Mechanical Air Distribution and Interacting Relationships

It has been determined from extensive testing conducted between May 1987 and May 1988, in the hot and humid climate of central Florida, that pressure differences within the envelope of residential housing exists. These can range from near neutral to pressures, either positive or negative, as great as 0.24" W.C. (60 pascals). Reasons sighted and discussed in this paper include duct system design, duct system failure, airtightness of the residence and human interactions. This testing further reveals that one of the largest driving forces in air change rates can be attributed to mechanically induced infiltration and exfiltration. Airtightness can also drastically affect this pressure difference within the envelope. In conclusion, the effects of these pressure differentials on energy consumption, indoor air quality, comfort, and degradation of building materials will be discussed. Possible solutions and practical field test protocol to correct these ill effects both in new and existing residential housing will be covered. FINDINGS The axiom, "the whole is the sum of all the partsl1, rings true in both new and retrofit construction. As residential housing becomes tighter it is becoming imperative that a house be viewed as a system. Extensive residential airtightness testing* was conducted between May 1987 and May 1988, on 371 single family homes, ranging from 1 year to 50+ years old. An estimated infiltration rate (ACH-50 / 20) from .35 to 1.95 was recorded (figure 1); 11.6% of these homes were mobile homes, 11.6% were frame construction and the remaining 76.9% were concrete block structures (figure 2). It has been determined that pressure differences within the building envelope* and across interior partitions of residential housing exists. The extent of airtightness of a residence can drastically effect this pressure difference within the building envelope. This testing has further revealed that one of the largest driving forces of air change rates in residential housing can be directly attributed to the operation of forced air ventilation systems. These factors can be major contributors to (1) excessive energy consumption, (2) poor thermal comfort, (3) degradation of bullding materials and ( 4 ) indoor air quality problems (i.e. homeowner ESTIMATED INFILTRATION RATES