When It’s Time to Replace a Duct

Less than 60% of U.S. HVAC contractors think that their customer’s comfort problems can be solved by simply changing equipment. When the new equipment is up and running, and the same comfort problem still lingers, it may be time to replace a duct or two. Let’s take a look at the steps involved in this simple maneuver.

The prime symptoms of inadequate airflow in a room are that it’s too hot in the summer and too cold in the winter. You’ll need air-testing instruments to quantify the volume of flow, but if you have these symptoms, you’ll need more conditioned air.

Diagnostics
Diagnostics can be extremely simple. If a room has insufficient airflow because of an undersized duct, add an additional duct or remove the existing duct and install a new oversized duct with a damper.

If you have the required tools, complete a full air diagnostic report to analyze the system and its need for additional ducts. Necessary diagnostic information includes airflow, static pressure, temperature, fan speed, and equipment specifications before an effective duct replacement decision can be made.

Remember, if the duct is in an unconditioned area, and there is substantial heat loss or gain due to a low duct insulation value. You might be getting all the airflow you need, but the BTUs you’re looking for may have been lost in a hot or cold attic.

Once you’ve confirmed the need for additional duct capacity, verify access to the area where the ducts are located.

Duct Materials
Duct replacement materials should match or exceed the existing duct material used in the system. Carefully inspect the existing system and evaluate the materials needed to make the transitions and hang the duct. Regardless of the original duct system material, “oversized” flexible duct with good quality sheet metal takeoffs and balancing dampers are the most commonly used duct materials.

Repair Procedure
Remove and dispose of the existing duct. Normally, a new takeoff is required at the trunk duct or directly from the plenum. Be certain to include a good sheet metal fitting and damper. Typically the can or boot may need replacement as well. Install the new duct according industry best practices. Run the duct as straight as possible and be sure to provide adequate support or suspension.

Return Duct Replacement
If adding a new return duct to relieve static pressure, be certain there is adequate plenum(s) at the equipment. Use both sides and the bottom of the air handler if necessary, and don’t be afraid to oversize the new return duct.

This is also a great time to add return air filter grilles to increase the filter surface area and reduce pressure drop through the filters. This will reduce the system’s total external static pressure of the entire system, which is another element of getting good airflow through a system.

Add needed insulation to the ducts and be sure bare sheet metal is not left exposed to an unconditioned area. Care should be taken to carefully seal at the boot and the ceiling, wall or floor and get a tight seal between the grille or register and the sheet metal fitting.

Test Out
At the completion of the duct renovation, test and balance the system and provide full performance testing to verify the system has achieved the desired results. Remember, If you don’t test, you’re just guessing.


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 a free duct replacement procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.

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