ASHRAE Journal - April 2009 - 54

there must be at least one pressure pickup point per 50,000 ft2 (4645 m2) of enclosure area, inside and outside. Exterior pressures must be manifolded and then used by each gauge as its reference pressure. Interior pressures must be measured on each gauge. The readings are recorded and averaged mathematically. Perform a Single-Point Door Fan Test. The purpose of this step is to get a quick ±25% reading of the building leakage. It is possible that the building is much leakier than the standard and does not warrant an accurate test. In that case, knowing that it fails by a wide margin may be all that is necessary since the test must be repeated. In other cases, the building may fail this single-point test because a door or window or other intentional hole has been accidentally left open. In that case, this is an opportunity to find the problem and set it right before repeating this step. Perform a 12-Point Door Fan Test, in Both Directions, Starting at +50 Pa to +75 Pa. This test can be performed with bias pressures up to 30% of the lowest test pressure and still deliver acceptable results. When both pressurizing and depressurizing, errors due to large bias pressures cancel out, making this procedure the most accurate way to test. Perform a 12-Point Door Fan Test in One Direction Starting at +75 Pa. This test can be performed with bias pressures up to 10% of the lowest test pressure and still deliver acceptable results. This option acknowledges that When setting up the building, seal continuous ventilation inlets, but leave intermittent ventilation open. super large buildings may require truck or trailer-mounted equipment that will not easily test in both directions. Because bias pressures will have a greater impact on single-direction tests, the maximum allowable bias pressure under these circumstances is reduced to 5 Pa. However, the test pressure achieved must be 75 Pa. At these pressures, the bias pressure is somewhat masked by the high test pressure, and extrapolation is no longer an issue. Because buildings often leak more in one direction versus the other, testing in only one direction must be considered less accurate. the building before any additional test pressures are applied. It is difficult to remove the effect of a bias pressure when applying a test pressure with a door fan apparatus. These bias pressures are created by an unpredictable combination of effects and cannot be subtracted. ASTM E779-03 merely subtracts these bias pressures, which works if they are small when compared to the test pressure. Houses typically experience bias pressures of 2 Pa to 5 Pa whereas larger buildings can experience 10 Pa to 20 Pa. Taking results at higher pressures helps achieve a more consistent result. A practical limit to the test pressure is about 75 Pa because above that test pressure, door fan power must be substantially increased, and the risk of damage due to higher test fan wind velocities, and test pressures, increases. A test pressure of 75 Pa is about the maximum pressure that a well-built suspended ceiling can withstand without tearing it down during depressurization or blowing the tiles out while pressurizing. Recommended adjustments to the ASTM standard to achieve meaningful and repeatable results on high-rise buildings are as follows: Test Procedure 1. Test in both directions from 50 Pa to 25 Pa to neutralize the effect of bias pressure and to cancel out temperature and barometric effects. Maximum bias pressure must not exceed 30% of the test pressure. Test Procedure 2. Test in one direction from a higher starting pressure of +75 Pa to 50 Pa. Since the test is in one direction 54 ASHRAE Journal Wind Pa = 0.12 x mph2 Warm Air Warm Air Cool Air Cool Air Figure 1: Driving forces on air leakage control. only, bias pressure must be recorded and additional corrections must be made for temperature and barometric pressure. Maximum bias pressure must not exceed 10% of the test pressure. Both of these procedures will achieve approximately the same accuracy but Test Procedure 1 requires less fan power and causes less air velocity disturbance in the building and can be performed in a wider range of weather conditions. High-rise building test results have focused on airflow at 75 Pa, which is the lowest reference pressure that yields stable results. Reporting airflow in cfm at 75 Pa per square ashrae.org April 2009

ASHRAE Journal - April 2009

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