Bonus-Room Coil Icing From High External Static
A 2-ton R-410A split system with a TXV serves a bonus room. The complaint is that the indoor coil ices after about 30 minutes and airflow is low. Outdoor ambient is 92°F; indoor is 78°F DB / 64°F WB. Suction pressure is 104 psig, suction line temperature 54°F, liquid pressure 330 psig, liquid line temperature 96°F. Superheat is 21°F and subcooling is about 10°F. The filter is new. You measure external static pressure: return 0.48 in. w.c., supply 0.42 in. w.c.; total 0.90 in. w.c. The air handler is rated for 800 cfm at a maximum of 0.50 in. w.c. external static. The blower is on high speed. Supply registers feel weak; the return is a single 14x20 in. filter grille with a restrictive stamped face. Delta T is 13°F. The evaporator coil is clean, but the drain pan is full and the condensate line is clear.
1. Calculate total external static pressure and explain how it explains the freezing coil. What CFM do you expect, and how does that affect the TXV and superheat?
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Total external static pressure is 0.48 + 0.42 = 0.90 in. w.c. The air handler is rated for no more than 0.50 in. w.c., so the blower is operating at nearly twice its allowable external static. The 0.48 in. w.c. return-side drop points to a restrictive return grille/filter grille. At high static, the blower cannot move 800 cfm; expect roughly 500-650 cfm, well below the 400 cfm/ton target. Low airflow reduces the heat load on the evaporator. The TXV throttles to hold superheat, so the suction pressure falls to 104 psig (about 33°F saturation) and superheat reads 21°F. With coil temperature near freezing and long runtime, frost and ice form. Ice then insulates the coil and cuts airflow further, making the problem worse.
2. What repairs and verification steps would you perform, and why is adding refrigerant or adjusting the TXV inappropriate?
Read the worked answer
Fix the airflow first: enlarge or replace the restrictive return grille, add return capacity, and verify the duct design. Then measure static pressure again and use the blower performance table to set the speed for 800 cfm. Confirm the coil is clean, the drain is clear, and the blower wheel is clean. Only after airflow is corrected should you check charge: with a TXV, verify subcooling at the liquid line and superheat at the suction line. Adding refrigerant would raise pressures and might stop the icing temporarily, but it would not correct the low airflow and could overcharge the system. Adjusting the TXV would mask the real problem and could cause flooding. The root cause is duct/return restriction, not charge.