What is Superheat?

Improper charge is the biggest non-electrical cause of call-backs in a fixed restrictor air conditioning system.  If all the components are functioning properly, comparing actual superheat to target superheat on a fixed restrictor air conditioner is the best way to determine if the system is charged correctly.

Superheat is a temperature rise above the saturation (boiling) temperature of a substance for a given pressure.  If heat is added to refrigerant when it is a 100% saturated vapor, then it will become superheated.  Imagine a pressure cooker full of water and steam.  As we continually add heat to the water, it will evaporate into steam. While the water evaporates into steam at a constant pressure the environment in the pot will remain at the same temperature.  The energy from the heat goes into turning the water into steam.  Once the water is all turned to steam then additional energy (heat) introduced into the pot will cause a temperature rise, this temperature rise above the saturation temperature (boiling point) for that constant pressure is called superheat.

Actual Superheat

Actual Superheat = Suction Line Temperature – Suction Line Saturation Temperature

Actual superheat is calculated from your suction line pressure and temperature measurements.  The suction line saturation (boiling) temperature is subtracted from the actual measured suction line temperature.  A properly charged fixed restrictor air conditioner has an actual superheat which matches the target superheat for the current outdoor and indoor environmental conditions.  Adjusting the actual superheat can be done by adding or recovering refrigerant from a fixed restrictor system.

Not Enough Superheat

Superheat is an essential safety measure to ensure that liquid refrigerant never enters the compressor.  Whether the compressor be reciprocating, scroll, etc. all compressors are designed to compress vapor refrigerant.  Various compressor types and brands have different tolerances to liquid exposure but one thing is certain, they will all prematurely fail if they are exposed to liquid refrigerant for an extended period of time.

Too Much Superheat

Too much superheat reduces the real world cooling capacity of your system (tonnage).  To maximize the capacity of an indoor evaporator coil you would want 0 °F of superheat and a 100% saturated vapor right at the exit of the evaporator coil.  This in fact would allow for the greatest cooling potential of the coil because the coil absorbs much more heat from the air in turning the liquid refrigerant into vapor than it does in superheating the refrigerant. However, this would put your compressor in great risk for premature failure because it leaves you with an unknown and minimal margin of safety.  If you charged a system this way a small change in the indoor or outdoor environment or suction line pressure change would expose the compressor to liquid refrigerant.  Too much superheat could also affect compressor cooling since refrigerant is used for heat dissipation of the compressor.

Target Superheat

Target superheat is one of the least understood topics in the HVAC service industries.  Target superheat is the recommended superheat for the given indoor and outdoor environmental conditions.

Environmental conditions affect the amount of actual superheat found in a system.  For that reason the target superheat chart of a system must take into account all the environmental conditions under which the system is expected to run.  This ensures that even if the environmental conditions change the system will operate with good capacity and the compressor will not be exposed to liquid refrigerant.

Most target superheat charts including the one used in California’s legislature under title 24 require indoor wet bulb and outdoor dry bulb measurements to yield a target superheat.

Figure 1. Title 24 Target Superheat Chart.  This is a copy of the chart found in the California Title 24 legislation.

Chart Properties

The target superheat tables all have limits for the maximum and minimum outdoor dry bulb temperatures and indoor wet bulb temperatures used to obtain a target superheat.  If you are outside of these ranges then you will not be able to properly charge the air conditioning system using these tables.  These tables were developed to accommodate the most typical operating ranges of the air conditioning equipment as specified by the manufacturers of the equipment.

There are target superheat tables and charging procedures which allow for the charging of air conditioners outside the normal operating range.  These tables and algorithms are typically used for preventative maintenance or for charging air conditioners in climates which fall outside of the typical range.  These tables and algorithms are even built into some automated superheat and subcooling tools.

Because small pressure drops in the pipe, measurement inaccuracies and other unknowns the minimum recommended target superheat for most charts is between 4-5°F for the environments with low indoor wet bulb and high outdoor dry bulb.

The following graph shows the effects of a changing indoor wet bulb and an outdoor dry bulb on target superheat.  The target superheat increases rapidly with an increase in indoor wet bulb.  Conversely the target superheat decreases slowly with an increase in outdoor dry bulb.

Figure 2.  Effect of changing indoor wet bulb and outdoor dry bulb on target superheat.  The effects of changing indoor and outdoor environmental conditions on the target superheat can be seen from the lines on the above chart.

Indoor Conditions

The indoor evaporator coil is absorbing heat from the air in the duct system.  This heat goes into the refrigerant and causes the refrigerant to evaporate and then become superheated.  In addition to lowering the temperature of the indoor air, the evaporator also dehumidifies the air.  The condensate line then takes that moisture out of the coil.

