Superheat and subcooling introduced

The dental clinic's rooftop unit had tripped on high pressure again. Dana reset it, let it run for fifteen minutes, and handed Theo a clipboard. "Four numbers," she said. "Suction pressure and suction line temperature, liquid pressure and liquid line temperature. From those, work out the superheat and the subcooling, and tell me what the system is doing." Theo wrote down the readings, looked at the P-T chart on his gauge, and did the subtraction. He came back with two numbers. Dana nodded. "Now you can see inside the pipes."

Superheat and subcooling are the two most useful measurements in refrigeration service. This lesson defines them, shows how to measure them, and introduces what they say about the charge and the feed of a system. RAC-220 builds on this when it covers metering devices and charging.

Superheat

Superheat is the number of degrees a vapor is above its saturation temperature at its pressure. In the evaporator, the refrigerant boils at the saturation temperature until the last of the liquid is gone. From that point to the evaporator outlet, the vapor keeps absorbing heat and its temperature rises. The difference between the actual vapor temperature and the saturation temperature is the superheat.

To measure it, read the suction pressure with a gauge at the suction service port, convert it to the saturation temperature with the P-T chart for the refrigerant, and measure the temperature of the suction line with a contact thermometer clamped to clean bare pipe and insulated from the surrounding air. Subtract. Superheat is always a temperature difference, so it is the same whether measured in kelvins or Celsius degrees, and it converts to Fahrenheit by multiplying by 1.8.

Technicians distinguish two kinds of superheat. Evaporator superheat is measured at the evaporator outlet, close to the coil, and it is the one an expansion valve controls, because its sensing bulb is clamped there. Total superheat, also called compressor superheat, is measured at the compressor suction or the condensing unit's service port, and it includes the heat the vapor picks up in the suction line. On a split system with a long line set, total superheat is larger than evaporator superheat. Both are useful; the important thing is to know which one you measured and which one the manufacturer's target refers to.

What superheat tells you

Superheat shows how well the evaporator is fed. If the metering device lets too little refrigerant into the evaporator, the liquid boils off early, the rest of the coil only superheats vapor, and superheat is high. The evaporator is starved: part of its surface is wasted, capacity falls, and the compressor, which relies on cool suction vapor to cool its motor, runs hot. If the metering device lets too much refrigerant in, liquid reaches the outlet, superheat falls toward zero, and droplets of liquid can carry over to the compressor. The evaporator is flooded, and the compressor is at risk.

Superheat also responds to the load. More heat into the evaporator boils the liquid off sooner and raises superheat; less heat, for example from a dirty filter or a failed indoor fan, leaves liquid in more of the coil and lowers superheat. A system with an expansion valve adjusts its feed to hold superheat roughly steady as the load changes. A system with a fixed metering device, such as a capillary tube or fixed orifice, cannot adjust, so its superheat swings with the load and with the charge, and manufacturers give target superheat charts for those systems based on the indoor and outdoor conditions.

As a general guide for teaching, many systems with thermostatic expansion valves run with an evaporator superheat of about 5 to 12 °F. Freezers often run toward the low end and air conditioners toward the middle. Always use the manufacturer's target for the actual equipment.

Subcooling

Subcooling is the number of degrees a liquid is below its saturation temperature at its pressure. In the condenser, the vapor first gives up its superheat, then condenses at the saturation temperature. Once the last vapor has condensed, the liquid keeps losing heat to the cooling air or water, and its temperature drops below saturation. The difference is the subcooling.

To measure it, read the pressure at the liquid line service port, convert it to the saturation temperature with the P-T chart, and measure the liquid line temperature near the same point. Subtract the line temperature from the saturation temperature. Some systems have only a discharge service port; the discharge pressure is close to the liquid pressure in most small systems, but the manufacturer's instructions say which port to use.

Subcooling does two jobs. It guarantees that the liquid line is full of liquid, not a mix of liquid and vapor, so the metering device receives a solid column of liquid. A line that loses pressure on the way, from a long run, a vertical lift or a restriction such as a clogged filter drier, could otherwise start to flash before it reaches the metering device, which starves the evaporator. And it increases capacity: colder liquid flashes less at the metering device, so more of each kilogram is left to boil in the evaporator, as Lesson 6 showed.

What subcooling tells you

Subcooling shows how much liquid is backed up in the condenser. In systems with an expansion valve, the valve holds the superheat steady, so extra refrigerant collects as liquid in the condenser, where it fills tubes that would otherwise condense vapor, and subcooling rises. A system short of refrigerant has little liquid in the condenser and low subcooling. That is why manufacturers of equipment with expansion valves usually give a target subcooling for charging. Very high subcooling with low suction pressure, on the other hand, can point to a restriction: refrigerant cannot get through the liquid line or metering device, so it stacks up in the condenser while the evaporator starves.

