Diagnostics Under Load

The core of the 608 exam is the ability to read a running refrigeration system and state its charge condition from the readings. This chapter covers superheat and subcooling interpretation, and the two most common charge errors: overcharge and undercharge.

Why it matters: the exam is not a memorization test — it is a reading test. Read the four numbers, compare them to spec, and state the charge condition. That is the skill.

The four numbers you need:

1. Suction pressure — at the evaporator outlet.
2. Discharge pressure — at the condenser outlet.
3. Superheat — the evaporator outlet temperature above its saturation temperature.
4. Subcooling — the condenser outlet temperature below its saturation temperature.

Superheat tells you how the evaporator is performing. Subcooling tells you how the condenser is rejecting heat. Together, they tell you the state of the system without opening it.

• Superheat too high — the evaporator is starving. Charge is low, the expansion device is

undersized, or the suction line is restricted.

• Superheat too low — the evaporator is flooding. Charge is too high or the expansion device

is oversized.

• Subcooling too low — the condenser is not rejecting heat. Raw-water loop restricted,

condenser fouled, or charge low.

• Subcooling too high — overcharge, or the condenser is rejecting heat too well for the load.

• Correctly charged — superheat and subcooling both within the manufacturer's specification.
• Undercharged — high superheat, low subcooling. Evaporator starving, condenser not full.
• Overcharged — low superheat, high subcooling. Evaporator flooding, condenser full.