Air Conditioning Repair Installation and Service Procedures 5 — Questions and Answers
Question 1: During a system commissioning, the technician measures a wet-bulb temperature of 67°F entering the evaporator and a dry-bulb of 75°F. What is the approximate entering relative humidity?
- About 40%
- About 55% (Correct answer)
- About 70%
- About 85%
Correct answer: About 55%
A 67°F wet-bulb with 75°F dry-bulb corresponds to approximately 55% relative humidity, which is typical summer indoor design conditions.
Question 2: What is the most common cause of a newly installed air handler producing excessive noise at startup?
- Refrigerant overcharge
- Blower wheel set screw not tightened or wheel rubbing the housing (Correct answer)
- Low refrigerant charge
- Incorrect thermostat wiring
Correct answer: Blower wheel set screw not tightened or wheel rubbing the housing
A loose blower wheel set screw allows the wheel to wobble or shift, causing scraping or rattling noise that disappears once the set screw is properly torqued.
Question 3: When charging a system by weight, the technician should zero the refrigerant scale:
- After connecting the charging hose to the cylinder
- Before connecting the hose, with the empty hose weight accounted for (Correct answer)
- After opening the cylinder valve
- Only if the manufacturer requires it
Correct answer: Before connecting the hose, with the empty hose weight accounted for
Zeroing the scale with the hose already attached accounts for the hose weight, so the display shows only the refrigerant transferred rather than hose plus refrigerant.
Question 4: A condensing unit is located in a pit below grade level. What is the primary concern with this installation?
- Refrigerant lines will be too short
- Hot air discharge will recirculate into the unit, reducing efficiency and capacity (Correct answer)
- The electrical disconnect is inaccessible
- The unit cannot be properly leveled
Correct answer: Hot air discharge will recirculate into the unit, reducing efficiency and capacity
A condensing unit in a below-grade pit tends to trap its own hot discharge air, causing high head pressure, reduced efficiency, and potential high-pressure lockout.
Question 5: After replacing a TXV on an R-410A system, the technician finds the superheat is 25°F instead of the target 10°F. What is the most likely cause?
- The TXV sensing bulb is not properly clamped to the suction line (Correct answer)
- The system has too much refrigerant
- The condenser is oversized
- The filter is clogged
Correct answer: The TXV sensing bulb is not properly clamped to the suction line
An improperly secured TXV sensing bulb reads ambient temperature instead of suction line temperature, causing the valve to restrict flow and drive up superheat.
Question 6: When pressure testing a newly installed system with nitrogen, the test pressure for an R-410A system should not exceed:
- 150 psig
- 300 psig
- 450 psig (Correct answer)
- 600 psig
Correct answer: 450 psig
ASHRAE 15 and most manufacturers limit nitrogen pressure testing of R-410A systems to 450 psig (the low-side design pressure) to avoid damaging system components.
Question 7: A homeowner reports the new system cools well but the house feels humid. Airflow measurements confirm CFM is above manufacturer specifications. What is the recommended corrective action?
- Add refrigerant charge to improve latent capacity
- Reduce blower speed to allow the coil more contact time for dehumidification (Correct answer)
- Install a larger condenser
- Increase the thermostat setpoint by 2°F
Correct answer: Reduce blower speed to allow the coil more contact time for dehumidification
Excessive airflow reduces coil contact time, limiting latent (moisture removal) capacity; reducing blower speed lowers coil temperature and increases dehumidification.
Air Conditioning Repair Installation and Service Procedures 5
During a system commissioning, the technician measures a wet-bulb temperature of 67°F entering the evaporator and a dry-bulb of 75°F.
What is the approximate entering relative humidity?