Ductwork is the part of a heating and cooling system nobody sees, so its problems get blamed on the furnace or the AC. If some rooms never get comfortable, the registers in them blow weakly, or the house gets dusty fast while the equipment tests fine, look at the ducts. In many older homes in Menlo Park, Atherton and Portola Valley they run through an attic or a crawlspace, where heat, cold, pests and decades of settling work on them.
Signs worth taking seriously
- Rooms that stay too hot or too cold, usually at the end of the longest runs or over the garage. A disconnected or crushed duct can starve a room completely.
- Weak airflow at some registers. With the fan running, hold your hand to each one. A register clearly weaker than the others points to kinked flex, a closed damper or a run pulled loose.
- Dust that comes back quickly, or dark streaks around ceiling registers. Leaks on the return side pull air from the attic, crawlspace or wall cavities and spread their dust through the house.
- New noises. Whistling at a grille suggests a restriction or a return that is too small; rattling, a loose section or hanger; a pop or bang as the system starts or stops, sheet metal flexing under pressure.
- Musty or garage smells when the system runs. A return leaking in a crawlspace or an attached garage can draw those smells, and car exhaust or chemical fumes, into the house. Treat a garage smell as a priority.
- Longer run times and higher bills. If the system runs longer to hold the same setting and the equipment checks out, conditioned air may be escaping before it reaches the rooms.
- Visible damage in the attic or crawlspace: torn outer jackets, flex lying flat under stored boxes, runs sagging between supports, old cloth tape hanging off joints, signs of rodents.
Why older homes show these problems
Many mid-century ranch homes started with a furnace and ducts sized for heating alone. Air conditioning added later often needs more airflow than those ducts were built for, so the far rooms come up short on hot afternoons. Flex duct in an attic gets stepped on and crushed during other work, and settling over the years, plus the occasional strong earthquake, can pull joints apart. Crawlspace ducts live with damp winters and rodents.
Large homes in Atherton often have long runs from equipment in an attic or basement, so one leak far down the line costs a lot of air. In Portola Valley, dust, pollen and smoke days make a leaky return easy to notice: it pulls unfiltered attic or crawlspace air into the system past the filter. And in any hot spell, an attic gets far hotter than the outdoors, so cool air in a leaky or poorly insulated duct arrives warm, which is why the back bedrooms never catch up.
What a duct inspection checks
A useful inspection measures, not just looks.
- Every reachable run in the attic and crawlspace: connections at the plenum, the takeoffs and the boots behind each register; flex that is kinked, crushed or sagging; torn jackets and thin insulation.
- The return side: the size of the return grille and filter, leaks into the return from a garage, closet or crawlspace, and whether closed doors cut rooms off from the return.
- Static pressure across the system, compared with what the blower is designed for. High pressure means restricted ducts or filter; it explains noise and weak airflow and makes the blower work harder.
- Temperature and airflow at the registers against the air leaving the equipment. A big temperature change along one run points to a leak or missing insulation.
- A duct leakage test when sealing or replacement is on the table. The registers are covered and the ducts are pressurized with a calibrated fan to measure how much air escapes. Done before and after the work, it shows whether sealing worked.
- A camera in runs that can’t be reached, to find a collapse or disconnection inside a wall or under a floor.
Repair, sealing or replacement
| What the inspection finds | Usual fix | Note |
|---|---|---|
| One disconnected, crushed or torn run | Repair: reconnect, replace the section, add supports | Often a single visit |
| Sound ducts with many leaky joints | Sealing with mastic or duct-rated foil tape; hidden leaks sealed from inside | Retest leakage afterwards |
| Bare or thin insulation in the attic | Re-insulate or replace those runs | Matters most in hot spells |
| Return too small or leaking | Larger return, new grille, sealed return path | Often quiets the system too |
| Brittle flex throughout, wrong sizes, AC on heat-only ducts | Replacement with a new layout | Best timed with new equipment |
Repair makes sense when the damage is in a few places and the rest is sound. Sealing suits ducts in decent shape that leak at many joints. Replacement is the realistic choice when the ducts are worn out end to end, never fit the system they now serve, or will be opened up anyway for a new central air or heat pump system. In California, replacing the main heating or cooling equipment can require a duct leakage test, so the two jobs are worth planning together. For a room the ducts never reached, such as a converted garage, a ductless mini-split is often simpler than a new run.
One caution for very old systems: white or gray papery wrap or tape on old metal ducts can contain asbestos. Do not disturb it; have it tested before any duct work.
Our duct repair work covers leaks and disconnected, crushed or damaged runs. Before you approve a larger repair or new ducts, you get the diagnosis, parts, labor and options in writing.
Where duct cleaning fits
Cleaning removes what is inside the ducts. It does not close a leak, reconnect a run or fix a small return, and a leaky return will fill clean ducts with dust again. It is worth it after construction or remodel dust, after pests, or when debris has visibly built up; duct cleaning explains when it pays and when it does not. For dust and smoke, better filtration and a tight return usually do more; see indoor air quality. If you are weighing duct work against insulation and air sealing, an energy audit looks at the whole house first.