Low Superheat, Low Subcooling: Start at the Metering Device

The short version

Superheat below target with subcooling below target usually means the evaporator is getting more refrigerant than it can boil while the condenser is not holding enough liquid to build subcooling. David Richardson, writing in Contracting Business, gives the usual answer: “These conditions usually signal a metering device feeding too much refrigerant into the evaporator.” Usually is the operative word, because other faults produce the same two readings. The trap is reading the low subcooling on its own, calling the system low on charge, and adding refrigerant. The customer pays for that twice: the cooling stays short because the coil is flooded rather than starved, and the extra refrigerant sends more liquid down the suction line toward a compressor that may already be taking some, which is how a call that started as a piston or a valve ends as a compressor replacement.

Read suction pressure and head pressure before you decide anything. Richardson gives the pressure signature as “The evaporating pressure (low side) is higher, while the condensing pressure (high side) is lower.” Bryan Orr’s five pillars lists at HVAC School put “Metering device allowing too much refrigerant flow” on all four of the lists this pattern touches, which are Low Superheat, Low Subcooling, High Suction Pressure and Low Head Pressure. Two other faults are on all four of them: a compressor that is not pumping, and a reversing valve bypassing, which Orr names on its own on three of the lists and folds in with poor compression on the fourth. The one hint he gives is that very poor compression “will also be combined with VERY HIGH suction.” He puts no number on it and says nothing about what suction pressure an overfeeding metering device should read, so the degree of elevation is a clue rather than a separator.

What the pattern means

Superheat is the actual suction line temperature minus the saturation temperature for the suction pressure you measured. Saturation temperature is the temperature at which the refrigerant boils or condenses at a given pressure. Superheat measures how far the vapor warmed after the last liquid boiled off. A low reading at the evaporator outlet means liquid was still boiling late in the coil, and a reading near zero means the vapor is sitting at saturation, which is the condition under which liquid can still be in the line.

Subcooling is the saturation temperature for the liquid line pressure minus the actual liquid line temperature. It measures how far the liquid dropped below saturation before it left the condenser. A low reading means the condenser is not packed with liquid the way the design intends.

Those two definitions assume one saturation temperature at a given pressure, which is true of a pure refrigerant. A zeotropic blend is a mixture whose parts boil at different temperatures, so it changes phase across a range. HVAC School gives the rule for which end of that range each calculation uses: “When calculating superheat, we use the ‘dew point.’ We use the ‘bubble point’ when calculating subcooling. The saturation temperature is the range of temperatures between those two points.”

Read the two together and neither charge fault fits. Richardson describes the overcharge case in the same article, and it is the opposite of half of this pattern: “A system with low superheat and high subcooling has too much refrigerant.” An undercharge does not fit either, because “Low on charge” is the first entry on Orr’s Low Subcooling list and it does not appear anywhere on his Low Superheat list. A system short of refrigerant starves the evaporator, and a starved evaporator runs superheat up. If the metering device turns out to be the fault, what you are looking at is refrigerant that is in the system and in the wrong part of it.

Superheat is a difference, so the number moves when either end of it moves. Suction pressure well above normal raises the saturation temperature you subtract, which pulls superheat down even when the suction line is not especially cold. That is why the pressures belong in this diagnosis from the beginning rather than as a confirmation at the end.

Where you took each temperature determines what it proves. At the evaporator outlet you read evaporator superheat, which is the number a thermostatic expansion valve controls. A thermostatic expansion valve (TXV) is a metering device that adjusts refrigerant flow to hold superheat near a set value. On the suction line at the condensing unit you read total superheat, which includes whatever heat the line picked up on the way. A low reading at the evaporator outlet with a normal reading at the condensing unit tells you the line added heat between the two points. It does not clear the coil, because Copeland puts the two readings to different uses: “The superheat that the thermal expansion valve is controlling is the evaporator superheat. This is measured at the outlet of the evaporator.” The number the valve is judged on is still the low one.

Superheat at the compressor is the reading that matters for the risk, because that is the point where liquid does damage. Copeland publishes a floor for one family of its own compressors: “Emerson recommends a minimum of 20F (11C) superheat, measured on the suction line 6 inches (152mm) from the suction valve, to prevent liquid refrigerant floodback.” That figure is written for Copeland scroll variable speed horizontal compressors and does not transfer to another manufacturer’s compressor, but it tells you that the low end of superheat has a limit somebody publishes, and that the limit is set at the compressor rather than at the coil.

