182 Energy Project Financing: Resources and Strategies for Success
gas such as sulphur hexafluoride into an area. The efficiency of an
exhaust system can be determined by measuring the rate of tracer gas
removal. Building infiltration/exfiltration rates can also be estimated
with tracer gas.
Blower Door
The blower door is a device containing a fan, controller, several
pressure gauges, and a frame which fits in the doorway of a building. It
is used to study the pressurization and leakage rates of a building and
its air distribution system under varying pressure conditions. The units
currently available are designed for use in residences, although they can
be used in small commercial buildings as well. The large quantities of
ventilation air limit blower door use in large commercial and industrial
buildings.
An air leakage/pressure curve can be developed for the building by
measuring the fan flow rate necessary to achieve a pressure differential
between the building interior and the ambient atmospheric pressure over
a range of values. The natural air infiltration rate of the building under the
prevailing pressure conditions can be estimated from the leakage/pres-
sure curve and local air pressure data. Measurements made before and
after sealing identified leaks can indicate the effectiveness of the work.
The blower door can help to locate the source of air leaks in the
building by depressurizing to 30 Pascals and searching potential leakage
areas with a smoke source. The air distribution system typically leaks
on both the supply and return air sides. If the duct system is located
outside the conditioned space (e.g., attic, under floor, etc.), supply leaks
will depressurize the building and increase the air infiltration rate; re-
turn air leaks will pressurize the building, causing air to exfiltrate. A
combination of supply and return air leaks is difficult to detect without
sealing off the duct system at the registers and measuring the leakage
rate of the building compared to that of the unsealed duct system. The
difference between the two conditions is a measure of the leakage at-
tributable to the air distribution system.
Airflow Measurement Devices
Two types of anemometers are available for measuring airflow:
vane and hot-wire. The volume of air moving through an orifice can be
determined by estimating the free area of the opening (e.g., supply air
register, exhaust hood face, etc.) and multiplying by the air speed. This