Ductwork and Airflow in Nevada City

Cooling leads the year in Nevada City, and the installer sizes the duct around the cooling load — where anybody sized it at all. That works in your favor more often than not: a system with a furnace moves less air in heating than it moves in cooling, and a heat pump moves the same air in both, so a run sized to carry the house through August usually carries it through the few cold weeks either way.

The short winter decides where the loss lands rather than whether there is one. A joint in an unconditioned attic or a vented crawl space leaks at the same rate whatever the month; here it leaks through a cooling season that runs most of the year and a heating season measured in weeks. So the bill a duct leak writes here is almost entirely a summer one, and the time to get a contractor under the house is before that season rather than in the middle of it.

The heating season here is short enough that a weak room gets tolerated — a few weeks of a back bedroom nobody sits in, and then it is spring. The same run is the one that fails in July, and by then it has been wrong for a year. It is usually the room at the end of the longest run, which is the one the air reaches last in either season.

The July dew point here sits below the temperature of the air inside a supply run, so a badly insulated attic run warms what it carries instead of sweating on it. What a leaking return does instead is carry dust. It draws attic or crawl space air into the system, and the filter only catches what came through it, which is why a house that is kept clean can still have a black filter and gray register faces. Dry air also makes the measurement cleaner: with almost nothing going into moisture, the temperature split across the coil reads as airflow and little else, and one reading separates a duct problem from an equipment problem.

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 for the one job it does not hold. 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 need the joint clean and dry, which is what a crawl space makes difficult.

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.