Why the metering device is the first suspect

Orr organizes this diagnosis around five readings he calls the five pillars: suction pressure, head pressure, subcooling, superheat, and delta T. He is explicit that no single reading decides anything: “This list must be utilized by taking all five calculations and matching up the potential problems until you find the most likely ones.”

Run this pattern through his cause lists and the metering device entry appears under every reading in this pattern. The wording is identical on all four lists: “Metering device allowing too much refrigerant flow; piston too large, TXV failing open, piston seating improperly.” It is on Low Superheat, on Low Subcooling, on High Suction Pressure and on Low Head Pressure. Two other entries reach all four, a compressor that is not pumping and a reversing valve bypassing, and both of them are more expensive to be wrong about.

Richardson describes the mechanism behind the subcooling half, which is the half that gets misread as a low charge. “When a metering device overfeeds, there is little back pressure to keep liquid refrigerant in the condenser. As a result, you will see lower condensing pressures and subcooling.” Ruling the charge out is worth the trouble, because “Low on charge” is the first entry on Orr’s Low Subcooling list. Once the charge is ruled out, the refrigerant is not missing. It is on the low side of a system whose metering device is passing more than it should, so there is too little back pressure to stack liquid up in the condenser where subcooling gets measured.

He also names both versions of the fault: “If the system uses a TXV, it is overfeeding or stuck open. If there is a piston to meter the refrigerant, it is likely oversized.” Those are two different repairs, and the metering device in front of you determines which conversation you are about to have with the customer.

The bulb is the first thing to check on a TXV, because it is the cheapest and it is often the answer. The valve does not measure superheat directly. It reads the suction line through a sensing bulb clamped to the pipe, and anything that makes the bulb read warmer than the line makes the valve open further. Jeffrey Staub, writing in HPAC Magazine, states the failure directly: “If the bulb mounting is loose, it will cause the TXV to overfeed, possibly flooding the compressor.”

Insulation is the second half of the same problem. Staub writes that “Overfeeding or underfeeding can also occur if the sensing bulb is not adequately insulated,” and his remedy is that “Insulation tape or foam should be liberally applied to shield the sensing bulb from ambient temperatures.” He attributes as much as 50 per cent of the heat the bulb absorbs to the strap holding it, and tells you to use the strap the equipment manufacturer recommends rather than whatever is in the truck.

A valve that is not responding to its bulb is the next case. Orr’s list has it as “TXV failing open,” and a valve held off its seat by debris behaves the same way as one that has failed internally. Richardson is blunt about the field options: “Unless you have an adjustable TXV, there is no simple solution to an overfeeding metering device.” On an adjustable valve, the manufacturer’s instructions for that valve give the direction, the increment per turn, and how long to wait between adjustments, and no generic number substitutes for them.

On a fixed orifice system the metering device is a piston or a capillary tube, and the failure is usually somebody’s earlier work. Orr’s wording covers both a piston that is too large and one that is “seating improperly.” A piston that came with a different condensing unit, or one left in place after a coil change, is sized for equipment that is no longer there. The mismatch runs both ways, because Orr puts “piston too small” under High Superheat and High Subcooling, so confirm the size instead of assuming it is oversized. A piston that is genuinely too large overfeeds every hour the system runs. Check what the outdoor unit’s literature requires before you conclude the valve or the charge is the problem.

Orr says of his own lists that “This is an incomplete list designed to help you.” Use them to order the suspects rather than to close the call.

The faults that read the same way

Low indoor airflow and low load explain half of this pattern and not the other half. Orr’s Low Superheat list carries “Low airflow/load; dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower, etc.” It also carries “Low return air temperature” and “Abnormally low humidity.” None of those entries appear on his Low Subcooling list. Less air across the coil also drops suction pressure rather than raising it, so an airflow fault and this pattern do not match on the pressures either.

Verify airflow anyway, because a system can carry an airflow fault and a metering fault at the same time. What an airflow fault does not do is account for the low subcooling in front of you.

A compressor that is not pumping produces all four readings. Orr’s Low Subcooling list gives the modes as “Compressor not pumping properly; leaking suction valve, leaking discharge valve, bad or broken crank.” His Low Superheat list carries the same family with a caveat attached: “Very poor compression (compressor, reversing valve issues) but will also be combined with VERY HIGH suction.” A compressor that is not moving vapor cannot pack the condenser, so subcooling falls, and it cannot pull the low side down, so suction pressure climbs and the saturation temperature climbs with it.

