Ducted Rooftop Air Conditioner Icing at High Ambient
A 2020 travel trailer has a ducted 13.5 kBtu roof air conditioner that ices the evaporator after about 40 minutes at 95°F ambient and 50% relative humidity. Supply-to-return temperature difference is only 8°F, and two of the five ceiling registers have noticeably weak airflow. The filter is clean. The blower draws 1.6 A against a nameplate 2.4 A. A flexible duct behind the refrigerator cabinet is crushed almost flat where a cabinet screw pinched it. The customer wants a pound of R-410A added because a neighbor's unit was low on charge. The unit has no service ports, and the data plate states factory-sealed, critical charge.
1. Decide whether this is an airflow problem or a charge problem. Show the evidence you would use and the measurements that separate the two.
Read the worked answer
The evidence points to airflow, not charge.
Temperature split. An 8°F supply-to-return difference at 50% RH is low; a healthy ducted RV unit typically runs about 15–20°F under those conditions. A low split with a freezing coil is the classic signature of low evaporator airflow, because the coil runs below freezing and ice further blocks the fins, which lowers airflow again.
Blower amps. 1.6 A against a 2.4 A nameplate is well below expected. A centrifugal blower moving less air does less work and draws fewer amps. That is consistent with a crushed duct and high external static pressure.
Physical findings. Two weak registers and a pinched duct are direct evidence of a distribution restriction.
Measurements to confirm. Record return and supply temperatures at the unit, external static pressure, blower wheel condition, evaporator cleanliness, register velocities at all five outlets, and duct static. Compare static pressure to the nameplate limit. Only after airflow is corrected should charge be judged. Without service ports and a weighed charge, there is no legitimate way to diagnose charge by gauge readings anyway. Adding refrigerant now would raise head pressure and amps at 95°F while the real restriction remains.
2. Explain why adding refrigerant to this unit is not a valid first step, including the equipment and documentation requirements.
Read the worked answer
The unit is factory-sealed and critical-charge. There is no suction or liquid port, so there is no superheat or subcooling measurement to support a charge decision. To add refrigerant at all, a technician would have to braze access valves into the system, recover the existing charge, evacuate to a deep vacuum, and weigh in the exact charge from the data plate. That is a sealed-system repair, not a top-off.
Refrigerant handling also carries regulatory requirements: Section 608 certification for anyone who opens the refrigerant circuit, recovery equipment, and records of refrigerant recovered and charged. Venting R-410A is prohibited.
Diagnostically, overcharging a critical-charge system at 95°F ambient raises discharge pressure and compressor current, reduces capacity, and can trip the high-pressure switch. It does not fix a frozen coil caused by low airflow, and it makes the system harder to evaluate later because the charge is now unknown.
Tell the customer the sequence: correct airflow first, verify temperature split and amp draw, and only then, if performance is still poor, install ports per the manufacturer's procedure and weigh in the charge. Document the airflow readings that justify that decision.
3. Give the repair plan and the acceptance tests you would use before signing off the job.
Read the worked answer
Repair plan.
- Remove the cabinet obstruction and replace the crushed duct section with the same or larger diameter, smooth-bore, insulated duct. Do not tape over a flattened section; replace it and support it so it cannot collapse again.
- Check every duct run for kinks, disconnected collars, torn insulation, and leaks at the ceiling plenum.
- Remove the shroud, clean the condenser and evaporator coils, and inspect the blower wheel for dirt and missing balance clips.
- Verify the blower motor is on the correct speed tap and that the run capacitor is within tolerance.
- Confirm all five registers are open and that the return air path is not blocked by stored gear.
Acceptance tests.
- External static pressure within the nameplate range.
- Blower amps near the nameplate value, not far below it.
- Supply-to-return split in the 15–20°F range at about 50% RH, measured after 20–30 minutes of stable operation.
- No frost on the evaporator or suction line after at least 60 minutes at 95°F ambient.
- Condensate drains freely and no water carries into the duct.
- Compressor amps within the data plate range. Record all readings and compare them with the before-repair numbers on the work order.