Identifying and Solving Duct Leak Problems

In a typical home, 20 to 30 percent of the air moving through the ducts is lost to leaks, holes and poorly connected joints, according to ENERGY STAR. In Sacramento, most of that air escapes into the attic. You pay to cool it, and the room it was meant for stays warm.
Leaky air ducts rarely announce themselves. They show up as one bedroom that never cools down, a power bill that keeps climbing, or more dust than usual. Here's how to spot the signs, check your ducts yourself, and know when it's time to have them tested and sealed.

Why duct leaks cost more in Sacramento

Most Sacramento-area homes are built on slabs or crawl spaces, so the ductwork runs through the attic. On a July afternoon, that attic can reach 130°F or more. Any gap in a supply duct dumps cooled air into that heat, and any gap in a return duct pulls hot, dusty attic air back into the system.

The result is an AC that runs longer, works harder and still can't keep up. In winter it works the other way: heated air leaks out before it reaches the far rooms, which is one reason some homes have [a furnace that seems to blow cold air](link when post goes live) at the vents furthest from the unit.

Signs you have leaky air ducts

• Rooms that never match the thermostat. Bedrooms at the end of a long duct run are usually the first to feel it.
• Higher bills with no change in habits. If your summer bills are up and the system checks out, the ducts are a likely suspect. See more ways to cut your heating bill.
• Weak airflow from some vents. Strong air at one register and barely a whisper at another often means a disconnected or crushed run.
• More dust than usual. Leaky return ducts pull in attic dust and insulation fibers. If you're dusting more often or changing filters more often, check the ducts.
• Stuffy rooms or musty smells. Poor airflow and air pulled from the attic or crawl space can affect indoor air quality.
• An AC that runs nonstop on hot days. Ducts that lose cooled air make the system run longer to reach the set temperature, which wears it down faster and leads to more AC repairs.

Where duct leaks usually hide

• Joints and connections. Where ducts meet the plenum, the furnace or air handler, and each register boot. Tape dries out and lets go over time.
• Flex duct runs. Kinked, crushed or sagging flex duct restricts airflow, and torn outer jackets let conditioned air escape into the insulation.
• Disconnected runs. A run that has slipped off its fitting can blow straight into the attic for years before anyone notices.
• Return ducts and the filter cabinet. Gaps here pull unfiltered air into the system. A return leak in a garage can also pull in car exhaust and fumes.
• Pest damage. Rodents in attics and crawl spaces chew through flex duct.
• Poor installation. Short-cut connections, missing mastic and undersized runs leak from day one.

How to check your ducts yourself

You can do a basic check in about 30 minutes if your ducts are reachable:
1. Turn the system fan on so air is moving through the ducts.
2. In the attic or crawl space, run your hand along joints and connections. Moving air you can feel is a leak.
3. Hold a lit incense stick near seams. Smoke that blows sideways or gets pulled in points to a leak.
4. Look for dirty streaks on the insulation around ducts. Dark patches often mean air has been leaking there for a while.
5. Look for flex duct that's crushed, kinked, torn or lying disconnected.
Be careful in the attic. Step only on joists, and skip it on hot afternoons. A DIY check finds the obvious leaks, but most leakage sits in places you can't see or reach.

How leaky ducts get sealed

DIY fixes for small, reachable gaps. Use duct mastic or UL 181-rated foil tape. Don't use regular cloth duct tape. Despite the name, it dries out and falls off ducts within a few years.
Professional testing and sealing. A technician pressurizes the duct system with a calibrated fan and measures how much air escapes. That number shows how leaky the system is and whether sealing actually worked. From there, the tech seals joints and boots with mastic, reconnects loose runs, replaces damaged flex duct, and checks that the ducts in the attic are insulated.

Sealing pairs well with an HVAC tune-up, since both affect how much air your system actually delivers. If dust has built up inside the ducts, air duct cleaning should come first so the system starts clean.

Replacing your furnace or AC? Ducts get tested too

Under California's Title 24 energy code, replacing a central AC, heat pump or furnace in the Sacramento area generally triggers a duct leakage test, verified by a third-party HERS rater. If the ducts leak more than code allows, they need to be sealed as part of the job.

If you're planning a furnace installation or AC installation, ask how the duct test fits into the quote. A new high-efficiency unit hooked up to leaky ducts won't deliver the savings on the label.

Seal, repair or replace?

• Seal when the ducts are in decent shape and the leaks are at joints and connections. This is the most common fix.
• Repair sections that are crushed, torn or chewed.
• Replace when flex duct is brittle, falling apart or 20+ years old, or when the layout was undersized from the start. Many Sacramento homes from the 1980s and 1990s still have original flex duct.

Duct leaks aren't the only source of lost air. Gaps around windows, doors and the attic hatch matter too. See our guide to finding air leaks in the home.

If some rooms never feel right or your bills keep rising, your ducts are worth a look. Ace Plumbing Heating & Air Conditioning has served Sacramento homeowners for 54 years. Our technicians can test your ducts, find the leaks and seal them properly. Schedule a duct inspection, or check our current HVAC specials first.