Three-Ton System With 0.92 in. w.c. External Static
A 3-ton R-410A split system in a single-story house has a measured total external static pressure of 0.92 in. w.c. with a new 20×25×1 filter. Component drops: filter 0.28, return grille 0.12, evaporator coil 0.34, supply plenum and trunk 0.18. The blower table shows 1,200 CFM at 0.50 in. w.c. and about 850 CFM at 0.90. Return air is 75°F dry bulb/63°F wet bulb; supply air is 58°F dry bulb/57°F wet bulb. Suction pressure is 118 psig, subcooling 10°F, superheat 9°F. The homeowner says the house feels humid and the far rooms never satisfy the thermostat.
1. Use the static readings and blower table to judge airflow, and identify where the pressure is being lost.
Read the worked answer
Airflow is far below design.
- Total external static is 0.92 in. w.c. against a 0.50 in. w.c. design point.
- Interpolating the blower table: 1,200 CFM at 0.50, about 850 CFM at 0.90 - roughly 71% of design.
- Pressure breakdown: coil 0.34 (matted/dirty), filter 0.28 (high for a new 1 in. filter), return grille 0.12, supply 0.18.
The coil and filter together account for 0.62 in. w.c. and are the correctable items. The return path is also marginal for 3 tons; 0.12 at the grille plus undersized return duct limits the blower.
2. Calculate the temperature split and decide whether refrigerant should be added.
Read the worked answer
Charge is not the problem - do not add refrigerant.
- ΔT = 75 − 58 = 17°F, low for a 3-ton system at 63°F wet bulb (expect roughly 19–21°F at full airflow).
- Subcooling 10°F and superheat 9°F are normal for a TXV, so the charge is in range.
- Low airflow means low sensible capacity, long run times, poor dehumidification, and a cold coil that can ice over.
Adding refrigerant would push subcooling high and raise suction pressure, masking the symptom without restoring capacity. The correct move is to restore airflow and re-evaluate.
3. Prioritize corrective actions and state the measurements that would prove the fix.
Read the worked answer
Fix the pressure drop, then verify.
- Clean or replace the evaporator coil; re-measure coil drop (target roughly 0.20–0.30).
- Use a low-pressure-drop filter or increase filter area; 0.28 on a clean 1 in. filter is too restrictive.
- Improve the return: enlarge or add a return, and check return duct sizing and grille free area.
- Re-measure total static; target ≤0.50–0.60 in. w.c. and about 1,200 CFM.
- Re-check ΔT (should rise to about 19–21°F), superheat and subcooling, and room-by-room airflow; then balance the far rooms.
Verify with the fan table plus actual measurements, not by feel.