On three-phase equipment, one version of this fault is free to check and easy to miss. Copeland lists among the signs of a scroll compressor running backwards that “Discharge and suction pressures do not change,” and gives the check at startup: “If it is a three-phase scroll, determine the proper rotation at start up by observing that the suction pressure drops and discharge pressure rises.” Copeland writes that for its own scroll compressors. A compressor wired backwards after a service call is a wiring repair rather than a compressor replacement, provided it was caught early. The same page warns that “Prolonged operation in reverse result in insufficient lubrication,” so ask how long it has been running that way before you promise anyone a cheap fix.

A reversing valve bypassing internally does the same thing on a heat pump. Orr lists “Reversing valve bypassing” on Low Subcooling, on High Suction Pressure and on Low Head Pressure, and groups reversing valve issues with very poor compression under Low Superheat. Discharge gas that crosses inside the valve never does any work, which moves the same four readings in the same directions as a compressor that is not pumping.

Measurement error produces this pattern by more than one route. With temperature probes alone it is harder to fake than an undercharge, because producing it takes a suction line probe reading cold and a liquid line probe reading warm at the same time. Pressure is the easier route: a suction pressure reading biased high pulls superheat down, and a liquid pressure reading biased low pulls subcooling down. Selecting the wrong refrigerant will not usually produce both halves, because a saturation table that reads high at the low side reads high at the high side too, which pushes superheat down and subcooling up. Read the refrigerant off the data plate and select it before you convert anything regardless, since the wrong table makes both numbers wrong. The last version is comparing an evaporator outlet superheat reading against a target written for total superheat, which is a target mismatch rather than an equipment fault, and a genuinely low reading at the coil still needs its own answer.

A row below is a hypothesis, not a diagnosis. Let the system settle before you read it, and follow the equipment’s own procedure where it gives one.

Subcooling	Superheat	Suction pressure	Head pressure	Suspect	Confirm with
Low	Low	High	Low	A metering device feeding too much refrigerant	Bulb mounting and insulation on a TXV, or the piston size against the outdoor unit’s literature
Low	Low	Very high	Low	A compressor not pumping, or a reversing valve bypassing on a heat pump	Running current against the compressor’s published performance data at the pressures and voltage you measured, and, on a three-phase scroll, suction and discharge pressure at start up to confirm rotation
Normal to high	Low	Low	Normal	Low indoor airflow	Filter, blower speed, evaporator coil, and total external static pressure
High	Low	High	High	Overcharge	The manufacturer’s subcooling target, with condenser airflow verified first
Low	High	Low	Low	Undercharge, usually a leak	A leak search, then the weighed charge for that system, which is the data plate charge adjusted for the line set
Normal to high	High	Low	Normal to low	A restriction between the condenser outlet and the evaporator	Temperature drop across the liquid line filter drier
Normal	Low at the evaporator outlet, normal at the condensing unit	Normal	Normal	Where the superheat was taken, though the evaporator outlet reading still has to meet its own target	A contact probe at both places, with the refrigerant selected from the data plate

How to confirm it at the equipment

These checks narrow a metering device feeding too much refrigerant against a compressor that is not pumping, and they do it before anybody opens the system. Both faults show low superheat, low subcooling, high suction pressure and low head pressure, and one of them costs a piston while the other costs a compressor. Work in order, because each step costs less than the one after it.

Let the system settle, then confirm the readings you already have. Give it ten to fifteen minutes of steady runtime. Read the refrigerant off the data plate and select that refrigerant on your instrument before you convert a single pressure. Clamp contact probes to clean copper and insulate them from the surrounding air, because every number after this rests on those two temperatures.

Check your two temperature probes against each other. Clamp both to the same spot on the same pipe, let them settle, and read them. The gap between them is the disagreement you are carrying into every superheat and subcooling calculation for the rest of the call. The comparison is not a calibration check, because two probes can agree with each other and share the same error.

Take superheat in two places. Read it at the evaporator outlet, which is the number a TXV controls, and again on the suction line near the compressor, which is the number that tells you whether liquid is reaching it. Copeland’s published floor of 20°F at 6 inches from the suction valve applies to its own scroll variable speed horizontal compressors, so treat it as evidence that the compressor end has a limit rather than as a target for the equipment in front of you. The limit for that compressor comes from the manufacturer of that compressor.

