THE TECHNICAL TIMES
Louver Sizing
Keeping the Weather Out of Your Building
In the exciting world of HVAC systems and application
based design, not only must a HVAC unit be sized
correctly for the heating/cooling capacity demand, but
also the wall opening size and louver must be specified
such that water will not be drawn into the unit. Water
entrainment in a unit can often lead to serious water
damage inside the building, thus it is crucial to know the
threshold size of the wall opening being dealt with, which
will depend on the specific brand and type of louver
being used.
How do I make sure my wall opening and
louver are sized appropriately to keep rain
out of my building?
Before we can get into the calculations necessary for
solving this problem, it is important to have a
fundamental understanding of the key terms used to do
so. First, let’s take a look at what is called the Free Area
of a louver. This is defined as the area that air will
actually be able to flow through, essentially the area not
taken up or blocked by the louver blades. Obtaining this
value is most commonly done through the use of a Free
Area Chart (Figure 1.1 below) given by a supplier where
the interpolation of values may be required (Free Area
varies for a louver depending on the specific dimensions
of the area). A typical range of values for Free Area is
35-60% of the total area.
Figure 1.1. Free Area Chart for Greenheck ESD-435.
Next, the Split of the unit must be known. The split is
defined as the ratio of the area devoted to bringing
outside air in (intake) to the total area of the louver. With
single wall opening applications, splitter plates are used
within the wall sleeves to separate the specific
airstreams. For example, a 55 split would have 55% of
the opening dedicated to incoming outside air, as shown
in Figure 1.2. These splitter plates are located behind
the louvers at slight angle such that water can drain out
of the unit, and will usually not be set up for more than a
60 split due to the increased pressure drop on the
exhaust side.
Figure 1.2. Wall
Sleeve with
Splitter Plate. As
shown in this
picture, the intake
section has 55% of
the louver area
dedicated to it.
Adjusting the splitter
plate downward will
result in an increase
in this area. Note
also that the splitter
plate is slightly
angled downward to
help prevent water
draining into the
unit.
THE TECHNICAL TIMES
Another key term that is crucial in sizing the wall opening
and louver of a HVAC unit is Free Area Velocity,
defined as the volumetric flow rate of air divided by the
cross-sectional area it is flowing through.
So now that we’ve established our HVAC vocabulary,
let’s dive into the process of figuring out whether or not a
unit is going to bring in undesired water. Let’s consider
an example where we are using a Modine ClassMate®
(CMP48) unit with a single louver configuration.
Figure 2.1. Equation to Calculate the Free Area Velocity.
Lastly, Water Penetration Velocity can be defined as
the threshold velocity at which the unit will begin to not
only draw air, but also water in through the louver and
possibly into the building. This value is often given by the
supplier in their catalog. According to AMCA, “The
AMCA Water Penetration Test provides a method for
comparing various louver models and designs as to their
efficiency in resisting the penetration of rainfall under
specific laboratory test conditions. The beginning point of
water penetration is defined as that velocity where the
water penetration curve projects through 0.01 oz. of
water (penetration) per sq. ft. of louver free area” (1).
Figure 2.3. Side View of a ClassMate® Heat Pump Unit with a
Single Louver Configuration.
Figure 2.2. Water Penetration Graph for Greenheck ESD-435. Water
Penetration data gathered using the AMCA Water Penetration Test.
THE TECHNICAL TIMES
The maximum airflow CFM specified in the technical
manual for a 48 [MBH] (4 [ton]) heat pump unit is 2800
(Note: We always want to use our worst case
scenario values, which are typically condenser airflows
in the case of single-packaged equipment). If we are
using a Greenheck ESD-435 Drainable Blade Louver,
our Free Area will be right around 53% of our given wall
opening cross-sectional area, giving us a Free Area
Ratio (FAR) of 0.53. The Rated Water Penetration
(RWP) Velocity associated with the ESD-435 is 989
.
We will also assume a 50 Split. For now, let’s assume
we are choosing a size and not retrofitting (this will be
discussed later). The width of the wall opening used for
this example must remain constant (43 inches / 3.6 ft.),
thus we must vary the height in order to adjust our area.
