Arkansas HVACR NewsMagazine January 2019

S tate, National, Chapter News Feature Story

b. most homes being built by the 2009 Arkansas Energy code have less, some far less, NACH than the minimum of 0.35. In a test of 50 homes, one RESNET rater found new construction homes in the less than 0.1 range. Some even lower—a factor of less than 1/3 the minimum. Even though the standard provides for a mechanical ventilation rate that combines the NACH and mechanical ventilation, ASHRAE 62.2, 4.1.2, it is impractical to design the mechanical ventilation considering the NACH since the amount cannot be determined until the structure is completed. Therefore, design of the ventilation system should provide for the total needed volume. The actual amount allowed can be dampered to combine the NACH and the mechanical if a blower door test is performed after the structure is completed. 2. Mechanical Ventilation and or Combination of NACH and mechanical The Arkansas Energy Code, Table 405.5.2, Specifications for the Standard Reference and Proposed Designs, Air Exchange Rate “…not be less than 0.01 × CFA + 7.5 × (Nbr+1) where: CFA = conditioned floor area Nbr = number of bedrooms.” For example: using a house with 2,000 square feet of conditioned space and three bedrooms. Qtot = 0.01 x CFA + 7.5 x (N br +1) Qtot = 0.01 x 2000 + 7.5 x (3 + 1) Qtot = 0.01 x 2000 + 7.5 x 4 Qtot = 0.01 x 2000 + 30

Qtot = 20 + 30 Qtot = 50 cfm

In this example the proper amount of planned mechanical ventilation is 50cfm: however, there are other standards as well. ASHRAE 4.1.1 uses the formula : Qtotal = 0.03 x A floor + 7.5 (N br +1) Where Afloor is the area of the dwelling unit floor area and Nbr is number of bedrooms but not less than one. Using the same example: Qtot = 0.03 x A floor + 7.5 x (N br +1) Qtot = 0.03 x 2000 + 7.5 x (3 + 1) Qtot = 0.03 x 2000 + 7.5 x 4 Qtot = 0.03 x 2000 + 30 Which formula should a designer use? Let’s make a comparison with these two formulas and the .35 NACH calculation. Let’s assume the house has 8 foot ceilings since this formula requires air exchange of the total volume. Qtotal = 0.35 x A floor x ceiling height Qtotal = 0.35 x 2000 x 8 Qtotal = 0.35 x 16,000 Qtotal = 5,600 CFH Since we are looking for CFM, we divide by 60. Since AMC 2010, Table 403.3, Private Dwellings states “0.35 ACH but not less than 15 per person”, the ASHRAE 4.1.1 formula comes closest to agreement. Additionally, ASHRAE 4.1 states, “A mechanical exhaust system, supply system, or combination thereof shall be Qtot = 60 + 30 Qtot = 90 cfm Qtotal = CFH / 60 Qtotal = 5,600 / 60 Qtotal = 93.3 cfm