Ductwork in Florida: Leaks, Airflow and Installation

What ductwork is, and the three kinds in most homes

In a house with forced-air heating and cooling, ducts distribute conditioned air, which means air that has been heated or cooled, throughout the house. Supply ducts carry that air out to the rooms, and return ducts bring room air back to the equipment.

Carrier's ductwork guide names the three most common kinds in homes: flexible duct, sheet metal duct and duct board. Flexible duct is a tube of wire coils covered in plastic or metalized polyester. Sheet metal duct is rigid and made of galvanized steel or aluminum. Duct board is a rigid product made of compressed fiberglass insulation with a foil facing.

How much air ducts lose, and where it escapes

The Duct Retrofit Decision Guide, a 2024 PDF hosted by the Department of Energy, repeats the 20 to 30 percent range and credits it to ENERGY STAR. None of the duct-loss and airflow sources cited here reports measurements taken in Tampa Bay homes.

ENERGY STAR's duct sealing fact sheet lists leaky, torn and disconnected ducts, poorly sealed registers and grills, leaks at the furnace and filter slot, and kinks in flexible duct. Registers and grills are the vent covers in each room. ENERGY STAR lists signs that ducts may need work: high utility bills, rooms that are hard to heat and cool, stuffy rooms, and ducts that run through an attic, crawlspace or garage. Our reading: these signs fit other problems too, so they are reasons to check the ducts, not a diagnosis.

The Department of Energy-hosted guide says duct compressions that look minor to the eye can severely restrict airflow. Its example is that a compression of just 15 percent can raise pressure loss by 400 to 800 percent. Pressure loss means the blower must push harder to move the same air, so a squashed section of flexible duct matters more than it looks.

Why ducts sweat in a Florida attic: humidity, dew point and mold

The National Weather Service says the dew point marks the temperature at which water vapor turns into liquid water droplets, a process called condensation. A surface colder than the dew point of the air around it collects water, the way a cold glass does on a summer day. The Tampa Bay office describes the rainy season as humid, with persistent dew point values above 70°F. Our reading: a cold supply duct in a humid attic can meet both conditions, a cold surface and moist air.

Researchers at the Florida Solar Energy Center wrote in a 2017 research proposal on the Florida Building Commission's website that duct condensation, or sweating, is most likely from July through November. The proposal says the potential for condensation increases if porous insulation, such as blown-in loose-fill insulation, surrounds the duct. The proposal also says there has not been adequate published research on how deeply buried ducts, which sit under a layer of attic insulation, perform over time in hot, humid climates. The document is a proposal for a study, not a finished study.

ENERGY STAR's contractor list calls for insulation with an R-value of 6 or higher on ducts in attics, crawlspaces and garages. R-value measures resistance to heat flow, so a higher number means more insulation. EPA also says insulated duct that gets wet or moldy cannot be effectively cleaned and should be removed and replaced. EPA adds that before ducts are cleaned, retrofitted or replaced, the cause of the problem must be corrected or it will likely recur.

Water at a ceiling can also come from the condensate drain, covered in air conditioner leaking water. Growth inside the system has its own page: mold in HVAC systems.

Airflow and static pressure: the numbers to ask for

Airflow is the amount of air the blower moves, measured in cubic feet per minute, or cfm. Static pressure is how hard the blower has to push to get that air through the filter, coil and ducts, measured in inches of water column. More resistance in the ducts raises the static pressure and cuts the airflow. External static pressure is the part of that resistance outside the equipment cabinet.

A 2024 Lennox manual for one air handler says that for best air performance the external static pressure drop must not exceed 0.5 inches of water column. Our reading: the right limit depends on the model, so use the one in the manual for your equipment. The Department of Energy-hosted guide tells installers to check the manufacturer's recommended total external static pressure for the model installed and compare the measured value to it. The same guide says high static pressure has various causes, and the most common is a dirty or incorrectly placed filter.

ENERGY STAR says poor airflow can mean reduced efficiency, higher utility bills, overly damp air, reduced comfort and a shorter equipment life. A common rule of thumb is about 400 cfm of airflow for each ton of cooling capacity. The Air Conditioning Contractors of America (ACCA) blog says many factors can change that figure. It adds that in hot, humid climates, a lower airflow like 350 cfm per ton can ensure the customer is comfortable. Our reading: ask what airflow the installer set and which manufacturer table it came from, instead of accepting a round number.