Read suction and head pressure against the pattern rather than against memory. Richardson’s signature for an overfeeding metering device is a higher evaporating pressure with a lower condensing pressure. Orr’s caveat attaches very poor compression to suction pressure that is far above normal, without saying where normal ends, and he lists an overfeeding metering device under high suction as well. So the degree of elevation narrows the field rather than settling it, and the delta T reading and the compressor checks cover the rest.

Take delta T while you are inside. Delta T is the temperature drop of the air across the evaporator coil, return air temperature minus supply air temperature. Orr describes it as “a nice calculation because it gives you a good look at system performance and airflow,” and puts the ordinary range at “between 16 and 22 degrees’ difference from the return to the supply.” A narrow split fits a coil that is flooded and a compressor that is not pumping equally well, so it narrows the field rather than determining the answer. A wide split points back at airflow instead.

Inspect the sensing bulb on a TXV system before you touch anything else on the valve. Five checks: the clamp is tight and the bulb does not shift when you push it; the bulb’s whole length contacts clean copper; a proper bulb strap holds it rather than tape or a hose clamp; insulation covers it completely; and it sits where Staub puts it, “securely mounted on the evaporator outlet downstream from the refrigerant header, in a position on the pipe least affected by liquid refrigerant and oil.” Any one of those, corrected, can be the whole repair.

Then test whether the valve still responds to its bulb. Staub’s published test warms the bulb by hand: “Simply hold it in your hand for a minute or two.” He gives the result to watch for as “If the bulb charge is fine and the valve is not stuck or seized, it should respond by opening.” Watch for suction pressure to rise and superheat to fall. Cooling the bulb is the mirror of that test, and on a valve that is overfeeding, cooling is the more direct test, because a valve that responds should throttle closed and drive suction pressure down and superheat up.

A valve that does not move in either direction is not controlling the coil, and that is the finding you take into the repair.

Check the compressor before you condemn the metering device. Compare running current against the compressor manufacturer’s published performance data at the suction pressure, discharge pressure and voltage you measured. Rated load amps on the data plate is not that number, and Copeland describes RLA as a “Reference current used in contactor and wire selection for US Customers.” Watch suction and discharge pressure separate at start up, and on three-phase equipment confirm rotation the way Copeland describes. A weak compressor call is made against the manufacturer’s performance data for that compressor at the conditions you measured, not against a rule of thumb, and it is the most expensive diagnosis on this page.

What to do once you have the answer

A tight bulb, a proper strap and full insulation are the repairs that do not open the system. Correct any of those, let the system run again, and re-read superheat and subcooling before you go further. A valve that was reading the air around it is not a valve that needs replacing.

Replacing a valve or changing a piston means opening the refrigerant circuit, and that is a recovery job. The Environmental Protection Agency (EPA) sets the requirement at 40 CFR 82.156(a): “Before opening appliances (except small appliances, MVACs, and MVAC-like appliances) or disposing of such appliances, technicians must evacuate the refrigerant, including all the liquid refrigerant, to the levels in Table 1 using a recovery and/or recycling machine certified pursuant to § 82.158 unless the situations in paragraphs (a)(1) or (2) of this section apply.” MVAC means motor vehicle air conditioner.

The same section adds a step people skip when the recovery machine sounds finished: “A technician must verify that the applicable level of evacuation has been reached in the appliance or the part before it is opened.”

Identify the refrigerant before any heat comes near the system. Getting this wrong can injure a person as well as damage a part, because heat applied to a sealed volume of refrigerant raises its pressure and because a flammable refrigerant needs ignition sources taken away rather than brought closer. Copeland publishes its flammable refrigerant guidance as “general information to air conditioning and refrigeration service technicians about certain requirements for working with systems charged with flammable refrigerants,” and its instruction on the tool is absolute: “Never use a torch to remove compressor components or tubing. Always use tubing cutters.”

Identifying a flammable refrigerant is not permission to apply heat carefully. It is the point where a different procedure starts. The same page tells you to “Always check the area around the system with a combustible gas monitor rated for the specific class of flammable refrigerant (Class 3, 2 or 2L) in the system.” It attaches its clearance figure to venting rather than to service in general: “If controlled venting of flammable refrigerants is permitted, venting should take place in a well-ventilated area, far from all sources of ignition. Maintain at least a 10-foot radius around the compressor free of sources of ignition, people, and anything that can burn.”

