Iced Coil, Low Airflow, and Three Extra Pounds of R-410A
Your company takes over maintenance on a 2,600 ft² single-story house built in 2004. The owner says the 3-ton R-410A heat pump "runs all day" and the indoor coil iced over twice in July. Another contractor cleaned the coil and added 3 lb of R-410A; the icing returned within a week. You measure: return air 75°F dry bulb, supply air 51°F, outdoor ambient 82°F. Suction pressure at the outdoor service port is 100 psig with a suction line temperature of 46°F. Liquid line pressure is 320 psig with a liquid line temperature of 90°F. Total external static pressure is 0.90 in. w.c. against a 0.50 in. w.c. rating, and return static alone is −0.45 in. w.c. The return is a single 14-inch flex duct feeding a 20×25 filter grille with a MERV 13 filter that drops 0.35 in. w.c. Measured airflow is about 900 cfm on the highest blower tap.
1. Interpret the readings. Is the system overcharged, undercharged, or neither, and is the icing an airflow problem or a charge problem?
Read the worked answer
Work the numbers before touching refrigerant. Saturation temperature at 320 psig for R-410A is about 112°F; the liquid line is 90°F, so subcooling is roughly 22°F against a normal 8–12°F target. That is a solid indication the system is overcharged, which fits the 3 lb that was added. Suction at 100 psig is about 29°F saturation with a 46°F line, so superheat is about 17°F, high for a TXV at that load. Supply-to-return temperature split is 24°F. Airflow is 900 cfm divided by 3 tons, or 300 cfm/ton, against a target near 400 cfm/ton (1,200 cfm).
Read together, these point to one root cause and one introduced fault. The starved, low-load evaporator (high superheat, low suction, coil icing) is a symptom of insufficient airflow, not low charge - the previous technician misread the icing and added refrigerant to a system that was already full. That raised subcooling and head pressure and likely explains the high-pressure lockout. Total static at 0.90 in. w.c. against a 0.50 rating confirms a badly restricted air path: 0.45 in. w.c. of the loss is on the return, which is the classic signature of an undersized return and an aggressive filter. Do not recover charge until airflow is corrected; a starved coil distorts charge readings.
2. What do you change to reach roughly 1,200 cfm at or under rated static, and in what order?
Read the worked answer
Attack the return first, because that is where 0.45 in. w.c. is being lost. A single 14-inch flex run is not a 1,200 cfm duct; at that velocity the friction and turbulence eat far more pressure than the design allows. Replace it with an 18-inch or 20-inch flex run, or two 14-inch runs, and shorten or eliminate sharp bends and crushed sections.
Second, deal with the filter. A 20×25 grille at 1,200 cfm pushes roughly 350 fpm face velocity, and a MERV 13 pleat at that velocity commonly drops 0.30–0.40 in. w.c. - the entire remaining static budget. Convert to a larger filter grille or, better, a 4- to 5-inch media cabinet sized for 1,200 cfm with a lower MERV if the occupants do not need MERV 13. If they do need it, the media must be bigger, not thicker and denser in the same opening.
Third, re-measure before changing blower speed. Correcting the ductwork often brings the blower back onto its curve and drops total static without a tap change; jumping to a higher tap on a restricted duct just adds noise and motor heat. Recheck dry-coil and wet-coil static separately, confirm the evaporator coil and secondary heat exchanger are clean, and record the before/after numbers.
3. Airflow is now corrected. How do you handle the extra refrigerant, and what do you verify and record before leaving?
Read the worked answer
Once airflow is right, the coil can boil refrigerant normally and the readings become trustworthy. Because you know roughly 3 lb was added and subcooling is about 10°F above target, do not guess by bleeding vapor. Recover the charge, and either weigh in the nameplate charge (best practice when the exact history is unknown) or charge to the manufacturer's subcooling target of 8–12°F with the compressor stable for at least 15 minutes.
Verify as a package: 350–450 cfm/ton measured by a true airflow method, not by blower tables alone; total external static at or below the 0.50 in. w.c. rating with a wet coil; superheat in the 8–12°F range at the TXV bulb; subcooling 8–12°F; and a temperature split around 18–22°F at 75°F return. Watch that the coil no longer ices, which may take a longer run than a single service call - schedule a follow-up.
Document everything: the original readings, the extra charge you found, the duct and filter changes, the recovered weight, and the final numbers. This protects you if the owner later claims the icing was never fixed, and it is the record that shows the previous contractor's top-off was the accessory fault rather than the cause. Note on the invoice that the return duct was the underlying defect and that future filter changes should use the same size and MERV rating.