Three Steps to Efficient HVAC Design

Looking to design an efficient HVAC system? Here are three steps you can take with your louvers and dampers. Read more about HVAC on the MCDLG Newsstand.

Pressure drop is an issue we must live with in HVAC design. We need louvers to protect ductwork and interior spaces from water damage. And we need dampers throughout the HVAC system to manage and direct airflow to different parts of the building.

These are necessary to keep your system running smoothly, but they also create pressure drop. System fans can help make up for the loss, but that comes at a cost. If left unchecked, this cost can add up across your HVAC system. You can minimize pressure drop by designing with efficiency in mind. Here are three things to think about for designing efficient HVAC.

Number 1: Your requirements come first

We say it all the time on the Newsstand, but it always bears repeating: your requirements always come first. If an opening in your building requires AMCA 540/550 protection, then you will need a louver that meets those standards. You will look for the listing label found on the submittal.

If airflow in a length of duct is expected to be 1800 feet per minute, then you need a damper that can close and open in those conditions. You will look for the damper’s maximum velocity.

You can begin to make design choices for the damper or louver once you know it will meet the requirements of the application.

Number 2: Look for smooth profiles

When air moves through your louver, it moves across the blades. This is also true with dampers, when they are fully open. Smooth surfaces provide the least resistance to airflow.

For louvers, this means blades with few features and a mostly flat blade profile. Drainable troughs and blade catches are excellent for capturing water, but they also create resistance for the air stream.

The exception is chevron blades, which protect the interior with minimal effect on airflow. Chevron blades are commonly used for severe weather and hurricane louvers, because they reject heavy rainfall while maintaining the necessary airflow. Read our article on chevron blades to learn how they achieve this balance.

Unnecessary protection can drive up costs. If you need these drainable features to keep your interiors dry, then you will need drainable louvers. Even so, look for louvers with a higher free area. This gives the airstream more space to move through the louver, which can help minimize the overall effect on the airstream.

A higher free area generally means less pressure drop, assuming the louvers you’re comparing have similar blade designs. When making the choice, check that the air intake speed won’t exceed the louver’s beginning point of water penetration. Otherwise, your system will draw in rainwater with the air. It is more important to protect the opening from water damage.

For dampers, there are typically two options for blades, and they both have smooth profiles. Control dampers can have either single-thickness blades or airfoil blades. There are additional options for each blade type, like core insulation, but their profiles will maintain the same smoothness. The goal of a damper is to pass air through the system, then close off the opening when it isn’t in use.

Both blade types are designed to minimize resistance to the air stream. However, if you want airflow efficiency, then you should look for dampers with airfoil blades. Single-thickness blades are relatively flat, but there are features along the profile that create a small amount of resistance. Airfoil blades provide the least resistance to the airstream.

There is one more thing to think about when designing for efficiency.

Number 3: Keep accessories out of the airstream

Accessories can add functionality to your louver or damper. They can be important additions, but they can also restrict airflow.

Think of a screen for a louver. These accessories keep uninvited guests out of your louvers, especially in places where they are easily accessible by birds or insects. However, they also create a barrier for airflow. The airstream will cross over the blades and through the screen to get to the interior. Louver filters create a similar issue. Although they help keep the airstream clean, they also create a secondary barrier.

These accessories may be necessary for your project, but they also add to the cost required to draw air through the louver. You should only consider these accessories if the application will require them. If you expect uninvited guests at the louver, then you should add a screen to keep them out.

For damper accessories, your options come down to where you want the accessory installed: “on face” or “on jamb”. You can choose to place the actuator in front of the damper, behind it, or on either side of the damper. The first two options will place the actuator inside the airstream with the damper. That means the airstream will move over the actuator and its mounting bracket as it passes through the open damper. This can create excess resistance.

This decision comes down to space and accessibility. Is there enough space around the damper for the actuator? Can a technician easily reach the actuator to test and maintain it? You don’t want the actuator to be out of reach. A working actuator in the air stream is better than a malfunctioning actuator that you can’t service.

In many cases, “on jamb” actuators are mounted to the duct itself, for easier access. This also removes the accessory from the air stream. However, there may not be enough room for the actuator on the outside of the duct. Or it may put the actuator in a hard-to-reach place. If an on-jamb actuator won’t fit, then you should consider an “on face” installation.

Try to make decisions that keep accessories out of the airstream. But remember, your requirements come first.

Bonus: The submittal is your friend

Product submittals tell you what the product can do and what options are available. They help you understand how the louver or damper will perform in your building. This can help you make the best decisions for efficient airflow.

Focus on keeping the air stream clear, look for products with smooth blade profiles, and always make sure the product meets your requirements. The best first step is knowing what you need, and the second step is checking the submittal.