Air Distribution: The Most Underrated Aspect of HVAC System Design

When people talk about installing a new HVAC system, a lot of the time they’re really just thinking about the HVAC unit itself. There is more to HVAC than the new heat pump, air conditioner, or furnace. But an HVAC system is much more than just the heating and/or cooling unit.

The HVAC system also includes the ductwork that circulates the air through the home or other building. And what many people don’t realize is that the design, layout, and installation of that ductwork can have as much or more impact on their comfort and their system’s efficiency as the unit itself.

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The Easier Part of HVAC System Design

Determining the ideal size for the new unit is actually the easier part of HVAC system design. The HVAC professional performs a Manual J load calculation that takes into account not just the size of the building but the sizes of individual rooms.  It also takes into account the placement and number of windows, the type of insulation, and other factors.

This calculation tells us how much heating and cooling the building needs in BTU/hour. Then it’s a matter of determining what size unit (or units) will suit it the best.  It considers if it can provide that amount of heating and cooling most efficiently and effectively, without being oversized and wasteful.

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Here’s Where It Gets Tricky

Choosing a new unit is only the first step in the process of good HVAC system design. Now we have to figure out how to get the air to circulate properly. Because if the air doesn’t circulate correctly, then it doesn’t matter how perfectly-sized the unit is. The people in the home or other building won’t be comfortable.  The system also won’t run as efficiently as it could. According to the U.S. Department of Energy, in many homes the efficiency of the air distribution system is 60-75% or even lower.  This is due to poor design and installation.

How Do We Fix It

There are several common issues caused by poor design of air distribution systems. One is uneven heating and cooling. If some areas are getting much more conditioned air than others.  This means you can have one room that’s freezing while another is simultaneously too hot. How many times have you seen this in an office environment? One worker’s desk is directly under a vent and she’s constantly bundled up in a sweater and shivering. Two cubicles away, her coworker is sweating because none of that cool air is reaching him. To create more even heating and cooling, good air distribution design will take into account things like the placement of windows and exterior walls. For instance, a room with a west-facing window will need more cool air in the summer to prevent it from getting overheated.

In addition to delivering air in the right amounts to the right places, a good air distribution system has to effectively and efficiently bring air back to the unit through the return ducts. If the system isn’t properly balanced, then the building won’t stay at a neutral air pressure. This can lead to more outside air getting pulled into the building. That makes the whole system less efficient. Plus, if the air handler has to work too hard to draw air into the system, then it will run less efficiently and wear out faster. Pressure imbalances also increase the leakiness of the air ducts. That means conditioned air can get lost in unconditioned spaces like the attic. Also, dirty air from the attic or crawlspace can get pulled into the system, sending dust, mold, and other unwanted contaminants into your home or office.

Finally, poor duct design can also make your system louder. Undersized duct systems or ones with more bends than necessary will increase the velocity of the air traveling through them, leading to more noise. While we usually think of comfort in terms of temperature and humidity, a noisy system can reduce comfort in a different way.

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Basic Elements of Good Air Distribution Design

There’s a lot that can go wrong if an air distribution system isn’t designed and installed correctly. HVAC professionals spend years learning how to properly put together these systems.

So we can’t cover everything involved in this article. But here are a few of the things that HVAC professionals take into consideration when designing an air distribution system.

1. Where should the air handler be located? Can it be placed inside of conditioned space? Can it be centrally located or will it need to be at one end of the house?
2. What type of supply duct system to use. If using trunk-and-branch, should the trunk be reduced (made narrower) as it gets further from the air handler?
3. What type of return duct system to use and methods for keeping the system balanced when interior doors are closed.
4. What type of ductwork to use. Where should turns and bends be placed and how can they be minimized to avoid reducing airflow?
5. Can ductwork be placed in conditioned space to increase efficiency?
6. Where should supply registers be placed for greatest comfort and efficiency? Where should the return register(s) be placed? How will they affect the movement of air in those rooms?

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