The same page puts identification first: “One of the first steps in safely working on a system is to identify the refrigerant used in the system and whether it is flammable.” It also sets a qualification ahead of the work: “EPA certification is not enough. Servicing HVAC/R systems with flammable refrigerants requires additional certification.”

Close the system back up the way the repair requires. Richardson’s instruction after replacing a valve or changing a piston is short: “Be sure to follow proper evacuation procedures and measure vacuum with a micron gauge.” A micron gauge measures the vacuum in the system in microns of mercury, and it is what tells you whether the moisture the open circuit let in has come back out.

Re-read the system before you leave, and expect all four readings to move. Superheat should come up toward the target for that metering device, subcooling should come up toward the manufacturer’s target, suction pressure should fall and head pressure should rise. Readings that stay where they were mean the part you changed was not the fault.

Then look at what the flooded coil may already have cost the compressor. Richardson states the consequence for the overcharge case, and it applies to any coil that cannot boil what it is being fed: “The excess refrigerant could easily make it back to the compressor and cause permanent damage from flood back and, in severe cases, slugging.”

Danfoss describes the same mechanism from the compressor side in its compressor checkup guidance, written for refrigeration and air conditioning compressors. It gives the reason compressor manufacturers publish a minimum suction superheat: “The purpose of these values is to eliminate the return of liquid refrigerant to the compressor while it is operating which can dilute oil and result in bearing failures.” Danfoss describes a second route to the same place, off cycle migration, where refrigerant vapor collects in a compressor that is colder than the rest of the system: “The oil will absorb the refrigerant which upon start up, boils out and causes the oil to foam, greatly reducing lubrication.” A system that spent a season flooding its coil may have spent a good deal of that season on diluted oil, which is worth telling the customer before you quote the repair.

Common mistakes

Read suction pressure and head pressure alongside superheat and subcooling, since both an overfeeding metering device and a compressor that is not pumping are on all four of Orr’s lists and the degree of the suction elevation is the first thing that leans one way.
Check the bulb mounting, the strap and the insulation on a TXV system first, since Staub describes a loose bulb as a cause of overfeeding and possible compressor flooding, and correcting it does not open the system.
Select the refrigerant printed on the data plate before you convert any pressure, because the saturation table for the wrong refrigerant makes both calculations wrong in the same step.
Verify airflow even though it does not explain this pattern, since a system can carry an airflow fault and a metering fault at the same time.
Take superheat at the compressor as well as at the evaporator outlet, because the compressor is where liquid refrigerant does damage and where manufacturers publish a floor.
Recover the refrigerant to the levels the EPA requires before you open the circuit to change a valve or a piston, and verify the level was reached before the part comes off.

Don’t

Add refrigerant because subcooling is low, since an overfeeding metering device produces low subcooling on a system that holds every ounce it needs.
Call this an overcharge, because Richardson puts the overcharge signature at low superheat with high subcooling and this pattern has the subcooling going the other way.
Condemn a TXV before you have checked the bulb clamp, the strap, the insulation and the bulb’s position on the pipe.
Leave an oversized or badly seated piston out of the suspect list on a fixed orifice system, since Orr names both a piston that is too large and one seating improperly.
Bring a torch to a system whose refrigerant you have not identified, which Copeland rules out for flammable refrigerants in favor of tubing cutters.
Treat a narrow temperature split as proof of a flooded coil, because a compressor that is not pumping narrows it the same way.

Frequently asked questions

What does low superheat with low subcooling mean?

It means the evaporator is receiving more refrigerant than it can boil while the condenser is not holding liquid back. David Richardson, writing in Contracting Business, states that “These conditions usually signal a metering device feeding too much refrigerant into the evaporator,” and names the two versions as a TXV that is overfeeding or stuck open and a piston that is oversized. Treat it as a hypothesis rather than a diagnosis. A compressor that is not pumping produces the same four readings, and so does a reversing valve bypassing on a heat pump. Bryan Orr at HVAC School attaches very high suction pressure to very poor compression, but he lists an overfeeding metering device under high suction too and draws no line between them, so the degree of elevation is a clue rather than an answer.

Does low superheat mean the system is overcharged?

