Nonstop Runtime at 95°F: Undercharge, Restriction, or Overcharge?
A 3-ton R-410A split system with a TXV and a rated airflow of 1,200 cfm serves a second-floor zone. On a 95°F day the homeowner says the unit runs nonstop and the house will not drop below 78°F, though the design indoor temperature is 75°F. You measure 105 psig suction with a 58°F suction line at the service valve, and 310 psig discharge with a 91°F liquid line at the condenser outlet. The condenser coil is clean, the blower is on the correct tap for 1,200 cfm, the filter is new, and there is no measurable temperature drop across the liquid-line filter drier. The sight glass is clear. No one recorded a weigh-in charge at start-up; the nameplate charge is 8.5 lb.
1. Using the readings, decide whether the system is undercharged, restricted, or overcharged. Show the numbers that support your conclusion and state the corrective step.
Read the worked answer
Convert pressures to saturation temperatures first.
- Suction 105 psig ≈ 33°F evaporating. Superheat = 58 − 33 = 25°F (a TXV system normally holds 8–12°F at the evaporator outlet).
- Discharge 310 psig ≈ 98°F condensing. Subcooling = 98 − 91 = 7°F (target is typically 10–12°F at design conditions).
Interpretation:
- High superheat plus low subcooling = low charge. The condenser is not backing up liquid, so the TXV is starving the coil.
- A restriction (plugged drier, kinked or undersized liquid line) would show normal or elevated subcooling ahead of the restriction and a temperature drop or flash point at the drier/TXV inlet. There is no drop across the drier and the glass is clear.
- Overcharge would show elevated subcooling, not 7°F.
- Load conditions argue against a false reading: 95°F outdoor with 78°F indoor is above design load, which should raise, not lower, subcooling.
Corrective step: recover the remaining charge, repair/verify the leak source, evacuate to 500 microns, and weigh in the nameplate 8.5 lb as liquid. Then confirm 10–12°F subcooling and an 8–12°F superheat at design conditions before leaving.
2. The homeowner asks you to 'just add a couple of pounds' because the system is only two years old. What do you do, and what does the refrigerant regulation require of you?
Read the worked answer
You cannot top off and leave. At 8.5 lb the system is at or above the 5-lb threshold, so it is a regulated appliance under Section 608 leak-repair rules for comfort cooling.
What that means in practice:
- Find and repair the leak - the charge loss on a two-year-old system is not normal seepage.
- Repair within 30 days of discovering the leak (sooner if the owner agrees), and keep records.
- Initial verification of the repair within 30 days after the repair, using a documented method (pressurize with nitrogen, electronic leak detection, or evacuation to a specified micron level).
- Follow-up verification roughly 10 months later if the repair was not verified by the initial test alone.
- If the leak cannot be repaired, the options are retrofit, replacement, or documented retirement - not repeated top-offs.
Also, adding liquid R-410A by guesswork to a system with a TXV will not restore correct subcooling reliably. The customer should be told the cost difference between a documented leak repair and an unbilled top-off that will fail again.
3. Same system, but the service call happens on a 65°F day with a 72°F indoor temperature. How do you verify the charge, and why is the method different?
Read the worked answer
At 65°F outdoor with a mild indoor load, the system cannot build design condensing conditions, so the 10–12°F subcooling target is not directly transferable.
Acceptable approaches:
- Weigh-in is the primary method. With no recorded start-up charge and a recent leak repair, recover, evacuate, and charge to the nameplate 8.5 lb by scale. That is the only value that is condition-independent.
- If a weigh-in is not practical, use the manufacturer's subcooling chart if it publishes one that covers low-ambient operation, and note that subcooling usually reads lower at reduced ambient, so hitting 10°F at 65°F may indicate an overcharge once the day warms up.
- Do not judge charge by superheat alone on a TXV system; the TXV will hold superheat by modulating and will mask an overcharge until the condenser is flooded.
- If you must run the system, add load (doors open, fans on the indoor coil) and take the reading only after pressures stabilize for 10–15 minutes; a low-load subcooling reading taken too early will cause an overcharge.
Final verification should include a return-to-service check on a warm day or a documented weigh-in.