ASTM E741-11

Effects of Air Change—Air change often accounts for a significant portion of the heating or air-conditioning load of a building. It also affects the moisture and contaminant balances in the building. Moisture-laden air passing through the building envelope can permit condensation and cause material degradation. An appropriate level of ventilation is required in all buildings; one should consult ASHRAE Standard 62 to determine the ventilation requirements of a building.

Prediction of Air Change—Air change depends on the size and distribution of air leakage sites, pressure differences induced by wind and temperature, mechanical system operation, and occupant behavior. Air change may be calculated from this information, however, many of the needed parameters are difficult to determine. Tracer gas testing permits direct measurement of air change.

Utility of Measurement—Measurements of air change provide useful information about ventilation and air leakage. Measurements in buildings with the ventilation system closed are used to determine whether natural air leakage rates are higher than specified. Measurements with the ventilation system in operation are used to determine whether the air change meets or exceeds requirements.

Known Conditions—Knowledge of the factors that affect air change makes measurement more meaningful. Relating building response to wind and temperature requires repetition of the test under varying meteorological conditions. Relating building response to the ventilation system or to occupant behavior requires controlled variation of these factors.

Applicability of Results—The values for air change obtained by the techniques used in this test method apply to the specific conditions prevailing at the time of the measurement. Air change values for the same building will differ if the prevailing wind and temperature conditions have changed, if the operation of the building is different, or if the envelope changes between measurements because of construction or deterioration. To determine air leakage sites, follow Practice E1186.

Fan Pressurization—A related technique (Test Method E779) uses a fan to pressurize the building envelope. Measurements of corresponding air flows and pressure differences across the envelope characterize envelope airtightness as either the air leakage rate under specified induced pressure differences or the equivalent leakage area of the envelope. These factors permit modeling natural air change due to wind and temperature differences. However, direct measurement of natural air change is not possible with Test Method E779. Test Method E779 permits comparison of different buildings, isolation of leakage sites, and evaluation of retrofit measures.

1.1 This test method covers techniques using tracer gas dilution for determining a single zone's air change with the outdoors, as induced by weather conditions and by mechanical ventilation. These techniques are: (1) concentration decay, (2) constant injection, and (3) constant concentration.

1.2 This test method is restricted to any single tracer gas. The associated data analysis assumes that one can characterize the tracer gas concentration within the zone with a single value. The zone shall be a building, vehicle, test cell, or any conforming enclosure.

1.3 Use of this test method requires a knowledge of the principles of gas analysis and instrumentation. Correct use of the formulas presented here requires consistent use of units, especially those of time.

1.4 Determination of the contribution to air change by individual components of the zone enclosure is beyond the scope of this test method.

1.5 The results from this test method pertain only to those conditions of weather and zonal operation that prevailed during the measurement. The use of the results from this test to predict air change under other conditions is beyond the scope of this test method.

1.6 The text of this test method references notes and footnotes which provide explanatory material. These notes and footnotes (excluding those in tables and figures) shall not be considered requirements of this test method.

1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.