Not with subcooling this low. Richardson puts the overcharge signature at “low superheat and high subcooling,” because refrigerant that will not fit in circulation stacks up in the condenser and drives subcooling up. Low superheat with low subcooling has the subcooling moving the opposite way, which is why the metering device is the first suspect and adding refrigerant makes the situation at the compressor worse rather than better.

How do I tell an overfeeding metering device from a compressor that is not pumping?

Suction pressure is the first clue rather than the answer. Orr’s five pillars lists put very poor compression under Low Superheat with the note that it “will also be combined with VERY HIGH suction,” while Richardson describes an overfeeding metering device as raising the evaporating pressure and lowering the condensing pressure. Orr lists both faults under High Suction Pressure and gives no threshold between them, so what settles it happens at the compressor. Compare running current against the compressor manufacturer’s published performance data at the pressures and voltage you measured rather than against rated load amps, watch suction and discharge pressure separate at start up, and on a three-phase scroll confirm rotation. Copeland’s check is to observe at start up that suction pressure drops and discharge pressure rises.

Can low airflow cause low superheat and low subcooling?

Low airflow drives superheat down, and Orr’s Low Superheat list carries a long entry for it covering a dirty filter, a dirty evaporator, a restricted or undersized return, a dirty blower wheel and a blower running at the wrong speed. It does not appear on his Low Subcooling list, and less air across the coil lowers suction pressure rather than raising it. So low airflow accounts for half of this pattern and contradicts the pressure half. Verify airflow anyway, because a system can carry both faults at once.

What is the repair for an overfeeding TXV or an oversized piston?

On a TXV, check the bulb first, since a loose clamp, a missing strap, missing insulation or a bad position on the pipe can all make a healthy valve overfeed, and correcting any of those does not open the system. If the valve still will not control superheat, Richardson’s answer is that “Unless you have an adjustable TXV, there is no simple solution to an overfeeding metering device,” and the repair is replacement or adjustment per the valve manufacturer’s instructions. On a fixed orifice system the repair is the correct piston for that equipment. Both mean opening the circuit, which the EPA requires you to recover before doing.

Glossary

Bubble point: the saturated liquid end of a refrigerant blend’s phase change at a given pressure, which is the temperature subcooling is calculated from.
Contracting Business: a trade publication cited here on the superheat and subcooling combinations.
Copeland: a compressor manufacturer, cited here on compressor superheat, scroll rotation and flammable refrigerant service.
Danfoss: a components manufacturer, cited here on what liquid refrigerant does to compressor oil.
Delta T: the temperature drop of the air across the evaporator coil, return minus supply.
Dew point: the saturated vapor end of a refrigerant blend’s phase change at a given pressure, which is the temperature superheat is calculated from.
EPA: Environmental Protection Agency, which sets the recovery requirement quoted here.
Evaporator superheat: superheat measured at the evaporator outlet, which is the number a TXV controls.
Fixed orifice: a metering device that cannot adjust, such as a piston or a capillary tube.
Floodback: liquid refrigerant reaching the compressor while it runs.
HPAC Magazine: a trade publication cited here on expansion valve bulbs.
HVAC School: a practitioner training and reference organization cited here.
Micron gauge: the instrument that measures a system’s vacuum in microns of mercury during evacuation.
MVAC: motor vehicle air conditioner, carved out of the recovery paragraph quoted here.
Piston: the fixed orifice metering device used in much residential equipment, sized per model.
RLA: rated load amps, which Copeland describes as a reference current used in contactor and wire selection. It is not the current the compressor manufacturer’s performance data predicts at the pressures and voltage you measured.
Saturation temperature: the temperature at which refrigerant boils or condenses at a given pressure, and on a zeotropic blend the range between the bubble point and the dew point.
Sensing bulb: the bulb clamped to the suction line at the evaporator outlet, which senses the line temperature and supplies the pressure a TXV balances against evaporator pressure and spring pressure.
Slugging: liquid refrigerant entering the compression chamber, where it cannot be compressed.
Subcooling: how far the liquid refrigerant sits below its condensing saturation temperature.
Superheat: how far the refrigerant vapor sits above its evaporating saturation temperature.
Total superheat: superheat measured on the suction line at the condensing unit, including heat the line picked up.
TXV: thermostatic expansion valve, the metering device that adjusts flow to hold superheat.
Zeotropic blend: a refrigerant mixture whose parts boil at different temperatures, so it changes phase across a range rather than at one temperature.

Drafted with AI assistance and reviewed by the author.

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