Start at the register, not the attic
The first question is whether the problem is one room or every room. Weak air at one or two registers usually means something is wrong with the runs that feed them. Weak air everywhere points at the system: a clogged filter, a dirty coil, a struggling blower, or a return that can’t pull enough air. That’s an AC repair, not a duct job, and we’ll tell you so.
For rooms that stay too hot or too cold, ENERGY STAR says “Improper equipment operation, duct problems or inadequate insulation could be the cause.” So we rule out the equipment, then trace the run from the register back through the attic, looking for the spot where it’s crushed, kinked, sagging or loose.
Why flex duct gives out in Houston attics
Many houses have flexible duct: a plastic inner liner, a blanket of insulation and an outer jacket, hung in a vented attic. It holds up when it’s hung right. The Department of Energy’s Building America program calls for supports no more than 4 feet apart, no more than ½ inch of sag per foot between them, and straps that don’t pinch the inner core.
Real attics can drift from that. Straps loosen, runs get stepped on, and storage boxes can end up sitting on the ducts. Building America says sagging restricts air flow and “Crushed ducts can eventually fail causing air leakage and large energy losses,” with symptoms like weak airflow at a register, extra noise, even a burned-out fan.
Repairs that fix one run
Many duct problems can be fixed without a new system. Typical fixes for one run include reconnecting a run that pulled off its collar or boot, re-hanging a sagging run, replacing a crushed or torn section, or sealing a boot to the ceiling so the register stops pulling attic air around its edges.
ENERGY STAR’s contractor list says the same: “Repair damaged and disconnected ducts and straighten out flexible ducts that are tangled or crushed.” For how much air leaky ducts lose, see our air duct cleaning and sealing page.
When a repair stops making sense
A repair is worth it when the rest of the ductwork is sound, and not when the problems are everywhere: the outer jacket cracks when it’s handled, the inner liner is torn on more than one run, the insulation has flattened or soaked through across the attic, the plenum is rusting or splitting at the seams, or the ducts were sized for a smaller system than the one you have now. Fixing one run at a time there just schedules the next repair.
Your technician will tell you which side of that line your ducts are on, and why. If both are reasonable, ask for both prices in writing.
What a full ductwork replacement covers
A full replacement can run from the air handler to the rooms: the supply plenum, the main trunk line, each supply run out to its boot and register, the return duct and grille, and the collars (the fittings where a run connects). Joints should be fastened and sealed, the ducts insulated, and the runs hung so they don’t sag. Your written price lists which parts are included.
Afterward, ENERGY STAR says to “evaluate air flow after repairs are completed” and to include a new filter with any duct improvement. We check the airflow after our duct work, too.
The return side: noise, closed doors and stuffy bedrooms
Supply ducts get the attention, but the return side causes plenty of complaints. A return that’s too small for the blower can whistle at the grille and leave the system short of air.
Closed bedroom doors are another one. A bedroom with a supply vent and no return has nowhere to send its air once the door shuts. Building America says such a room is typically pressurized with the door closed, meaning air is pushed in with nowhere to leave, and that poor return paths can cause drafts, temperature differences between rooms and equipment that works harder than necessary. It doesn’t recommend counting on the gap under the door; the fixes it describes are a transfer grille (an open vent through the wall), a jump duct (a short piece of insulated flex duct in the attic that connects a ceiling register in the bedroom to one in a common space) or a dedicated return. Ask us which fits your house.
Replacing ducts along with a new AC
A new system doesn’t automatically mean new ducts. Sound, sealed ducts that are big enough can stay. If they leak, are worn out or are too small for the new equipment, doing both at once means one project instead of two. Our AC replacement and installation page covers how the system side is sized; ask to see the duct work as its own line on the quote.
New runs for additions and problem rooms
Sometimes a room needs a run it never had: a converted garage, an addition, or a back bedroom fed by one small duct at the end of a long trunk. Whether the system can feed one more room is an equipment question too, so your technician checks both before quoting.
What we don’t do
We don’t install or service ductless mini-splits. For mold, attic insulation or attic ventilation questions, ask when you call and we’ll tell you whether it’s something we handle. For dryer vents, ask when you call.
Sources: ENERGY STAR, when is it time to replace · ENERGY STAR, hiring a professional duct contractor · U.S. DOE Building America Solution Center, support intervals for flex ducts · U.S. DOE, support for flexible ducts · Building America Solution Center, jump ducts · Building America Solution Center, undercut doors. Checked November 2026.
