Plenum Leakage Functional Test Description
Link to test forms for a plenum leakage functional test. The text below describes this test.
A key factor in achieving success with an under floor plenum system is to make sure that the plenum is as leak-free as possible. Achieving a leak-free plenum can be very difficult. For instance, the 1/4” gap between the drywall and the floor that is usually required to prevent moisture migration into the drywall amounts to a potential unsealed opening of over 8 square feet if it occurs around the base of a 100 foot by 100 foot plenum in an office building. Left unsealed, that would constitute a major leak that could prevent the system from functioning as intended. Even with due diligence during construction, it is not unusual to test the plenum for leakage and discover that 20%-30% of the system capacity is being lost. In these situations, the construction team usually finds itself confronted with 800 leaks of 5 cfm rather than two 2,000 cfm leaks. This situation can be challenging and can require some ingenuity to identify the problems. Figure 1illustrates some techniques that have been used with success.
In Figure 1 on the left, a ventilation damper associated with a natural ventilation cycle was covered with plastic. A tube led from the plastic to the atmosphere. This concentrated the relatively low leakage rate from the entire damper area into the tube to provide a velocity high enough to measure. The test result showed that the leakage rate for the dampers was an order of magnitude greater than their specifications. The culprit wan an inadequate damper actuator torque for preloading the seals. In the center picture of Figure 1, a similar arrangement is being used to concentrate the leakage from a floor communications and power outlet box for measurement. In the picture to the right, a tissue paper strip quickly detects significant leakage around a receptacle.
Testing for plenum leakage is similar to a blower door test and can often be accomplished using the HVAC equipment that serves the area. The plenum leakage test procedure linked to at the beginning of this document describes such a test. This leakage test was written for the system illustrated
Figure 2 and Figure 3. This system includes a natural ventilation cycle.
Figure 2 System Diagram for the Plenum Leakage Test Example
Figure 3 Continuation of the System Diagram for the Plenum Leakage Test Example