulb equals the Table 1A or 1B dry-bulb plus 10°
• For cool ing, the default value for grains of mois ture dif fer ence (for the out door air and return air) is equal to the Table 1A or Table 1B value.
5) The aver age leak age rate for duct sys tems that are care fully sealed by approved meth ods may be sub stan tial lower than the sealed leak age rates listed by the pre vi ous note. Per for mance should be cer ti fied by test or qual ity con trol pro gram before tak ing credit for this type of seal ing effort.
6) When using Table 7, use the load fac tors and latent gain val ues for unsealed duct sys tems ( 0.35 / .070 sce nario) when duct tape is used to seal the leak age points. (Duct tape is not and approved seal ing method. Seal ing work must con form to indus try stan dards.)
7) If a duct run is located in a garage, use Table 7G. If a duct run is located behind an attic knee wall or between the joists in a roofceil ing sand -wich, use Table 7A.
8) The Table 7 load fac tors are com pat i ble with the ambi ent tem per a tures listed by Fig ure 23-6. If the load esti mat ing soft ware per forms an energy bal ance on an uncon di tioned space, the esti mated space tem per a ture should be used to gen er ate load fac tors for the duct runs that pass through the space (pro vid ing the soft ware use the Man ual J duct load model, see note 12).
9) Table 7 can be used to pro duce load fac tors and latent gain val ues for duct runs that pass through dif fer ent types of spaces, and run seg -ments that have dif fer ent leak age rates and insu la tion R-val ues. See Worksheet G1.
10) When duct runs are in an exposed wall, the duct load fac tor for the riser or drop is added to the load value for the sys tem (see Worksheet G1).
This pro ce dure applies to all the exposed-wall load fac tors (heat ing per cent age, cool ing per cent age or latent load value.). Note that the load fac tor for a two-story riser or drop is twice the sin gle-story value, etc..
same with the J1AEform. Then repeat the process for Sections 8 and 9. Refer to Sections 3 and 6; and Appendices 4, 7, 8 and 9, as required.
n Learn to use the MJ8AE spreadsheet (redo the example dwellings in the book, or investigate a simple dwelling that is available for survey).
n Read Sections 2 and Appendix 6 internalize this guidance
n Go to Appendix 2 and read the capabilities and sensitivities material. Make sure that you under-stand the limitations of MJ8AE. Use advanced Manual J procedures for applications that are not compatible with MJ8AE.
n Read Appendix 3 and make sure you understand these concepts. Make sure you understand the limitations of MJ8AE. Use advanced Manual J procedures for applications that are not compati-ble with MJ8AE.
n Read the Introduction of this manual and internal-ize this guidance.
n Read Section 10 and acquire the necessary knowl-edge and skills. Pay particular attention to the error checking procedure.