The Refrigeration Cycle, Superheat, and Subcooling
Refrigeration questions are about 20 percent of the practice bank, and they split cleanly into two halves. This chapter is the physics: what each component does, what saturation means, and how to calculate and interpret superheat and subcooling, which are the two numbers that tell you almost everything about a running system. The next chapter is the regulatory and handling half, covering refrigerant types, blends, and EPA Section 608. Almost every troubleshooting question later in the guide resolves back to a superheat or subcooling reading, so this is the chapter to be fluent in.
Four Components, Four Jobs
The evaporator absorbs heat from the conditioned space. Liquid refrigerant boils inside it at a low temperature, and the heat required to boil it comes from the air passing over the coil. The compressor raises the pressure of the vapor leaving the evaporator and circulates it; in a residential hermetic compressor, the returning suction gas also passes through the motor windings and cools them, which is one reason a system starved of suction gas overheats its own compressor. The condenser rejects that heat outdoors and condenses the high-pressure vapor back to liquid. The metering device, a thermostatic expansion valve, an electronic expansion valve, a fixed orifice, or a capillary tube, drops the pressure sharply so the liquid can boil cold again in the evaporator. The pressure drop is the whole point: refrigerant boils at whatever temperature corresponds to its pressure, so lowering the pressure lowers the boiling point until it is below the temperature of the air you want to cool. As refrigerant passes through the metering device, part of the liquid flashes to vapor immediately, which is what chills the remainder down to the evaporator temperature. On an operating system the discharge line, from compressor to condenser, is hot and at high pressure, while the suction line, from evaporator to compressor, is cool and at low pressure. Latent heat of vaporization is the term for the heat absorbed as refrigerant changes from liquid to vapor at constant temperature, and it is where nearly all the cooling capacity comes from.
Saturation, Pressure-Temperature Charts, and What They Mean
Wherever liquid and vapor coexist in a refrigerant circuit, the temperature and pressure are locked to one another. That relationship is the saturation curve, and a pressure-temperature chart or app for that specific refrigerant is how you convert a gauge reading into a temperature. Two practical consequences follow. First, a gauge on the low side is really a thermometer for the evaporator: whatever suction pressure the compressor and metering device establish determines the evaporator's saturation temperature, which is why an undercharged system can freeze a coil on an 80 degree day. If suction pressure falls far enough, the saturation temperature drops below 32 degrees F and the condensate on the coil turns to ice. Second, you must use the chart for the refrigerant that is actually in the system. Different refrigerants have entirely different saturation curves, and the data for a new A2L blend comes from the pressure-temperature data published for that specific refrigerant, not from an R-410A chart with a mental adjustment. The same physics explains cylinder behavior. As a refrigerant cylinder warms, its internal pressure rises along the saturation curve, which is why cylinders are kept out of direct sun and away from heat sources and are never filled beyond the fill limit for the cylinder.
Superheat, Worked
Superheat is the number of degrees a vapor is above its saturation temperature at the existing pressure. It exists only after the last drop of liquid has boiled, so any superheat at all proves the refrigerant leaving that point is entirely vapor. That is the safety function: superheat at the compressor inlet is what guarantees no liquid is entering the compressor. To measure it, read the suction pressure, convert it to a saturation temperature with the chart for that refrigerant, then measure the actual suction line temperature and subtract. Example: the saturation temperature at the measured suction pressure is 40 degrees F and the suction line reads 52 degrees F, so the superheat is 52 minus 40, which is 12 degrees. Same arithmetic with an R-410A gauge reading of 118 psig whose saturation temperature is 40 degrees F and a 52 degree suction line: 12 degrees of superheat. Interpretation is where the exam lives. Very low superheat, say 5 degrees, means the evaporator is nearly flooded and liquid may be reaching the compressor. Very high superheat, say 30 degrees, means the coil is starved: the refrigerant boiled off early, the rest of the coil is doing little work, and the compressor is taking in hot gas, which raises discharge temperature and cooks the oil. When you are specifically hunting for flooding, measure superheat at the compressor rather than at the evaporator outlet, because heat picked up along a long, poorly insulated suction line can make a coil with correct superheat still return liquid.
