Plenum Leakage Test Procedure

Before starting, the systems in adjacent spaces should be operating and stable in their normal cycles so that the pressure relationships for the rest of the building are established at their normal values. The exhaust fans for non-normal functions like chiller refrigerant purge cycles or fire pump room ventilation should be off. 

The steps in the process are as follows:

1   Shut down RTU-e and its related systems.

2   Force the under floor plenum outdoor air dampers closed.

3   Force the terminal unit primary air flow dampers closed.

4   Force the passive supply plenum dampers closed.

5   Bock off the computer room diffuser that provides ventilation air to the computer room directly from the under floor plenum.

Steps 2-5 should completely isolate the under floor plenum so that there is no flow path out of it and the only flow path into it is through the RTU-3 supply duct.  This configuration will allow us to use the RTU-3 supply fan to pressurize it.

6   Force the RTU-3 return dampers fully closed.

7   Force the ceiling plenum relief dampers fully closed.

Steps 6 and 7 should completely isolate the occupied space and ceiling return plenum so that there is no flow path into or out of it.  This configuration will allow us to open the terminal unit primary air dampers after the under floor plenum test and use the RTU-3 supply fan to pressurize the under floor plenum, the occupied space, and the ceiling plenum.

8   Force the RTU-3 outdoor air dampers fully open.

9   Place the RTU-3 supply fan drive in a mode that will allow us to manually control the fan speed. 

10 Set the fan speed to 0 Hz.

11 Start the supply fan and gradually increase the fan speed until the plenum reaches a pressure of 0.2 inches w.c. (the maximum operating pressure anticipated by the design.)

12 Document the outdoor air flow rate at this setting.  This is the plenum leakage rate.  If it seems excessive, then we will need to try to understand where the air is going so the leaks can be fixed.

13 After documenting the leakage rate, begin opening terminal unit primary air valves one at a time.  Open a valve, wait for stabilization and then increase the supply fan speed until the under floor plenum pressure is back to 0.2 inches w.c..

14 If the pressure in the occupied space relative to the outside is less than 0.05 inches w.c., then open another terminal unit primary air valve and repeat steps 13 and 14 until a condition is reached where the under floor plenum is at 0.2 inches w.c.. relative to atmosphere and the occupied space is at 0.05 inches w.c. relative to atmosphere.

15 When the required test condition is reached document the current outdoor air flow rate.  This is the overall leakage rate. 

16 Document the primary air flow rate through the fan terminal unit air valves that are open.  This is the leakage rate for the occupied space and ceiling return plenum.  It should be very close to the same number you get when you subtract the under floor plenum leakage rate from  step 12 from the current outdoor air flow rate.

17 If this rate is excessive, we will need to try to understand where the air is going so that the leak can be fixed.

18 Return the system to normal operation.

If the plenum really is leak-free, this test should not take that long as long as all parties involved know what they are supposed to do and are ready to do it.  Thus it is possible to do the test while the building is occupied and not be too disruptive.  It is desirable to do it early or late in the day to minimize the impact on the occupants and minimize the testing problems associated with people going through doors while readings are being taken.  Towards that end it may be desirable to station people by the doors to control traffic through them while a reading is being taken. 

If the plenum leaks, then the system may be placed back in the test configuration to find the leak or leaks.  This test may need to be done very early in the morning or late in the evening to minimize impact on the occupants. 

Smoke stix and a borozine gun will help identify leaks by allowing air flow patterns to be detected.  Foggers are also helpful but more expensive and may be difficult to find.  Sometimes, laboratory facilities or clean room facilities have them to monitor the flow patterns around hoods and in clean spaces and to look for leakage. A Shortridge™ air data meter and an inclined manometer or magnehelic will be useful to verify the pressures or flow rates, but many times the sensors on the DDC system will provide the necessary data.