Ductwork Replacement and Duct Sealing in San Diego, Tested, Not Guessed

A furnace, air conditioner or heat pump only delivers what the ducts carry. In the 1970s–1990s tract homes that make up most of Mira Mesa, Rancho Bernardo, Santee and Chula Vista, the ducts are flex duct run through an attic that reaches 130°F or more on a summer afternoon. Joints were taped, not mastic-sealed; boots were never fastened to the ceiling; and the return was sized for a furnace, not for the air conditioner added later. When we test these systems, leakage of 20–30% of the system's airflow is a common measurement. That is conditioned air paid for and dumped into the attic, and attic air pulled into the house on the return side.

Signs your ducts are the problem

• One or two rooms that never cool or heat, usually the ones at the end of the longest run or above the garage.
• Dust that comes back a day after cleaning, especially on the return side, meaning the return is pulling attic air.
• Bills that climb while the equipment is running longer to make up for what it loses.
• Whistling or hissing registers, a sign of high static pressure from undersized or restricted ducts.
• A new system that does not perform the way the ratings suggest. The equipment is rarely the cause.
• Musty smells at startup from condensation inside uninsulated or poorly insulated duct.

Duct leakage testing and California's Energy Code

The state Energy Code (Title 24, Part 6) treats ducts as part of the system. When heating or cooling equipment is replaced in a home with existing ducts, the ducts have to be leakage-tested by a third-party HERS rater and come in under the code's leakage limit, expressed as a percentage of the system's airflow. If they fail, they are sealed and retested. New duct systems are held to a tighter limit. We perform our own duct blaster test during the evaluation so you know where you stand before the official test, and we include the HERS verification in every changeout it applies to. Read more in our San Diego HVAC permit guide.

Sealing vs. replacement

Sealing with mastic and mesh at every joint, boot and plenum connection fixes leakage on ducts that are otherwise sound and sized correctly. Replacement is the answer when the flex duct jacket has gone brittle, the inner liner is torn, the runs are undersized for a modern high-airflow system, or the layout strangles the air with long runs and tight bends. On a full changeout, replacing tired ducts at the same time is usually cheaper than coming back for them later.

Sizing and static pressure

Modern variable-speed equipment wants low resistance. We measure total external static pressure at the air handler; a high reading means the ducts, filter or coil are restricting airflow, and the blower is working hard to push against it. Undersized returns are the most common cause in San Diego homes, where a single hallway return was built for a small furnace. Duct sizing follows the load calculation room by room, so a bedroom over the garage in Carmel Valley gets the airflow it actually needs. See how HVAC sizing works.

Insulation in hot attics

In inland Zone 10 attics (Poway, Escondido, El Cajon, Santee) the temperature around the ducts on a July afternoon is far above the air inside them. Thin or compressed insulation, or a torn outer jacket, lets that heat straight into the supply air. California's Energy Code sets a minimum insulation value for new ducts (R-6 in San Diego's climate zones); in hot inland attics we often recommend R-8. Coastal Zone 7 attics are milder, but the marine layer makes uninsulated metal duct sweat, which drips and stains ceilings.

Returns and filtration

A duct job is the right time to fix the return. Adding a second return, enlarging the filter grille, or installing a media filter cabinet at the air handler lowers static pressure, quiets the system and improves filtration. See indoor air quality for filter options.