Subcooling, Worked
Subcooling is the mirror image, measured on the high side at the condenser outlet, which is the liquid line. It is the number of degrees the liquid is below its saturation temperature at the condensing pressure. Read the discharge or liquid line pressure, convert to a condensing saturation temperature, measure the liquid line temperature, and subtract the liquid line temperature from the saturation temperature. Example: condensing saturation temperature 105 degrees F, liquid line temperature 95 degrees F, so subcooling is 105 minus 95, which is 10 degrees. Subcooling matters because a metering device is rated to meter liquid. If the column of refrigerant arriving at the valve contains vapor bubbles, the valve cannot pass its rated mass flow and the evaporator starves. Adequate subcooling is what guarantees a solid column of liquid. Now interpret. A system with 2 degrees of subcooling and 25 degrees of superheat on a TXV is undercharged: there is not enough refrigerant to fill the condenser and back up a liquid seal, and the starved evaporator shows it on the low side. A system with a condensing saturation temperature of 120 degrees F and a liquid line at 118 degrees F on a 95 degree day has only 2 degrees of subcooling with a very high condensing temperature, which points to a shortage of charge or poor condenser heat rejection. High head pressure with high subcooling and normal superheat on a TXV system is the classic overcharge signature: extra refrigerant is stacking in the condenser, reducing the surface available to condense. On a TXV system, subcooling is the reading that tells you the most about charge level, because the valve holds superheat roughly constant regardless of charge.
Metering Devices in Detail
A thermostatic expansion valve maintains a relatively constant evaporator superheat by balancing three forces on its diaphragm. Bulb pressure, generated by the sensing bulb strapped to the suction line, pushes the valve open. Evaporator pressure and spring pressure push it closed. When the coil is starved, the suction line warms, bulb pressure rises, and the valve opens further; when the coil floods, the suction line cools, bulb pressure falls, and the valve closes down. The external equalizer line exists because the pressure that should close the valve is the pressure at the evaporator outlet, not at the inlet, and on a coil with meaningful pressure drop those differ enough to make the valve misbehave. A TXV that hunts, with suction pressure swinging every 30 seconds or so, is usually being lied to by its own bulb, so the first thing to check is a loose, badly located, or uninsulated sensing bulb. An electronic expansion valve does the same job with sensors and a controller positioning the valve electronically, which gives a wider control range and faster response. A fixed orifice or piston has no feedback at all: it passes flow according to the pressure difference across it, so its superheat varies with load, which is exactly why those systems are charged to a target superheat from a chart rather than to a subcooling value. A capillary tube behaves similarly and has one useful property: system pressures equalize during the off cycle, so the compressor restarts against a low load and does not need a start assist.
Heat Pumps: Reversing Valve, Accumulator, and Defrost
A heat pump is a refrigeration circuit with a four-way reversing valve that redirects discharge gas either to the outdoor coil, for cooling, or to the indoor coil, for heating. When a customer reports cold air from the registers on a call for heat, the reversing valve or its solenoid coil is the prime suspect, because the machine is physically running in cooling. An accumulator sits on the suction line ahead of the compressor and does two things: it holds any liquid that returns from the coil so it cannot slug the compressor, and it meters oil back gradually through a small orifice so the oil does not stay trapped. Heat pumps need it because heating mode, defrost transitions, and low ambient operation all produce intermittent liquid return. Defrost is the operation candidates describe wrongly most often. On a call for defrost the reversing valve shifts to the cooling position so hot discharge gas flows through the outdoor coil and melts the frost, the outdoor fan stops so that heat is not blown away, and auxiliary electric heat is usually energized to keep the supply air from feeling cold indoors. A time-and-temperature defrost control initiates on a fixed clock interval when the coil is cold enough; a demand-defrost control initiates based on measured coil conditions, such as the spread between coil temperature and ambient, so it defrosts only when frost has actually accumulated. Head pressure control is applied on systems operating in cold ambient conditions to keep enough pressure difference across the metering device for it to feed the evaporator properly.
Compressors, Compression Ratio, and Oil
Compression ratio is absolute discharge pressure divided by absolute suction pressure. Absolute means gauge pressure plus atmospheric pressure, which is why a ratio computed from gauge readings alone is wrong. As the ratio rises, capacity falls, and the mechanism is worth knowing: gas trapped in the clearance volume at the top of a reciprocating compressor's stroke re-expands on the downstroke, and the higher the discharge pressure, the more of the cylinder that re-expanding gas occupies, so less fresh vapor is drawn in on each stroke. A scroll compressor works differently, compressing continuously between two mating spiral elements rather than with pistons and valves, which makes it quieter and more tolerant of small amounts of liquid, though not immune. Liquid causes two distinct failures that candidates conflate. Slugging is a sudden charge of incompressible liquid entering the cylinder, which can break valves and bearings immediately. Flooding is gradual liquid return that dilutes the oil, washes the bearings, and destroys the compressor over weeks. Polyolester oil, used with modern refrigerants, is hygroscopic: it absorbs moisture from the air quickly, so containers stay sealed until the moment of use. Moisture in a circuit is corrosive because it reacts with refrigerant and oil to form acids, and it can freeze at the metering device and block flow. Non-condensable gas such as air produces a characteristic symptom: head pressure higher than the condensing temperature would predict, because the air occupies condenser volume and adds its own partial pressure.
Last updated: September 2026