Fixed-Orifice System After a Homeowner Top-Off
On a 95°F afternoon, you respond to a 3-ton R-410A residential split system with a fixed orifice. The homeowner says a neighbor “added a little refrigerant” last week because the suction line felt cold. Current readings: outdoor 96°F DB, indoor 80°F DB / 67°F WB, suction pressure 142 psig, suction line temperature 51°F, liquid pressure 355 psig, liquid line temperature 108°F. The air filter is clean, the indoor blower is on the correct speed tap for 1,200 cfm, and the evaporator coil is clean. Supply air is 69°F, return air 80°F. The compressor is not cycling and amp draw is slightly above nameplate. You measure an 11°F temperature drop across the evaporator and a 2°F drop across the liquid-line filter drier. The owner wants you to “top it off again” so it cools better.
1. Using the readings, calculate superheat and determine whether the system is overcharged, undercharged, or correctly charged. Show the saturation temperature you used and explain how the 11°F indoor temperature drop supports your answer.
Read the worked answer
R-410A at 142 psig has a saturation temperature of about 48°F. Superheat is 51°F - 48°F = 3°F. For 67°F indoor wet-bulb and 96°F outdoor dry-bulb, the fixed-orifice charging chart calls for roughly 9-10°F superheat, so 3°F is far too low. A low superheat means liquid refrigerant is reaching the compressor - the evaporator is flooded. The 11°F indoor temperature drop is below the normal 18-22°F, which fits a flooded coil that is not absorbing enough sensible heat. The 2°F drop across the filter drier argues against a liquid-line restriction, and the clean coil and correct blower tap argue against low airflow. The high liquid pressure is consistent with overcharge but is not the primary charge indicator on a fixed-orifice system. Conclusion: the system is overcharged, most likely from the neighbor’s top-off, not undercharged.
2. What should you do before recovering or adding charge, and what is the final charging procedure? Include why “topping off” is not acceptable.
Read the worked answer
First verify the basics: correct orifice size for the outdoor unit, 1,200 cfm airflow, clean coils, and compressor amp draw. Then recover all refrigerant into a recovery cylinder; do not simply remove a little vapor. Replace the filter drier if the system is opened, evacuate to 500 microns, and weigh in the nameplate charge. Start the system, let pressures stabilize, and fine-tune charge to the target superheat from the manufacturer’s chart using actual indoor wet-bulb and outdoor dry-bulb. Check that superheat lands near 9-10°F and that the indoor delta T rises toward 18-22°F. Topping off is unacceptable because it is not measured, it can overcharge the system, and it hides the reason refrigerant was low. Also check for leaks and document the final weight.