Hot Master Bedroom, 95 cfm, and 0.85 in. w.c. of Static
An 1,800 sq ft ranch has a 3-ton system moving a nominal 1,200 cfm through a 20x8 trunk. Total external static reads 0.85 in. w.c.: 0.55 on the supply side and 0.30 on the return, including a 0.28 in. w.c. drop across a 16x25x1 filter. The blower is rated 1,200 cfm at 0.50 in. w.c. The master bedroom, 12x14 with one west-facing window, has a single 6 in. flex run to a 10x6 ceiling register; you measure 95 cfm at the register. A room-by-room load calculation gives it 4,200 BTU/h sensible. The homeowner says the bedroom runs 4°F warmer than the rest of the house on summer afternoons, and that closing the doors makes it worse.
1. Is 95 cfm adequate for the calculated room load? Show the math and state what the run should be.
Read the worked answer
Airflow required = sensible load ÷ (1.08 × design temperature difference).
- At a 20°F room-to-supply difference: 4,200 ÷ (1.08 × 20) ≈ 194 cfm
- Even allowing a generous 25°F difference: 4,200 ÷ (1.08 × 25) ≈ 156 cfm
The room is getting about half of what it needs, which matches the 4°F complaint.
A 6-inch flex run at typical residential friction rates carries roughly 100–120 cfm; it physically cannot deliver 190 cfm without excessive velocity and noise. This is a sizing problem, not a balancing problem - no damper adjustment elsewhere in the house will add 100 cfm to this branch.
Corrective action:
- Replace the 6 in. run with 8 in. flex or hard pipe for that room, and increase the register to roughly a 12x6 or equivalent.
- Provide a return path from the room (see the door question).
- Re-measure at the register after the change and rebalance the remaining branches, since removing restriction here will shift airflow in the trunk.
2. Interpret the static pressure readings. Which side do you address first, and what is your order of recommendations?
Read the worked answer
Total 0.85 in. w.c. against a 0.50 in. w.c. rating means the blower is operating to the right of its design point on the fan curve - actual delivered cfm is below 1,200, so the whole house is short of air, not just the bedroom. Every register reading you take in this house is suspect until this is corrected.
Breakdown judgment:
- Filter: 0.28 of 0.85 - about a third of the total. A 1-inch pleat at that drop is undersized or loaded. A 4–5 in. media filter with more face area, or a larger filter grille, is the cheapest cfm recovery available.
- Supply 0.55 vs return 0.30. The supply side is the larger restriction and gets attention first after the filter.
- Trunk velocity: 20x8 = 160 in² ≈ 1.11 ft². At 1,200 cfm that is roughly 1,080 fpm - well above the 600–900 fpm normally targeted in a residential trunk. Expect a high trunk loss and noise; an oversized branch run off an undersized trunk will still under-deliver.
Order of work: filter/grille first (lowest cost), then check the evaporator coil drop against the manufacturer's published value, then reduce supply-side restriction - enlarge or add a trunk section and shorten the flex runs. Re-read total static and room cfm after each step rather than doing everything at once.
3. The homeowner closes the bedroom door at night and says the room gets worse. Explain the mechanism and size a fix.
Read the worked answer
The room has supply but no return path. With the door closed, the 6 in. supply pressurizes the room slightly; once the pressure rises, delivered cfm falls off, and the room becomes a dead-air pocket with the supply air short-circuiting back under the door. The complaint that it gets worse with the door closed confirms a return-path problem rather than another supply-side problem.
Sizing the return path:
- Target roughly the room's supply airflow, about 150–200 cfm.
- A door undercut of 1 in. over a 30 in. door is only about 30 in² of free area - on the order of 60–70 cfm at normal pressure, not enough.
- Use two transfer grilles (or one jump duct) sized for ~200 cfm net free area with a slight offset so sound does not pass straight through, or add a dedicated return from the bedroom to the return trunk.
Verify after installation: door closed, measure register cfm and room pressure relative to the hallway. Supply should hold near the target and the room should be within about 1 Pa (a light tissue should not flap at the undercut).