Ductwork 101 — How Air Actually Moves Through Your Home

Ductwork is the pressurized air-delivery network that connects your air handler to every room. Two halves matter. The supply side carries conditioned air from the indoor coil to the registers in each room. The return side pulls warm humid room air back to the air handler so the coil can condition it. Without enough return air, the supply side has nothing to push.

Three numbers describe whether a duct system is doing its job. Static pressure measures how hard the blower has to work to move air through the ducts; it's the equivalent of blood pressure for a building. Manufacturers design air handlers for a total external static pressure (TESP) of 0.5 inches of water column. Most retrofit residential duct systems we measure in Greater New Orleans run 0.9-1.2 inches — roughly double the design assumption. Airflow (CFM) measures the actual volume of air moving through each register. Manual J tells you the room-by-room load (how much cooling each room needs), Manual S selects the right equipment for that load, and Manual D designs the duct system to deliver it.

If any of those four design steps is skipped, the homeowner pays for it forever in comfort, bills, or both.

More often than the equipment is. Four signals to recognize.

One room never gets comfortable. A back bedroom that's 6-8°F warmer than the thermostat-controlled space in summer, or a bonus room over a garage that's unusable for half the year. The AC isn't undersized — that one room's branch duct is undersized, restricted, or leaking into the attic before reaching the register.

The whole house "can't keep up" but the equipment is healthy. Manifold pressures look right, refrigerant charge is correct, the compressor is pulling normal current. If the system can't satisfy the thermostat, look at airflow. A low-airflow system has its capacity choked off — sometimes 20-30% of nameplate — by an undersized return or a high static-pressure trunk line.

The system runs constantly but the electric bill makes no sense. A blower fighting high static pressure burns more watts. A coil starved for airflow can ice up, which makes the bill worse. Both fail the same way: a thermostat that's almost-but-not-quite satisfied for 18 hours a day.

You can hear the air. A trunk line running 0.9+ inches static pressure roars at the air handler closet. A register running 800+ feet per minute whistles. These are not features of a healthy duct system. Quiet airflow is a sign of design discipline.

In older slab homes around New Orleans and ranch-style houses across Harahan and Covington, the duct system is the limiter at least half the time we're called in for a "the AC isn't working" complaint.

Four regional realities to know about.

Pre-2010 retrofit ductwork is everywhere. Many Greater New Orleans homes had central AC retrofitted into a structure that originally had window units or floor furnaces. The duct system was sized to the cheapest install, not the load. Trunk lines are often a size too small, return paths are starved (sometimes a single 14×20 grille for a whole 1,800 sq ft house), and supply branches were run wherever framing allowed.

Flex duct in unconditioned attics. Insulated flex duct from a hot attic carries three problems: insulation R-value is only R-6 or R-8 even when intact, the inner liner is often crushed where the duct turns over a joist, and the joints at boots and elbows are taped rather than mastic-sealed. Attic temperatures of 130-140°F in August conduct heat into the supply air, costing 8-15% of cooling capacity before air reaches the room.

Unsealed return-side leakage. Returns often run through that same hot attic. If the joints aren't sealed, the duct inhales 130°F attic air at every leak — measurable latent load the AC then has to remove, which we cover in why Louisiana homes hide humidity problems. A leaky return system can add 20-30% to the cooling bill.

Tape failure and single-return floor plans. Foil tape on duct joints fails predictably in attic heat — the adhesive cooks off in 5-8 years and the joint pops open. Mastic sealing doesn't fail that way. And older slab homes commonly have one large central return grille, so bedrooms with closed doors become positive-pressure boxes the air can't escape from. Adding transfer grilles or door undercuts is often a $300 fix for what felt like a $9,000 problem.

Four measurements separate a real duct diagnostic from a guess.

A digital manometer reads total external static pressure across the air handler. If TESP is above 0.7 inches, the duct system is fighting the equipment — and the equipment is losing. We measure this on every call where the complaint is comfort-related and the refrigerant numbers come back clean.

A hot-wire anemometer or velocity hood reads register airflow in CFM. Compare room-by-room to the design target from a Manual J load calc; rooms running below 70% of target are starving, and rooms above 130% are overcooling at the expense of their neighbors.

A duct-blaster pressure test surfaces total leakage. Healthy duct systems leak less than 6% of nominal airflow. Pre-2010 retrofit systems in this market routinely leak 25-40%. A smoke pencil at suspected joints shows leakage direction and lets us mark every spot that needs mastic, and an infrared camera scanning the ceiling reveals heat conducting through under-insulated supply runs.

You can do a homeowner version of this audit. Tape a tissue to each supply register; flutter direction and intensity tell you which rooms are starved. Walk the attic on a cool morning and look for dark streaks on insulation around boots and elbows — dust trails that mark air leakage. Read supply-air temperature at the register closest to and farthest from the air handler; more than a 5°F difference between them points to long-run heat gain.

Three actions before you sign any equipment quote.

First, ask whether the contractor measured static pressure and per-register airflow before they recommended anything. If they didn't, the recommendation is a guess.

Second, ask whether they're sizing equipment from a Manual J load calc or from a square-footage rule of thumb. Square-footage sizing in our climate produces oversized equipment, which makes humidity worse — see the comparison in our repair-vs-replace framework.

Third, if your real problem is one or two uncomfortable rooms, ask whether a ductwork fix or a ductless mini-split for that zone is the right answer instead of replacing the whole system. Sometimes it is. When you're ready for an unbiased read, the second-opinion diagnostic brings the manometer, the anemometer, and a duct-blaster to the house before we say a word about equipment.

The answer depends on three measurements: total static pressure, total leakage percentage, and how the design compares to current load.

Sealing only is the right call when sizing is approximately correct and the runs are in decent shape but leakage is 15-30% and joints are tape-only. Mastic-sealing every accessible joint, replacing crushed flex, and rebalancing registers can recover 15-25% of capacity for a small fraction of replacement cost.

Modification is right when one or two zones are starving but the trunk lines are adequate. Adding a return, upsizing a branch from 6" to 7", or installing transfer grilles between rooms often resolves comfort complaints without a redesign.

Replacement is right when trunk lines are undersized for current load, the layout is a single-return design that can't be balanced, or the duct material (fiberglass duct board, deteriorated flex, rusting galvanized) is at end of life. Replacement is the most expensive option but the only one that solves a fundamentally broken design.

The decision needs to be backed by measurements, not a sales pitch. If a contractor recommends full replacement without showing you static pressure and leakage numbers, that's not a recommendation — that's an estimate.