Why HVAC Ductwork Design Matters

On this page, you’ll learn about the importance of ductwork, four key factors in ductwork design and ways to improve ductwork performance and efficiency. When ductwork, aka duct work, is designed properly, it can eliminate hot and cold spots or areas in a small home and reduce them in large homes. 

Think of all the changes in home design and function over the last 100 years.  Combined heat and air systems have replaced large coal and wood stoves to heat, and open windows to cool.  Smart TV’s with WiFi remotes now provide family entertainment in the place of a large, console radio that the family gathered around in the evening.  Microwave ovens now heat prepackaged food in minutes where once a wood-fired cookstove cooked fresh meat and vegetables in hours.

The Importance of Heat and AC Ductwork Design

hvac ductwork design

But what hasn’t changed?  Ductwork.  Ducts built into the floors, wall and ceilings are what enable heated or cooled air to be distributed throughout the house.  HVAC ducts have been around a long time, but there have been some changes in the materials as well as design.  If you already have a home, you may need to be satisfied with the ductwork you have.  But if you are going to be building new, renovating, or just remodeling, you might benefit from learning more about heating and AC ductwork design.

Did you know? Both heated and cooled air are called “treated air.” And air conditioned air isn’t just cooled. It is dehumidified too. Drier air is more comfortable at a higher temperature than humid air, so you can keep your thermostat set a little higher in summer, still be comfortable and reduce energy use and cost. 

Does Ductwork Design Really Matter?

The short answer to that question is Yes.  It matters because the only way the heated air from your furnace or cooled air from your air conditioner can get to the different rooms in your home is through the ductwork.  Anything that affects the ability of the air to flow freely through the ducts has an effect on efficiency – and that translates into both the comfort and cost associated with heating and cooling your home.

How Does Ductwork Design Make a Difference?

There are at least four factors that make a difference in how the design of your HVAC ductwork functions: the weight of air, friction, turbulence, and distance. 

1. The Weight of Air

We don’t often think about how much air weighs, but in certain situations, it can make a big difference.  HVAC applications are one of those situations.  The air that is in your home actually weighs almost 1/10 pound per cubic foot.  That’s about an ounce and a half.  Big deal, you say? Well, let’s look at it in a little more depth.  An average-sized AC unit is rated at 2 ½ tons.  It will move approximately 1,000 cubic feet of air per minute.  That means the blower in your furnace or air handler will be pushing about 80 pounds of air through your ductwork every minute.

You cannot change the weight of air; it’s something we must live with.  But because air does have weight, it’s a factor in how the design of your HVAC ductwork makes a difference.  And just before we leave this topic, think about the fact that it’s easier to push 80 pounds sideways than it is to push it upwards.  If your HVAC unit is one or two levels below the rooms in your home, the air is always being pushed up before it can be pushed sideways.

Pro Tip on HVAC Equipment Location: Warm air rises. Cool air falls. More heated air is needed in cold climates. More air conditioned air is required where the AC runs spring into fall but the heat source gets little work. So…in cold climates, system efficiency is maximized when the furnace/air handler is located in the basement or lower level of a multistory home. And the best place for the air handler in a home in a warm climate is in the attic.

2. Friction in Ductwork

You have probably already thought about this after thinking about pushing that 80 pounds of air.  It’s a lot easier to push an 80 pound weight on a sheet of ice than it is on a gravel road, isn’t it?  If the ductwork in your home has a rough interior surface or is rusty, your HVAC system will have to work harder to push the air to the various rooms.  If the ducts are made of a flexible material, or have dampers in them, these design features create additional friction in your ductwork.  Once again, this makes it harder to push that air from your heating or cooling system through your home.  This impacts the efficiency of the system and will show up on your utility bill.

3. Turbulence

Closely related to the previous item, turbulence is the third factor in how your AC ductwork design will make a difference in how well it functions.  We all know that a stone or log situated at the edge of a flowing stream of water will cause an “eddy” on the downstream side of it.  In this eddy, a portion of the water swirls around rather than flowing smoothly downstream.  It is the same with any irregularity on the inside surface of your ductwork, except it’s some of the air that swirls.  Here are some examples of turbulence-causing irregularities:

  • Non-smooth surfaces (mentioned above).
  • Sharp angles at turns and corners rather than smooth, sweeping elbows.
  • Improperly designed or fitted joints – or loose joints sealed with tape.
  • Anything that interrupts the smooth flow of the air from your heating or cooling system fits into this category and is a negative factor in your ductwork design.

