Second Floor Never Cools: Static Pressure Tells the Story
Two-story, 2,400 ft² house, 4-ton variable-speed air handler in a second-floor closet, a single 20×25 return grille in the first-floor hallway, and all-flex ductwork in the attic. Complaint: upstairs bedrooms run 5–7°F warmer than downstairs and the system runs continuously. In high stage you measure: TESP 1.08 in. w.c.; supply static 0.64; return static 0.44, of which 0.28 is across the filter; blower rated 0.50 in. w.c. TESP. The bedroom branches are 6-in. flex with two sharp 90s each, and one is crushed between a truss and the decking. Measured bedroom airflow is 55 CFM each against a design of 110 CFM. Total measured supply airflow is 1,180 CFM.
1. Diagnose the system. Which measurements drive your conclusion, and how do you rank the causes?
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Total airflow is the disease, not the symptom. A 4-ton needs about 1,600 CFM at 400 CFM/ton; you measured 1,180 CFM - roughly 74% of design. At that airflow the coil runs cold, latent removal collapses, and the supply air temperature spread widens, so the equipment cannot deliver its rated sensible capacity even though it is running flat out.
The static profile explains the airflow:
- TESP 1.08 in. w.c. against a 0.50 in. w.c. rated blower - the blower is sitting at the far right of its fan curve, so it physically cannot move design air.
- Return side is 0.44 in. w.c., with 0.28 across the filter alone. The single downstairs return is undersized for 1,600 CFM and the filter is choking it.
- Supply side is 0.64, driven by long flex runs, sharp bends, and the crushed branch.
Ranking: (1) return/filter restriction, (2) crushed and over-bent branch runs, (3) gross undersizing of the distribution for 4 tons, (4) no zoning or balancing dampers, so the short downstairs path takes the air that the upstairs needed.
2. What is your repair plan, in order, and what improvement do you expect from each step?
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Work from the fan outward - fixing branches first while the return is still choked wastes the effort.
- Return and filtration. Replace the 20×25 filter grille with a properly sized return (or add a second return upstairs) and move to a low-pressure-drop media cabinet. Target: cut the 0.44 return static to roughly 0.15–0.20. This alone typically recovers 100–200 CFM.
- Trunk and branch correction. Replace the crushed branch, replace sharp 90s with radius elbows or sweep bends, straighten and support the flex, and upsize the bedroom runs to 6-in. or 7-in. short, taut runs sized per Manual D. Target: cut supply static from 0.64 toward 0.30 or less.
- Balance. Add volume dampers and set each register to its design CFM, prioritizing the west bedrooms.
- Verify equipment. After statics drop below the 0.50 in. w.c. rated TESP, set the variable-speed blower's airflow taps to the measured duct capacity and recheck charge.
Realistic expectation: TESP under 0.50, total airflow above 1,500 CFM, bedroom registers near 100–110 CFM, and the upstairs/downstairs split closed to 2–3°F. Do not promise 0°F split from duct work alone if the upstairs also has a load problem - check attic insulation and window area.
3. How do you prove the job is fixed, and what would make you recommend a zoning or equipment change instead of more duct work?
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Proof is measurement, before and after, on the same instruments:
- Static profile per component with a clean filter.
- Total airflow (flow hood or calibrated fan) and every register CFM against Manual D/Manual J room values, within about ±10%.
- Temperature split at the return and at the farthest register, plus room-by-room temperature at the same time of day.
- Blower tap settings and the final charge verification, documented on the invoice.
Go to zoning or equipment change when the numbers say duct work can't get there: if the two floors have meaningfully different loads (large west glass, vaulted ceiling, no attic insulation over the bedrooms), a single-zone system will always fight itself no matter how good the static is. Options, in cost order: more attic insulation and radiant barrier, then a two-zone damper system with a bypass or a variable-speed unit, then a second dedicated upstairs system. Also recommend a zoning or second system if the existing closet and duct chase cannot physically carry a larger trunk without major structural work.