As a general guide for teaching, many air-cooled systems run with about 8 to 14 °F of subcooling, and many residential split systems are charged to a manufacturer's target near the middle of that range. Always use the nameplate or installation instructions for the actual equipment.

Putting the four readings together

Superheat and subcooling are most useful together, alongside the suction and head pressures and the temperatures of the air or water across each coil. The table below shows the classic patterns. Real systems can show combinations, and good diagnosis always checks airflow and the condition of the coils before adding or removing refrigerant. RAC-220 and RAC-430 develop this into a full diagnostic method.

Dana showed Theo how the dental clinic's readings fit the table. The head pressure was high and the condensing temperature was about 22 degrees above the outdoor air instead of the usual 10 to 15. Superheat was normal and subcooling was slightly low, because hot, high-pressure liquid was not cooling well. The pattern pointed to the condenser, not the charge, and the condenser coil was matted with cottonwood fluff from a tree beside the building. After a careful cleaning, the head pressure dropped and the trips stopped. Had Theo added refrigerant because the subcooling looked low, he would have made the high pressure worse.

# Superheat and subcooling from field readings (R-410A, approximate table in psig)
import math
table = [(0, 48.6), (10, 62.3), (20, 78.3), (30, 96.8), (40, 118), (50, 142), (60, 170),
         (70, 201), (80, 236), (90, 274), (100, 317), (110, 365), (120, 418), (130, 476)]
P_ATM = 14.7

def t_sat(psig):
    p = psig + P_ATM
    for (t1, g1), (t2, g2) in zip(table, table[1:]):
        p1, p2 = g1 + P_ATM, g2 + P_ATM
        if p1 <= p <= p2:   # interpolate on the logarithm of absolute pressure
            return t1 + (t2 - t1) * (math.log(p) - math.log(p1)) / (math.log(p2) - math.log(p1))
    raise ValueError("pressure outside the table")

def check(name, suction, suction_line, liquid, liquid_line):
    sh = suction_line - t_sat(suction)
    sc = t_sat(liquid) - liquid_line
    note = []
    if sh > 18: note.append("high superheat")
    if sh < 4: note.append("low superheat")
    if sc > 18: note.append("high subcooling")
    if sc < 5: note.append("low subcooling")
    print(f"{name:20s} SH {sh:4.1f} F  SC {sc:4.1f} F  {', '.join(note) or 'within the guide range'}")

check("ductless, normal", 118, 51, 375, 100)
check("readings A", 88, 58, 276, 89)
check("readings B", 93, 63, 368, 86)

ductless, normal     SH 11.0 F  SC 12.0 F  within the guide range
readings A           SH 32.5 F  SC  1.5 F  high superheat, low subcooling
readings B           SH 34.9 F  SC 24.6 F  high superheat, high subcooling

Readings A show high superheat and low subcooling with low pressures on both sides: the classic undercharge pattern. Readings B show high superheat and high subcooling with low suction pressure: refrigerant is backed up in the condenser and not reaching the evaporator, which points to a restriction. The program only does the arithmetic and flags the numbers; the diagnosis still needs the technician to look at airflow, coils and the whole system.

Measuring well

Small measurement errors make large diagnosis errors, because superheat and subcooling are small differences between larger numbers. A gauge that reads a few kilopascals or a psi off, a thermometer probe that is loose or not insulated, or a reading taken before the system has stabilized can change the answer by several degrees. Good practice is to let the system run at least ten to fifteen minutes with doors closed and the load steady, clamp probes to clean bare copper and wrap them with insulation, take pressure and temperature at the same location, check gauges at zero before connecting, and repeat the readings a few minutes apart to make sure they are stable.

For zeotropic blends with glide, use the dew-point temperature for superheat and the bubble-point temperature for subcooling, as Lesson 5 explained. Most digital gauge sets handle this automatically once the refrigerant is selected, which is one reason they are now standard.

Superheat, subcooling and the charge

Superheat and subcooling are also the main tools for setting the refrigerant charge, which RAC-220 covers in detail. For systems with fixed metering devices, the manufacturer provides a chart of target superheat that depends on the indoor wet-bulb temperature and the outdoor dry-bulb temperature, because the right superheat changes with the load. Refrigerant is added if the measured superheat is above the target and recovered if it is below. For systems with expansion valves, the manufacturer usually gives a target subcooling, because the valve holds superheat steady and the extra refrigerant shows up in the condenser.

The best method of all, whenever the system allows it, is to weigh the charge: recover the refrigerant if necessary, evacuate the system, and weigh in the exact amount on the nameplate, adjusted for the length of the line set. Superheat and subcooling then confirm that the system is running as designed. Dana's rule for Theo was simple: never add or remove refrigerant on the strength of one number, and never before checking airflow, filters and coils.