Two-Story House, 0.92 in. w.c. TESP, and a Request to Upsize
A 2,400 sq ft two-story house has a single 3-ton split system with one thermostat downstairs. The upstairs bedrooms run 4–6°F warmer. With the blower on high and a clean coil you measure: return static 0.44 in. w.c., supply static 0.48 in. w.c., pressure drop across the 1-inch pleated filter 0.30 in. w.c., pressure drop across the evaporator coil 0.28 in. w.c. The blower performance table in the installation manual shows 1,200 CFM at 0.50 in. w.c. TESP and about 940 CFM at 0.90 in. w.c. The ductwork is mostly 6-inch flex with long runs, and the only return is a single 20×25 filter grille. The customer wants a 4-ton unit installed before the summer.
1. Compute the total external static pressure and the delivered CFM per ton. Is the system within specification?
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TESP = return static + supply static = 0.44 + 0.48 = 0.92 in. w.c.
From the blower table, 0.92 in. w.c. corresponds to roughly 940 CFM. Spread over 3 tons: 940 ÷ 3 = ≈313 CFM/ton, against a nominal 400 CFM/ton. The system is delivering about 78% of design airflow and about 30% short of the 1,200 CFM the unit is rated at.
Consequences are predictable and all visible to the homeowner: high temperature split across the coil, low sensible capacity, a coil that runs cold and wet (latent capacity is actually up, sensible is down), condensate blow-off risk, and long run times. Upstairs comfort gets worse, not better, because the same limited air is being pushed through the longest, leakiest runs.
2. Build a static pressure budget for this system. Where should the 0.50 in. w.c. be spent, and what does the measured data say about where the excess actually is?
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A workable budget for a residential 0.50 in. w.c. rated air handler:
| Component | Allowance | Measured |
|---|---|---|
| Filter | 0.10–0.15 | 0.30 |
| Wet evaporator coil | 0.20–0.25 | 0.28 |
| Supply duct + grilles + dampers | 0.10–0.15 | (≈0.20 by subtraction) |
| Return duct + grille | 0.05–0.10 | included above |
The filter is two to three times its allowance and is a 1-inch pleated element in a single filter grille - too small a face area for the CFM. The coil is at the top of its range but acceptable. Subtracting filter and coil from the 0.92 total leaves about 0.34 in. w.c. for all of the ductwork, which is also over budget and consistent with long 6-inch flex runs and one undersized return.
Conclusion: roughly two-thirds of the excess static is in the filter and the return path, not the supply trunk. Fixing the filter and return is also the cheapest work, so it goes first, with the duct redesign second.
3. The customer wants a 4-ton unit. Explain what that does to airflow, static pressure, and the upstairs temperature, and what you would propose instead.
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A 4-ton unit needs about 1,600 CFM through ductwork that cannot currently move 950. On the same blower curve, squeezing 1,600 CFM through a 0.92 in. w.c. system is not possible - static would rise further, the ECM/PSC motor would ride its limit, and the delivered CFM would still be short of nominal. The result is a system that is oversized on paper and undersized in practice.
Other consequences: a 4-ton coil/condenser matched to the same leaky, unbalanced ducts simply short-cycles; sensible capacity improves only slightly; latent removal drops as run times shorten; and the upstairs rooms stay warm because the problem is distribution and stratification, not gross capacity.
Proposal:
- Run a Manual J before discussing size - the existing 3 tons may be correct or even oversized.
- Fix the air side: 4-inch media filter, enlarge/add a return (ideally a return upstairs), replace or upsize the long flex runs per Manual D, seal the ducts, and balance with dampers.
- Consider zoning or a small ducted mini-split for the second floor; that addresses the 4–6°F stratification directly.
- Only after the air side is corrected and Manual S confirms the load should equipment size be revisited.