Almost every refrigeration diagnosis on the 313A exam, and on the job, comes back to two numbers. Superheat tells you what is happening at the end of the evaporator. Subcooling tells you what is happening at the end of the condenser. Read together, they separate faults that look identical on a suction gauge.
What is superheat?
Superheat is the temperature of refrigerant vapour above its saturation temperature at the same pressure.
Superheat = suction line temperature − saturated suction temperature
Say the suction line at the evaporator outlet measures 12 °C, and the suction pressure converts to 4 °C on the PT chart. The vapour is 8 K above saturation. In other words, the last liquid finished boiling somewhere before the outlet, and the vapour has picked up 8 degrees of sensible heat since.
- High superheat: the liquid ran out early. The evaporator is starved, either because there is not enough refrigerant or because something upstream is holding it back.
- Low superheat: liquid is reaching, or nearly reaching, the end of the coil. The evaporator is overfed for the heat it is getting, which risks floodback.
Say which superheat you mean. Evaporator superheat is read near the coil outlet, where a TXV bulb sits. Total superheat is read near the compressor and includes heat picked up in the suction line.
What is subcooling?
Subcooling is how far liquid has cooled below its saturation temperature at the same pressure.
Subcooling = saturated condensing temperature − liquid line temperature
If head pressure converts to 110 °F and the liquid line at the condenser outlet reads 98 °F, the liquid is subcooled by 12 °F. Subcooling tells you how much liquid has stacked up in the bottom of the condenser: more refrigerant, more stacked liquid, more subcooling.
Which one do you charge by?
It depends on the metering device.
| Metering device | Charge check | Why |
|---|---|---|
| TXV or EEV | Subcooling, to the manufacturer's target | The valve holds superheat steady, so the charge shows up in the condenser instead. |
| Fixed orifice (piston, capillary) | Superheat, from the manufacturer's chart | Nothing adjusts the feed, so superheat moves with the charge and the load. |
After evacuation, the first step on either kind is to weigh in the nameplate charge plus the line-set adjustment. Superheat and subcooling confirm and fine-tune; they do not replace weighing.
How do you read them together?
One reading narrows the problem; two readings name it. With good airflow across both coils:
| Suction pressure | Superheat | Subcooling | Likely cause |
|---|---|---|---|
| Low | High | Low | Undercharge (a leak) |
| Low | High | Normal to high | Restriction in the liquid line or metering device |
| High-ish | Low | High | Overcharge |
| Low | Low | Normal | Low evaporator airflow or load |
The first two rows are the classic trap. Both starve the evaporator, so the suction side looks the same. Subcooling breaks the tie: a system that is short of refrigerant has little liquid in the condenser, while a restricted system has refrigerant piled up behind the restriction. The undercharge vs restriction article works through that case.
Where do they sit on a pressure-enthalpy diagram?
On a P-h diagram, the saturation dome separates liquid (left), mixture (inside) and vapour (right). Superheat is the short run along the low-pressure line beyond the right edge of the dome, before compression. Subcooling is the short run along the high-pressure line past the left edge of the dome, before the metering device. The diagram above marks both.
What changes for blends with glide?
Zeotropic blends such as R-407C boil across a range of temperatures (glide), so the PT chart gives two values at each pressure. Use the dew point for superheat and the bubble point for subcooling. Using a single mid value on a high-glide blend can put either reading off by several degrees. See dew point and bubble point.
How does this show up on the 313A exam?
Charging and verifying operating parameters appear in Major Work Activity E (commissioning, 17% of the exam), and diagnosis sits in F (maintenance and service, 25%), per the public Red Seal breakdown. Expect calculation questions and "which reading would confirm..." questions. The quickest way to find out whether you have this down is to try it: the free sample includes charge-diagnosis questions with full explanations.
Sources
- Red Seal: Refrigeration and Air Conditioning Mechanic examination weightingsWork area weights and task codes
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.