313A System Troubleshooting & Safety

Question 1

A refrigeration system shows high discharge pressure, high suction pressure, and low superheat. What is the most likely cause?

Accepted Answer

Refrigerant overcharge

Answer

High discharge pressure, high suction pressure, and low superheat together indicate refrigerant overcharge — excess refrigerant floods the evaporator (low superheat) and overwhelms the condenser (high discharge pressure).

Question 2

A system has low suction pressure, low discharge pressure, normal subcooling, and high superheat. What is the most likely diagnosis?

Accepted Answer

Refrigerant undercharge

Answer

Low suction pressure combined with high superheat and low discharge pressure indicates insufficient refrigerant in the system — the evaporator is starved of liquid, causing high superheat, and the compressor is moving less vapour mass, lowering both pressures.

Question 3

What instrument is used to measure refrigerant pressure at service valves on a refrigeration system?

Accepted Answer

A digital manifold gauge set

Answer

A manifold gauge set (or digital manifold) connects to the high- and low-side service valves to measure suction and discharge pressures, allowing the technician to diagnose system performance and charge level.

Question 4

What does a bubbling sight glass indicate when the system is at steady-state operation?

Accepted Answer

Refrigerant shortage, moisture contamination, or a restriction causing partial vaporization of the liquid upstream of the sight glass

Answer

A bubbling or foamy sight glass at steady state indicates the liquid line contains vapour bubbles, which can be caused by low refrigerant charge, excessive pressure drop, or a restriction in the liquid line producing flash gas.

Question 5

A compressor is running but suction pressure is very high and the system is not cooling. The discharge line is not hot. What is the most likely cause?

Accepted Answer

Failed compressor discharge or suction valves, causing the compressor to pump poorly

Answer

If the compressor is running but the discharge line is not hot and suction pressure is very high, the compressor valves have likely failed — the compressor is spinning but not effectively compressing the refrigerant, so it is not moving refrigerant through the system.

Red Seal HVAC 313A Practice Test

Red Seal HVAC 313A Practice Test

313A System Troubleshooting & Safety