 4. Distance

Many HVAC duct designs are of the “trunk and branch” type.  There is a main trunk into which air is pushed by the air handler, and then several branches that are connected to the trunk.  Some of these branches are connected to only one or two air registers in just one room, while others might be connected to several registers in more than one room.  Some might be connected to vertical (often smaller) ducts that go up through walls to another level of the home.  Also sometimes registers are placed directly in the main trunk with no other ductwork involved at all.

Which register will have the greatest amount of air pressure – the one that is connected by 40 feet of ductwork two levels above the air handler, or the one that is 10 feet away directly in the side of the main trunk?  Pretty obvious, isn’t it?  

The further a register is from the air handler and the more turns the air has to take to get there, the less pressure it will have coming out. When heating, this is the primary cause of hot spots near registers or air vents and cold spots far from them. When cooling, the opposite is true. Neither is beneficial; proper ductwork design will reduce hot spots and cold spots by planning both duct runs and by the spacing/frequency of air vents.

Supplemental Heating and Air Conditioning in Larger Homes

When all these factors are put together, we can see why we need to have our HVAC ductwork designed for the maximum efficiency.  It can also explain why sometimes a home in a cold climate will have some rooms that are nice and warm, while others need supplemental heat to keep the occupants comfortable.  On the other end of the spectrum is the home in a very warm climate that needs a window air conditioner installed in an upstairs bedroom because the central A/C just doesn’t seem to reach that far. 

Improving Ductwork Performance

As mentioned earlier in the article, with an existing HVAC system, there isn’t a lot that can be changed without some pretty major retrofitting.  However, if you are going to be building new or remodeling, you can make sure that your new ductwork follows good design practices.  The U. S. Department of Energy has an extensive guide that your HVAC technician should be familiar with and hopefully will follow. If you are doing the work yourself, read up on these principles and use the materials that will ensure the most efficient airflow through your HVAC ducts.

As far as your existing ductwork is concerned, here are a few things you can do:

Inspect: Visually inspect all of the ductwork that is accessible.  Look for loose joints or taped joints that might be leaking air because the tape has dried and peeled off.

Not-so-fun Fact: According to Energy Star, “In typical houses, about 20% of the air that moves through the duct system is lost due to leaks, holes and poorly connected ducts. The result is an inefficient HVAC system, high utility bills, and difficulty keeping the house comfortable, no matter how the thermostat is set.” Making sure ducts are in good condition and insulated is a home improvement job with excellent return on investment.

flex ducts

Consider Replacing Flexible Ducting: If you find flexible ductwork, know that the inner surface probably is creating friction and turbulence to the airflow.  If it’s feasible, you might replace it with new ductwork that has a smooth interior.  At the very least, check any flexible ductwork for tears or leaks at joints.

Cleaning? Ductwork that is more than 10 years old could probably benefit from professional cleaning.  It will not only remove any clumps of debris that cause friction and can restrict airflow, but will get rid of a lot of dust that will constantly be coming into your rooms.

What about furniture? If air vents are covered by furniture, move the piece off the vent. If that’s not possible, use air deflectors to get treated air out where it can be felt and enjoyed.

Ductwork - Next Steps to Better Efficiency & Comfort

If you have been wondering why your furnace isn’t heating as well as you think it should, or why your air conditioner just doesn’t seem to be doing the job, it could be because your heat and AC ductwork design isn’t all it should be.  

Before shelling out really big bucks for a whole new system, try checking into the design of your ductwork.  You might just discover how you can improve the overall performance of the heating and cooling of your home by modifying, repairing and insulating the ductwork you already have.

You might also be interested in the comprehensive and one-of-a-kind PickHVAC Ductwork Cost Guide. As we say there, “This information is aimed at the homeowner who wants to fully understand the mechanical systems in their home – in this case, the ducts, their purpose, proper design and cost.”