Ducts, Registers & Filters
~10 min read · Trace distribution defects: crushed flex, disconnected runs, filter and return problems.
Ducts are the lungs' plumbing, and they fail unglamorously: crushed flex, disconnected boots blowing into crawl spaces, return systems starving the blower, and the prohibited-return list that keeps garages and bathrooms out of the airstream.
Distribution defects
Flex duct: crushed, kinked, over-length, unsupported sags (each bend and sag chokes flow), inner liner torn. Metal duct: disconnected joints (conditioned air heating the attic — energy AND comfort findings), unsealed seams (mastic beats cloth duct tape, which dies in heat), missing insulation in unconditioned spaces (condensation dripping from cold ducts in humid crawls). Boots and registers: disconnected at floors (blowing into the crawl — with dust-streak evidence), rusted boots (crawl moisture), and rooms with no supply at all.
- Crushed/kinked flex and sagging runs choke rooms
- Disconnections condition the attic, not the bedroom
- Uninsulated ducts in humid spaces rain condensate
Returns: the starved half
Blowers need return air equal to supply: undersized or blocked returns (furniture over the one big grille), closed-door bedrooms with no return path (no transfer grilles/undercuts — rooms pressurize, flow dies), and prohibited return sources — the stable list: no returns from closets, bathrooms, toilet rooms, kitchens, garages, mechanical/boiler/furnace rooms, or unconditioned attics (odors, moisture, and CO/fumes don't belong in the airstream; a garage return is a carbon-monoxide manifold). Panned joist returns leak from everywhere they pass.
- Return capacity and paths (grilles, undercuts) matter
- No returns from garages, baths, kitchens, mechanical rooms, attics
- Panned framing returns: leaky legacy practice
Air quality riders
Duct interiors (visible at registers): dust mats, mold staining on internal liner in humid climates, rodent evidence. Filters: location, fit (gaps bypass), collapse. Duct materials: asbestos-suspect wrap on old ducts (report, don't disturb), and buried/inaccessible duct sections get limitation language. Balancing complaints (hot/cold rooms) tie the section to the buyer's lived experience.
Worked example
A ranch with hot-and-cold-room complaints: the far bedroom's flex run sags in three deep loops and is kinked 90° at a truss; the master's supply boot lies disconnected in the crawl, its register sucking crawl dust upward on blower start (streaks on the carpet edge); and the seller proudly added a return grille in the attached garage 'to help the system breathe.' Sort comfort from hazard.
Comfort findings first: the sagging, kinked flex explains the cold far bedroom — support and re-run; the disconnected boot means the master is conditioned by crawl-space air (the dust streaks are the airflow's autograph) — reconnect and seal, and note the moisture/air-quality implication. Then the hazard: a garage return puts exhaust, fuel vapors, and stored-chemical fumes one blower cycle from every bedroom — CO delivery by design; this is a remove-immediately safety finding, stated with the prohibited-return principle. One system: two comfort defects and one genuinely dangerous 'improvement' — the report's job is making sure the last one doesn't read like the first two.
Common exam pitfalls
Filing the garage return under energy tips.
It's a CO/fume pathway — a safety finding with immediate-correction language, not an efficiency note.
Ignoring return paths for closed doors.
Supply without return path = pressurized dead rooms — check undercuts/transfer grilles when returns are central.
Accepting cloth duct tape as sealing.
It bakes, dries, and falls off — mastic/foil systems seal; report taped joints in hot spaces.
Straight, sealed, supported, insulated — and the return never breathes from the garage.
Recap
- Flex geometry: kinks, sags, crush = room-by-room comfort findings
- Disconnections condition crawls/attics; dust streaks tell
- Insulate ducts in unconditioned humid spaces
- Returns: capacity, door paths, prohibited-source list
- Garage/bath/kitchen/mechanical returns are hazards
- Asbestos-suspect wrap: report, don't disturb
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