Upon back-solving for the height through the use of
some handy algebra, we are left with the following
equation to solve for our minimum height:
Can I reuse an existing wall opening in my
building?
If there is currently a wall opening that you would like to
reuse when installing a new HVAC unit, you will need to
either calculate the minimum height required (as shown
in the previous example), or you can calculate what Free
Area Velocity you will have with your current area and
then compare it to the Rated Water Penetration Velocity
given in the louver supplier catalog.
If my louver is not sized appropriately, what
are my options?
First off, if the area of your louver meets the threshold
requirements, but the width of your wall opening is
smaller than our standard width, Modine offers a range
of Rear Extensions (transition plenums) that can be
installed to help fit the unit to the wall opening.
If it turns out that the area you are looking to use as a
wall opening is too small, there are a few options
available to you.
Figure 3.1. Equation to Calculate the Minimum/Threshold Height.
For our example:
Upon plugging in the correct values on the right hand
side of the equation, a minimum height value will be
obtained. With the given inputs for our example, we
calculate a minimum height of 36.64 inches. As long as
our wall opening has a height about this value, we have
obtained a louver dimension that will satisfy the louver’s
rated water penetration design airflow.
First Option: Adjusting the split.
If you have only done calculations for a 50 Split, see
what values you get with using a 60 Split (meaning
plugging in a 0.6 for the value of the split in the
equation). As the manufacturer, Modine does not
recommend going above a 60 Split due to the resulting
increased exhaust pressure drops. A simple Split
analysis can be done on an excel sheet, giving you the
range of threshold heights for their corresponding Split
value (Figure 4.1 on next page).
Upon finding a Split that allows you to keep your current
wall opening, it is very important that the person who will
be installing the wall sleeve and louver is well aware of
the splitter plate specification as they are the ones who
ultimately control whether the cross sectional area of the
intake section is correct.
THE TECHNICAL TIMES
Variable / Input
Volumetric Air Flow
Free Area
Free Area Ratio (0.35 –
0.60)
Rated Water Penetration
Velocity
Resource / How To Obtain
Modine technical/installation
manual
Louver supplier
performance data, usually
displayed in “Free Area
Chart” in submittal or
supplier catalog
Louver supplier catalog
Figure 4.2. Table of Needed Inputs.
What if the louver I want to use doesn’t have
water penetration data?
Figure 4.1. Threshold Analysis for Varying Splits via Microsoft Excel. The
Threshold Height (Heightmin) is calculated using the formula given in Figure 3.1,
and then iterated for different values of the Split. As shown on the table, at a split of
60 (0.6), the lowest height we could have is about 30”.
Second Option: Choosing a different louver.
All Modine School HVAC products are specified to use a
Greenheck ESD-435 Drainable Blade Louver. There is,
however, a wide variety of other louver options on the
market available to you. Most of these options will be
able to supply you with their Rated Water Penetration
Velocity as well as the Free Area for your wall opening
dimensions. Once you have the needed inputs, simply
plug them into the equations given in this handout.
Third Option: Constructing a bigger opening.
Although not always the most cost effective option, if
neither of the options above will work for you then
constructing a larger wall opening is the only option left.
Without a Rated Water Penetration Velocity it is
impossible to do a threshold analysis to find out a
minimum height/area for your louver. Thus, calculating
the Free Area Velocity for a given louver is useless
without having a RWP velocity to compare it to. Louvers
that undergo the AMCA Water Penetration Test tend to
have RWP velocities in the range of 900-1200
. Any
louver that isn’t AMCA certified most likely isn’t designed
to keep water out and will have a RWP velocity that is
much lower than the louvers actually designed for
keeping water out.
What if my question isn’t covered in this
handout?
If there are any questions or concerns not covered in this
handout, please contact your local sales representative.
See us on YouTube www.youtube.com/ModineHVAC
Follow us on Twitter @ModineHVAC
(1) AMCA Standard 500-L-12, Laboratory Methods of Testing Louvers
for Rating, 2012, Air Movement and Control Association International,
Inc., 30 West University Drive, Arlington Heights, Illinois 60004, U.S.A.
©2015 Modine Manufacturing Company
AIR75-800