, Question 1
A technician measures a condensing unit's liquid line temperature at 95°F and a
condensing pressure corresponding to 110°F saturation. The system uses
R-410A. What is the subcooling, and what does this value indicate about the
refrigerant charge?
A. 15°F; overcharged
B. 15°F; undercharged
C. 15°F; normal for a TXV system
D. 15°F; indicates a restriction in the liquid line
Correct Answer: C - 15°F; normal for a TXV system
RATIONALE
Subcooling = condensing temperature (110°F) minus liquid line
temperature (95°F) = 15°F. For a TXV system, normal subcooling is
typically 8-12°F, but 15°F can be acceptable depending on ambient
and load. It does not necessarily indicate overcharge or undercharge
without additional data. A liquid line restriction would cause high
subcooling and low discharge pressure, but here subcooling is within a
plausible range.
Question 2
Using the psychrometric chart, determine the enthalpy of air at 75°F dry-bulb
and 50% relative humidity (sea level).
A. ~20.5 Btu/lb dry air
B. ~28.2 Btu/lb dry air
C. ~32.0 Btu/lb dry air
D. ~35.5 Btu/lb dry air
Correct Answer: B - ~28.2 Btu/lb dry air
Page 2
, RATIONALE
At 75°F DB and 50% RH, the wet-bulb is about 62.5°F. The enthalpy
corresponding to this point on the standard sea-level psychrometric
chart is approximately 28.2 Btu/lb dry air. The other values represent
significantly different moisture contents or temperatures.
Question 3
A residential HVAC system has a total external static pressure of 0.82 in. w.c.
and a blower rated for 0.50 in. w.c. The system airflow is measured at 950
CFM, but the equipment requires 1200 CFM. Which of the following is the
most likely cause and appropriate corrective action?
A. Undersized return duct; increase return duct size or add a return
B. Overcharged refrigerant; recover charge to correct subcooling
C. Faulty blower capacitor; replace capacitor
D. Closed supply registers; open all registers fully
Correct Answer: A - Undersized return duct; increase return duct
size or add a return
RATIONALE
High external static pressure (0.82 in. w.c.) with low airflow (950
CFM vs. required 1200 CFM) indicates excessive duct resistance,
often due to undersized return ducts. Increasing return duct capacity
reduces static pressure and increases airflow. Refrigerant charge and
blower capacitor issues would not directly cause high static pressure.
Question 4
In a heat pump operating in heating mode, the outdoor coil is frosted. The
defrost cycle initiates based on which two primary sensor inputs in modern
demand-defrost controls?
A. Outdoor air temperature and outdoor coil temperature
B. Outdoor coil temperature and outdoor humidity
Page 3
, C. Outdoor air temperature and suction line pressure
D. Outdoor coil temperature and suction line temperature
Correct Answer: A - Outdoor air temperature and outdoor coil
temperature
RATIONALE
Modern demand-defrost controls typically use outdoor air temperature
and outdoor coil temperature to determine when frost is likely and to
initiate defrost only when necessary. Humidity and suction line
parameters are not primary inputs. This approach improves efficiency
by avoiding unnecessary defrost cycles.
Question 5
A 3-ton R-410A split system has a measured sensible heat ratio (SHR) of 0.75
at design conditions. If the total capacity is 36,000 Btu/h, what is the latent
capacity?
A. 9,000 Btu/h
B. 12,000 Btu/h
C. 27,000 Btu/h
D. 36,000 Btu/h
Correct Answer: A - 9,000 Btu/h
RATIONALE
Latent capacity = Total capacity × (1 - SHR) = 36,000 × (1 - 0.75) =
36,000 × 0.25 = 9,000 Btu/h. The other options incorrectly calculate
latent as total times SHR or misapply the formula.
Question 6
Which of the following best describes the purpose of a hard start kit in a
single-phase compressor?
