Superheat and Subcooling, Explained with Worked Examples

Why superheat and subcooling matter on a service call

A gauge set gives you two pressures. On their own they don't tell you much: suction pressure moves with indoor load, head pressure moves with outdoor temperature. Superheat and subcooling turn those pressures into temperatures you can compare against the pipe. That comparison is how you tell an undercharge from a restriction, and a dirty coil from an overcharge, without guessing or topping off.

What is superheat?

Superheat is the temperature of refrigerant vapor above the point where it finished boiling. Inside the evaporator, liquid refrigerant boils at a fixed saturation temperature set by the pressure. Once the last drop has boiled, any more heat the vapor picks up raises its temperature. That extra rise, measured on the suction line, is superheat.

What is subcooling?

Subcooling is the temperature of liquid refrigerant below the point where it finished condensing. In the condenser, vapor gives up heat and condenses at the saturation temperature for the head pressure. Once it's all liquid, any more cooling drops its temperature below saturation. That drop, measured on the liquid line, is subcooling.

Worked example: superheat on an R-454B system

Now the mistake. If you read the bubble column instead, 110 psig converts to 38.5°F, and superheat comes out at 13.5°F. That 2.7°F error is R-454B's temperature glide (about 2.7°F at this pressure). On a fixed-orifice system charged to a superheat target, it could lead you to add refrigerant the system doesn't need.

Worked example: subcooling on an R-454B system

What about single-component refrigerants?

R-32 and R-22 are single-component refrigerants, so the bubble and dew points are the same and there's only one column. At 125 psig, R-32 saturates at 41.7°F, so a 52°F suction line means 10.3°F of superheat whichever way you read it. R-410A is a blend too, but its glide is a fraction of a degree, which is why older charts showed one column.

What do high and low readings mean?

Common mistakes

• Charging by pressure alone. Pressure changes with load and weather. Without the line temperature you don't know the charge.
• Reading the wrong PT column on a blend. Dew for superheat, bubble for subcooling. On R-454B the error is a couple of degrees.
• Using the wrong refrigerant chart. R-454B runs lower pressures than R-410A. Check the nameplate.
• Clamping the probe in the wrong place. Clean the pipe, insulate the probe, and measure close to the service valve where you read pressure.
• Skipping airflow. A dirty filter or coil changes both readings. Fix airflow first, then judge the charge.
• Not letting the system settle. Let it run long enough to stabilize before you record readings.

Check yourself: 5 questions

1. An R-454B system reads 118 psig suction with a 55°F suction line. What's the superheat?
2. The same system reads 400 psig on the liquid line, and the liquid line is 112°F. What's the subcooling?
3. In question 1, what superheat would you get if you read the bubble column by mistake?
4. An R-22 system reads 70 psig suction with a 52°F suction line. What's the superheat?
5. Superheat is near zero and subcooling is high. Which is more likely: an undercharge or an overcharge?

Practice, licensing and what comes next

Refrigerant handling is regulated: EPA requires technicians who maintain, service, repair or dispose of equipment that could release refrigerant to hold Section 608 certification (40 CFR Part 82, Subpart F). Work through the EPA 608 Core practice questions next, then see what your state requires. In Texas, for example, TDLR licenses air conditioning and refrigeration contractors and registers and certifies technicians: see HVAC licensing in Texas.

Next in this module: recovery, evacuation and charging. Planning to run your own shop one day? The Tech-to-Owner path starts with how to start an HVAC business: licensing, startup costs, pricing and getting the phone to ring.