Testing for leaks: what the Florida code form records

A duct leakage test uses specialized equipment to measure airflow and find leaks, according to the Department of Energy-hosted guide. Florida's 2023 energy conservation code, in its 8th edition, includes a Duct Leakage Test Report form in Appendix RD. The form records leakage for each system in cfm25. Cfm25 means cubic feet per minute at a test pressure of 25 pascals. The form divides total leakage by the home's conditioned floor area, and the result is called Qn.

On the form's prescriptive method, which sets fixed limits, Qn must be 0.04 or less to count as substantially leak free when the air handler is installed during the test. If the air handler is not installed during the test, Qn must be 0.03 or less. Our arithmetic: 0.04 works out to 4 cfm25 for every 100 square feet, or 80 cfm25 for a 2,000-square-foot house. The form says the test must follow ANSI/RESNET/ICC 380, a national testing standard, and be done by individuals named in the Florida Statutes.

Pinellas County posts a Submit a Duct Leakage Test Report Form page. It describes the form as required by the energy conservation section of the Florida Building Code and names the 2017 edition. This page did not read the code's full text, so it cannot say whether a test is required when ducts are replaced or equipment is changed out in an existing house. Ask your building department, and see HVAC permits in Florida for how permits work.

Seal, repair or replace: how one guide sorts existing ducts

The Department of Energy-hosted guide is written for heat pump installers deciding whether to adjust, reuse or abandon existing ductwork. Our reading: a central air conditioner uses the same kind of ducts, so the questions carry over. The guide calls ducts acceptable for use when they pass a visual evaluation and the total external static pressure is within the manufacturer's acceptable range. They must also be sized for the airflow, reasonably balanced, free of leaks and compressions, and insulated or in conditioned space. Conditioned space is the heated and cooled part of the house.

The guide puts ducts in a middle group, to be modified, when branches have compressions or leaks or when insulation has gaps or tears. The guide says to replace or abandon ducts that are in poor condition or deteriorating, too small for the airflow, or uninsulated in an unconditioned space. Before any work, the guide tells installers to ask about asbestos and check insulation, duct, tape and other possible asbestos-containing materials.

How heating and air conditioning ducts are installed

Installation starts with design, not with cutting and hanging duct. ACCA's Manual D is its procedure for designing residential duct systems. Its procedures match duct system resistance, or pressure drop, to the blower's performance as the manufacturer's tables define it. The Department of Energy-hosted guide calls a Manual D calculation crucial in new installations or when a new air handler is added.

These items come from ENERGY STAR's list of what to look for in a duct contractor:

• Inspect the whole duct system, including the attic and crawlspaces.
• Evaluate the supply and return air balance.
• Repair damaged and disconnected ducts, and straighten tangled or crushed flexible duct.
• Seal all leaks and connections with mastic, metal tape or an aerosol-based sealant.
• Seal all registers and grills tightly to the ducts.
• Evaluate the airflow after repairs are completed.
• Make sure gas or oil-burning appliances do not backdraft, and run a combustion safety test after the ducts are sealed.

Backdrafting means combustion gases, such as carbon monoxide, are drawn back into the living space instead of leaving through the vent. ENERGY STAR says some homeowners take on duct sealing as a do-it-yourself project, and many choose a professional contractor. Work on refrigerant, electrical wiring and gas or other combustion appliances is for licensed professionals only, and this page gives no do-it-yourself steps for any of it.

Questions to put in writing before you hire

Put these eight questions to any contractor, in writing.

1. Will you test duct leakage after the work, who will run the test, and can I have the written result?
2. What airflow did you set, and which manufacturer table does it come from?
3. What was the measured total external static pressure, and what is the manufacturer's limit for this model?
4. Where are the return ducts and grills, and are they sized for the airflow?
5. Will joints and registers be sealed with mastic, metal tape or an aerosol-based sealant, and what R-value of insulation will cover the ducts in the attic?
6. If I have a gas furnace or water heater, will you run a combustion safety test after the ducts are sealed?
7. Who pulls the permit, and does my building department require a duct leakage test for this job?
8. If you tell me a duct or its insulation is wet, moldy or damaged, can you show it to me?

If a contractor cannot answer one of these, the answer is not on paper yet, and that is worth knowing before you sign.