Infiltration: Blower Door

Estimated Infiltration
 

To estimate the number of air changes per hour in the living room and dining room we used the following equation¹:
 

ACH = (A_L(C_S * T + C_W v²) ^1/2) / V

where

AL	= effective leakage area (in2)    Calculated by adding up the sized of potential leaks and multiplying them     by a coefficient.  (ex:  number of electrical outlets = 3 * .38 in2 ea.)
CS	= stack coefficient
T	= difference between inside and outside air temperature2
CW	= wind coefficient
v	= average wind speed
V	= volume of house (or room)

Measured Infiltration
 

During the actual test, we measured the cubic feet passing through the fan per minute at a particular pressure difference (50 Pa).  Using the equation below, we converted the number into ACH  @ 50 Pa or ACH50³

The equation for determining the average air changes per hour is similar to the theoretical equation above:

                    ACH50 / (C *H*S*L)
 
 

where C = climate factor
          H = height correction factor
          S = wind shielding factor
          L = leakiness correction factor

Infiltration vs. Conduction
 

The heat loss equations will tell us not only how much heat is lost, but also how much of this heat loss is due to infiltration rather than conduction.     Using the the area of each material and tables4 in which show the R value, a coefficient related to heat loss, of various materials, we will calculate the total R value of the wall.  The area of the wall, the average temperature difference, and the U value, which is the reciprocal of the R value, are multiplied to give a the total heat loss through conduction. From this number we determined the percentage of heat loss which is actually due to infiltration.  This varies on the estimated tightness of the building.

Opaque Element #1 Description: ____________________ Outside air film . . . . . . . . R = ______ ____________________ R = ______ ____________________ R = ______ ____________________ R = ______ ____________________ R = ______ ____________________ R = ______ Inside air film . . . . . . . . . R = ______                                  R total = ______ U = (1) / (R total) = ______________

 

Element	U:   Btu/h-ft2-° F	A (area):    ft2	D(degree day):t -°F	q (heat loss):  Btu/h
Opaque  Element #1

Heat Loss due to Infiltration = 1.1 * V * T

where    V= airflow rate in CFM
           T = difference between inside and outside air temperature²

^{And, of course,}

^{Heat Loss due to Infiltration + Heat Loss due to COnduction = Total Heat Loss}
 

1. equation and coefficients: ASHRAE Fundamentals I-P Ed. ASHRAE,  Atlanta, GA, 1997, p. 25.1-25.24
2. climate information:  Western Regional Climate Center
3. all equations using experimental data, and the corresponding coefficients, are from the Minneapolis
    Blower Door Manual
4. coefficient tables found in Mechanical and Electrical Equipment for Buildings 8th Ed..  Stein, Benjamin
    & Reynolds, John S. John Wiley and Sons, Inc.  New York.  1992