Excess Static Pressure on a 3-Ton Air Handler
A 3-ton R-410A system with a TXV, a PSC blower rated 1,200 CFM at 0.50 in. w.c. total external static pressure, and a 12 SEER-era air handler serves a 1,700 sq ft house. The owner complains the house is "cool but clammy" and the coil has iced twice this summer. Conditions: indoor 76°F DB / 64°F WB, outdoor 94°F. A magnahelic reads −0.30 in. w.c. on the return plenum and +0.60 in. w.c. on the supply plenum. The filter is new and drops 0.14 in. w.c.; the wet evaporator coil drops 0.42 in. w.c.; the supply trunk to the farthest register drops 0.30 in. w.c. There is one 20x25 return grille in a closet door and a single 20x25 filter. The customer asks whether the system just needs "a little more Freon."
1. Compute the total external static pressure and judge how it compares to the blower rating. What is the likely effect on delivered airflow and coil temperature?
Read the worked answer
- TESP = |−0.30| + |+0.60| = 0.90 in. w.c., against a 0.50 in. w.c. rating - 0.40 in. w.c. (80%) over the design point.
- On a PSC blower, airflow falls steeply as static rises; the fan curve typically gives roughly 80–85% of rated CFM at this overshoot, so expect about 950–1,000 CFM instead of 1,200.
- That is about 320–335 CFM/ton, below the usual 350–400 CFM/ton. Less air across the coil means a colder coil: the coil temperature drops toward and below 32°F, which explains the icing, and the reduced sensible capacity explains a house that feels cool but humid.
Do the arithmetic rather than eyeballing it: the excess static, not the refrigerant charge, is the primary fault here.
2. Using the component pressure drops given, where is the excess static coming from, and what would you check or correct first?
Read the worked answer
Break the 0.90 in. w.c. into its parts:
- Filter: 0.14 (acceptable, slightly high for a new filter)
- Wet coil: 0.42 (high; a clean wet A-coil is typically 0.20–0.35 on a 3-ton at 1,200 CFM)
- Supply trunk to farthest register: 0.30 (high; and this is only one branch of the supply side)
- Remainder: return grille/plenum and the rest of the supply, roughly 0.04 by subtraction - which means the measured branches already account for nearly the whole 0.90, and the true offending elements are the coil and the supply duct/registers.
Sequence of action:
- Verify the coil is actually clean, not just wet - a dirty or partially blocked coil raises its drop and is the cheapest fix.
- Inspect the supply duct for undersized trunk, crushed flex, tight 90° elbows, and closed/dampered registers; measure each branch takeoff to find the restriction.
- Check the return path: a single 20x25 grille/filter in a closet door is marginal for 3 tons (roughly 2 CFM per sq in. of free area) and often adds 0.10–0.15 in. w.c. by itself.
- If duct corrections bring TESP near 0.50–0.60 in. w.c., airflow returns toward 1,200 CFM and coil icing stops.
Only after the static is corrected do you evaluate charge - with a TXV, airflow errors and charge errors can look alike on gauges.
3. Explain to the homeowner why adding refrigerant would not fix icing or clamminess, and describe what you would expect to see on gauges now versus after the airflow fix.
Read the worked answer
- Now (low airflow, correct charge): low suction pressure and low suction saturation temperature (coil below freezing), superheat on the high side of normal to high because the TXV is throttling against a cold, small-load coil, subcooling normal to slightly high, and a large air-side ΔT (low CFM × high ΔT ≈ the same Btu/h). The icing is caused by coil temperature, not by a shortage of refrigerant.
- If you add refrigerant: head pressure and subcooling rise, the coil gets colder, and the icing gets worse while the compressor runs hotter. You have masked nothing and fixed nothing.
- After airflow is corrected: suction saturation climbs back toward roughly 40°F at these indoor conditions, superheat settles to 8–12°F, subcooling stays in the manufacturer's range, ΔT across the coil returns to about 18–20°F at 1,200 CFM, and the coil no longer drops below freezing.
Tell the customer plainly: the system is not low on refrigerant, it is being strangled for air; the fix is duct and coil work, and the charge will be verified last.