The crack method is the most accurate means of calculating heat loss by infiltration, because it is based on actual air leakage through cracks around windows and doors, and takes into consideration the expected wind velocities in the area in which the structure is located. The air-change method does not consider wind velocities, which makes it a less accurate means of calculation.

Calculating heat loss by air infiltration with the crack method involves the following basic steps:
1. Determine the type of window or door
2. Determine the wind velocity and find the air leakage
3. Calculate the lineal feet of crack
4. Determine the design temperature difference

The data obtained in these four steps are used in the following formula:

H = 0.018 X Q(t1 – t0) X L


H = Heat loss, or heat required to raise the temperature of air leaking into the structure to the level of the indoor temperature (t1) expressed in Btu’s per hour.

Q = Volume of air entering the structure expressed is cubic feet per hour

t1 = Indoor temperature

t0 = Outdoor temperature

0.018 = The specific heat of air (0.240) times the density of the outdoor air (approximately 0.075)

L = Lineal feet of crack.

Determine the infiltration heat loss per hour through the crack of a 3 X 5 ft. average double-hung, non-weatherstripped, wood window based on a wind velocity of 20 mph. The indoor temperature is 70°F and the outdoor temperature is 20°F. The air leakage for a window of this type at a wind velocity of 20 mph is 59 cubic feet per foot of crack per hour. This will be the value of Q in the air filtration formula. The lineal feet of crack is (2 X 5) plus (3 X 3), or 19 feet (the value of L in the formula). t1 = 70°F and t0 = 20°F. Substituting this data in the air infiltration formula gives the following results:

H = 0.018 X Q(t1 – t0) X L
= 0.018 X 59(70 – 20) X 19
= 1.062 X 50 X 19
= 1008.9 Btuh = 1009 Btuh

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