A. To increase starting torque by adding a start capacitor and potential
relay
Page 4
A technician measures a condensing unit's liquid line temperature at 95°F and a
condensing pressure corresponding to 110°F saturation. The system uses
R-410A. What is the subcooling, and what does this value indicate about the
refrigerant charge?
A. 15°F; overcharged
B. 15°F; undercharged
C. 15°F; normal for a TXV system
D. 15°F; indicates a restriction in the liquid line
Correct Answer: C - 15°F; normal for a TXV system
RATIONALE
Subcooling = condensing temperature (110°F) minus liquid line
temperature (95°F) = 15°F. For a TXV system, normal subcooling is
typically 8-12°F, but 15°F can be acceptable depending on ambient
and load. It does not necessarily indicate overcharge or undercharge
without additional data. A liquid line restriction would cause high
subcooling and low discharge pressure, but here subcooling is within a
plausible range.
Question 2
Using the psychrometric chart, determine the enthalpy of air at 75°F dry-bulb
and 50% relative humidity (sea level).
A. ~20.5 Btu/lb dry air
B. ~28.2 Btu/lb dry air
C. ~32.0 Btu/lb dry air
D. ~35.5 Btu/lb dry air
Correct Answer: B - ~28.2 Btu/lb dry air
Page 2
, RATIONALE
At 75°F DB and 50% RH, the wet-bulb is about 62.5°F. The enthalpy
corresponding to this point on the standard sea-level psychrometric
chart is approximately 28.2 Btu/lb dry air. The other values represent
significantly different moisture contents or temperatures.
Question 3
A residential HVAC system has a total external static pressure of 0.82 in. w.c.
and a blower rated for 0.50 in. w.c. The system airflow is measured at 950
CFM, but the equipment requires 1200 CFM. Which of the following is the
most likely cause and appropriate corrective action?
A. Undersized return duct; increase return duct size or add a return
B. Overcharged refrigerant; recover charge to correct subcooling
C. Faulty blower capacitor; replace capacitor
D. Closed supply registers; open all registers fully
Correct Answer: A - Undersized return duct; increase return duct
size or add a return
RATIONALE
High external static pressure (0.82 in. w.c.) with low airflow (950
CFM vs. required 1200 CFM) indicates excessive duct resistance,
often due to undersized return ducts. Increasing return duct capacity
reduces static pressure and increases airflow. Refrigerant charge and
blower capacitor issues would not directly cause high static pressure.
Question 4
In a heat pump operating in heating mode, the outdoor coil is frosted. The
defrost cycle initiates based on which two primary sensor inputs in modern
demand-defrost controls?
A. Outdoor air temperature and outdoor coil temperature
B. Outdoor coil temperature and outdoor humidity
Page 3
, C. Outdoor air temperature and suction line pressure
D. Outdoor coil temperature and suction line temperature
Correct Answer: A - Outdoor air temperature and outdoor coil
temperature
RATIONALE
Modern demand-defrost controls typically use outdoor air temperature
and outdoor coil temperature to determine when frost is likely and to
initiate defrost only when necessary. Humidity and suction line
parameters are not primary inputs. This approach improves efficiency
by avoiding unnecessary defrost cycles.
Question 5
A 3-ton R-410A split system has a measured sensible heat ratio (SHR) of 0.75
at design conditions. If the total capacity is 36,000 Btu/h, what is the latent
capacity?
A. 9,000 Btu/h
B. 12,000 Btu/h
C. 27,000 Btu/h
D. 36,000 Btu/h
Correct Answer: A - 9,000 Btu/h
RATIONALE
Latent capacity = Total capacity × (1 - SHR) = 36,000 × (1 - 0.75) =
36,000 × 0.25 = 9,000 Btu/h. The other options incorrectly calculate
latent as total times SHR or misapply the formula.
Question 6
Which of the following best describes the purpose of a hard start kit in a
single-phase compressor?
A. To increase starting torque by adding a start capacitor and potential
relay
Page 4
