Ductwork and Airflow in Pawnee

In Pawnee the system runs almost year-round, which makes duct leakage a standing loss rather than a seasonal one. And there is no quiet month in which to have the system down while somebody opens the duct up, which is why a contractor books this work around the weather rather than after it.

Where the duct runs through a vented attic, the attic is the hostile part: it sits far above the outdoor temperature through the middle of the day, and a supply run crossing it arrives at the register warmer than it left the coil. Nothing is leaking and nothing is broken — it is the tail of the run undoing part of the cooling, which is why the far room is the one that never quite gets there.

Where the heat comes from a heat pump, a long run costs more than the same run on a furnace. A heat pump delivers supply air at around 95°F and a furnace delivers it at around 120°F to 140°F, so there is less to give away before the air stops feeling warm at all. A room that cools fine and heats badly is often the same duct with less to spare — which is why what the system is belongs in the first minute of the call.

July here is humid enough that duct picks up a failure dry climates never see: a supply run crossing an unconditioned attic or crawl space carries air near 55°F through air whose dew point is above 70°F, and where the insulation is torn, compressed or missing, the duct sweats. Water runs downhill to the low point of the run, and it turns up a floor below, some distance from the joint that made it. A return leaking in the same space does the other half — it draws that outside air in at its own dew point, and the coil spends its capacity drying attic air while the house stays cool and clammy.

A clean filter is the one thing on the shelf that changes airflow at all, and where it has not been changed since the last season it is worth doing before the phone comes out. What it cannot do is reach past itself: a filter gives back the air the system already had, and it does nothing for a run that is crushed, pulled off its collar or leaking into a crawl space, and nothing for a return that was always too small. Where it goes the other way is the upgrade: a dense one-inch filter sold for allergens takes more pressure than the slot was built for, and a system already short of air gets shorter. If the room is still wrong with a clean filter in, the filter was never the answer.

Duct tape is named after the job it fails at. On a warm duct the cloth-backed roll dries out and lets go, usually inside a couple of years, and the joint is open again with nothing to show for the afternoon. What holds is mastic brushed on, or foil tape carrying a UL 181 marking on the roll — and both want the joint clean and dry, which is the hard part in a crawl space.

Closing the registers in rooms nobody uses is the fix that feels obvious and is not. The air does not move to the weak room, it raises the pressure in the duct, and higher pressure pushes more of it out through every leak the system already has. Trimming one register that overshoots is a different matter, and it is what a damper is for — the difference is one adjustment against half the house closed off. A booster fan at a register does anything at all only where the run behind it is sound, and on a run that is leaking or pulled off its collar it changes nothing.