We redesign duct systems because they don’t perform as they should. Have you ever stuck new equipment on an existing duct system assuming the old ducting worked just fine? If I were your daddy, I’d say “shame on you!”

The assumption that the old duct system is operating correctly is a major mistake. Less than one in 10 duct systems are installed well enough to allow a new system to perform above 80% of the equipment manufacturer’s rated capacity. That’s a little scary, isn’t it?

Furthermore, there are mountains of information about how to design a new HVAC duct system, but nothing much that addresses how to redesign an existing HVAC duct system so it performs properly. We’ve looked, but can’t find it.

Test Before Redesigning
Most of us redesign HVAC systems when its time to change out equipment. If you think about it, a critical first step is to assess the existing system operation. We call it testing. Without testing the operating condition of an existing system, you have no starting point.

Sure, you can assume an existing duct system was designed and installed perfectly, but most of us know where that leads. Pre-testing enables us to assess an existing system before redesigning it. System values that should be tested include total external static pressure; system component pressure drops; fan, system and grille register airflow; as well as system temperatures. Compare this data against the equipment manufacturer’s published engineering data to evaluate existing system performance.

This is the best possible starting point for a duct system redesign.

Design and Sell?
If possible, you should complete this process in the presence of your customer. This can be a terrific educational and sales opportunity that produces an average closing rate well over 70%.

Invite your customers to participate or at least to watch the testing and record the test data. Also allow them input into the design process.

Create a Floor Plan
Walk through the building with your customers and measure the dimensions of each room. You might try a digital measuring device. Customers are impressed and there is no metal tape to drag around. Sketch a floor plan containing all the key characteristics of the building.

Then record the key elements of each room that may affect airflow and heat loss or gain. Draw a floor plan and record the equipment nameplate data. Gather all the data you’ll need later to complete an ACCA Manual J load calculation. Next apply ACCA Manual S to select the right sized equipment.

Check Fan Size and Static Pressure Capacity
This step is an extract from Manual D that creates a static pressure profile of the system, enabling you to look at a static pressure budget, and see if you spent too much on system components. It’ll also help verify what fan capacity is left over for a rainy day.

This is also an essential step for duct sizing. By doing this, you become very aware of how essential it is to select coils and filters that encourage adequate airflow. This is another five-minute task that assures your system redesign will deliver a system that actually works.

Room-By-Room Airflow
This step is where the proverbial rubber hits the road. How much air does each room need? The required system airflow is divided among the rooms of the home. It’s an essential step to help customers interpret the balancing hood readings.

Measure System Air Properties
Air is the stuff that moves through the system. To see what the system is doing, measure the air properties. To improve what the system does, interpret the air property readings and combine those readings with redesign principles. Refer to the manufacturer’s engineering data to support and interpret your readings.

How lucky do you have to be to find a system good enough to simply plug in new equipment? Dream on. The future of the entire redesign hinges on your ability to interpret existing readings and apply corrective measures. If static pressure is above the proposed rated fan pressure, you’ll either have to prescribe a fan with more pressure capacity or add additional ducting to relieve static pressure.

Maybe the filtration system needs to be changed, either to less restrictive filters or to increase filter surface area to reduce pressure drop. System components may have excessive pressure drop and may not be affordable to the fan, and have to be removed or replaced.

If the temperature loss through the duct system exceeds three degrees, perhaps additional R-19 insulation is required on the duct system. Temperature changes at the supply registers may indicate leakage at the boot that will need to be sealed. A significant change in the return air temperature may reveal a return leak that should be tightened up.

Some rooms may have no or doubled airflow indicating ducts may need to be replaced or dampers installed. Perhaps an oversized fan is the best option. What if flex duct looks badly deteriorated? Replace it.

To assume the duct system is adequate places the odds 90% against you. When you test the system, defects are pinpointed and a redesign can correct the flaws that have reduced system performance. Most technicians or salespeople say this testing takes less than an hour for each per system.

Duct System Design
Finally, lay out a duct schematic according to local tradition and the building allowances, or just focus on the ducts that aren’t performing properly.
Follow the principles of good duct design across the industry. Don’t be afraid to oversize your ducts and install dampers. We find most systems are 20% to 30% undersized.

Keep in mind the room available for duct renovation and the type of duct material used. Follow the NCI Duct sizing tables and then select and size the registers for air distribution and flow.

Build and then Test the System
Design is a concept of what should happen. Testing verifies each idea of the design and confirms system performance. To design only is just to dream and hope a great system will leap into existence.

The final step in design may require tearing out a few ducts and replacing them with larger ones. If you’re angry about this statement, you’re an over confident redesigner detached from reality. If you chuckle a little at this statement, I love ya, man. You are building systems that others are only dreaming about.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in receiving a free copy of the NCI duct Sizing Tables, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.