Rain Penetration Functional Test Description

The following functional test can be edited to match the specific requirements of your project.  The equipment required section should be completed to indicate the specific requirements of the edited version of the checklist.  Similarly, the acceptance criteria section should be edited to match the specific project requirements. The “System Identifier” heading should be changed to the identifier used by the system to be tested.  Additional columns can be added to allow the checklist to be used for more than one system. 

Bevel: Rain Penetration Functional Test Form

Link to the Rain Penetration Functional Test form for field entry.  A completed form has been included as an example.  The test is described in the sections below.


Performing a quantitative functional test on an installed louver in the field to verify resistance to rain penetration is a practical impossibility.  However, the consequences of moisture penetration beyond the louver can be quite serious for some systems and buildings resulting in:

n    Building envelope failures.

n    Water damage to occupied spaces located adjacent to and below the intake location.

n    Water damage to filters and other system components in the air handling system.

For these reasons, including the outdoor intake verification checks listed previously as well as the passive rain penetration functional test may be desirable.

Functional Testing Benefits



Energy Efficiency Related Benefits

There are not direct energy related benefits associated with this test.  However, identifying and then correcting an undersized louver can lead to some energy savings since the pressure drop through the louver will most likely be decreased.

The potential savings associated with this reduction in pressure drop can be determined using techniques documented under Fan Energy Savings Associated with a Static Pressure Reduction in Appendix C Calculations.

Other Benefits

Mitigates water damage to the building envelope and surrounding spaces.

Mitigates moisture damage to internal air handling system components.

Mitigates moisture related problems with filters, especially during subfreezing weather.

Mitigates IAQ issues.


Functional Testing Field Tips



Purpose of Test

The purpose of this test is to verify the rain penetration resistance of an installed louver qualitatively under actual operating conditions. In most cases, it is reasonable to assume that the installed louver will provide satisfactory rain protection and only use this test as a retrocommissioning and troubleshooting tool if water penetration problems are encountered subsequent to functional testing.  However, there are instances were performing some or all of the steps in this test for new construction may be warranted.  These would include:

§        Systems with field erected louver sections, especially if the project contains multiple louver designs where a discharge louver could accidentally be substituted for an intake louver.

§        100% outdoor air systems.

§        Systems with filters in close proximity to the intake louvers.

§        Systems where indoor air quality is critical.

§        Systems where the pressure relationships in the areas served are critical and the stable performance of the system is critical to maintaining these relationships.

§        Systems where water damage to areas around or below the unit would be costly or intolerable if it were to occur.

§        Systems located where local environmental conditions tend to include driving rain and wind.


Instrumentation Required

Shortridge Air Flow Measuring Computer with Velgrid probe, rotating vane anemometer, or equivalent device to verify louver face velocity and wind velocity

Rain gauge and stop watch (optional) for documenting rainfall rate during the test.

Compass (optional) for estimating the wind direction.

Camera (optional) for documenting water penetration on louver blades and general conditions at the time of test.

Test Conditions

Louver system, flashing and other components of the building envelope penetration around and above the louver are complete.  This is important because some of the water the louver is trying to protect the system from is water that is cascading down the sides of the building.

System operating at design airflow.

System operating on 100% outdoor air if not a 100% outdoor air system.

Heavy rainfall occurring.

Time Required to Test

30 to 60 minutes.

Acceptance Criteria

Louver applied at or below its rated point of beginning water penetration.

Intake compartment remains dry during a heavy rain.

Water penetration that does occur during a driving rain is contained and drained from the system without adversely affecting the system or surrounding building.

Equipment and components located in the intake compartment have been selected and installed in a manner that will allow them to tolerate minor moisture penetrations that can occur during a driving rain or during heavy fog.

Potential Problems and Cautions

Operating the system on 100% outdoor air during a heavy rainfall in the summer could result in loss of humidity control for systems that have not been designed for this condition (i.e. economizer equipped systems that would be off economizer mode during hot and/or humid weather).  Since the system does not need to be in this mode for a very long time to perform the test, this may not be a problem.  However, for systems serving areas where humidity control is critical, it may be necessary to perform the test when the humidity will not impact the process in the space or perform the test during a heavy rain in cooler weather.

Operating the system at design flow could over-cool or over-heat some areas if the system is a VAV system.  Since the system does not need to be in this mode for very long to perform the test, this may not be a problem.  However, for systems serving areas where overcooling or overheating can not be tolerated even for a limited time, it may be necessary to perform the test at time when the deviations can be tolerated or to take steps to mitigate the deviations.  If the areas impacted represent a small (less than 5-10% of the total flow) load, then it may be possible to do the test with out forcing those particular zones to full flow.