Because it takes a lot of cooling capacity to dehumidify air the relative humidity of the indoor air plays a very large role in where in the evaporator the refrigerant begins to become superheated.  In order for target superheat charts to be accurate they must take indoor relative humidity into account.  This is most commonly done by taking indoor wet bulb measurements.

When 50% RH for the indoor conditions is assumed the effect of changing indoor heat load is not properly taken into account and the resulting target superheats will not be accurate.  If you charge assuming 50% relative humidity you could put the compressor in danger for those times when the air conditioner has been on long enough to have a low indoor load relative to the outdoor load or have reduced capacity during the most commonly operated conditions for the air conditioner.

An air conditioner is started when the indoor conditions are warmer than desirable.  As the air conditioner does its job the indoor cools and the heat is rejected to the outside. The outdoor conditions may not change between measurements but because you are cooling and dehumidifying the inside the indoor conditions could change between measurements.  As the air conditioner cools the indoor air, the indoor heat load goes down.  This causes the actual and target superheat to go down.  It’s important to verify the actual superheat and target superheat at the same time to prevent changing conditions from affecting the refrigerant charging procedure.

Outdoor Conditions

Outside air passes over refrigerant lines that are at a higher temperature than the environment and the heat in the refrigerant is rejected to the outside environment.  The temperature of the outside has a great influence on how much heat can effectively transfer to the outdoor environment.  The hotter it is outside, the harder it is for the condenser to dump heat into the outside ambient.  This reduces the capacity of your air conditioning system.  In fact air conditioning units are only specified to run up to a certain outside ambient temperature.  Because there is no change to the moisture level of the air going through a condenser only outdoor dry bulb measurements are necessary to asses the outdoor heat load.

Superheat and TXV/EXV Systems

The TXV/EXV valve is designed to maintain constant superheat despite changing indoor or outdoor environmental conditions.  Instead the subcooling in the air conditioning system changes with the environmental conditions.  Because TXV/TEV systems have greater variety in their designs manufacturer’s target subcooling should be used to determine the proper charge level for a TXV/TEV whenever possible.  An actual superheat measurement can be used with a TXV/TEV system to check performance of the TXV/EXV valve.

Taking the Measurements

Taking Measurements for target superheat

Target superheat charts usually require indoor wet bulb and outdoor dry bulb.  The measurements are really concerned with the air traveling over the evaporator coil and condenser coil respectively.  This air is the actual environment that the refrigerant coils are exposed to and taking these measurements accurately without interfering with the normal operation of the air conditioning system can be tricky.

Measuring Indoor Wet Bulb

Indoor wet bulb measurements should be taken as close to inlet of the evaporator coil as possible.  In many cases there is an air filter before the evaporator onto which a wet bulb thermocouple can be clipped.  Take the measurement as close to the center of the air stream as possible.  The system must be sealed as it is during normal operation when the measurements are taken.  The thin wires of the thermocouple make this much easier.  If there is no filter in front of the evaporator, a small hole can be made before the evaporator.  Make sure you do not damage any equipment when doing this and that you seal the hole once you have finished.

Figure 3. Indoor Wet Bulb Measurement. For the best measurement, clip the wet bulb thermocouple to the filter in the airflow before the evaporator coil and slide the filter back in.

Other measurements such as taking return air grill temperatures are often used but because there are temperature changes and infiltration of outside air on the return air duct before it arrives at the evaporator using these measurements for target superheat will not give you the best accuracy.  The best measurement is taken right before the evaporator.


Measuring Outdoor Dry Bulb

For best accuracy, take the outdoor dry bulb temperature in the center of the inlet fins.  Clipping a thermocouple to the fins themselves makes the measurement very convenient and repeatable.

Taking Measurements for Actual Superheat

Actual superheat requires that you get the temperature and saturation temperature (boiling point) of the refrigerant in the suction line.  The saturation temperature is traditionally looked up on a P-T from a measured pressure or could be right on the gauge along with the pressure reading.  Because pressure drops occur throughout the refrigerant lines taking the temperature and pressure measurements physically close to each other will minimize these errors.  Typically both of these measurements are taken near the suction side service port.

Figure 4. Actual Superheat Measurements.  The suction line temperature and pressure are measured to calculate actual superheat.

Taking the Suction Line Temperature Measurement

For best results on the suction line temperature measurement use a temperature probe which is specifically designed to take the temperatures of refrigerant pipes.  A probe that does not make good thermal contact with the refrigerant pipes or keep that temperature effectively isolated from the ambient environment will give you false pipe temperatures which could lead to improper charging.  There have been some advances in products designed to accomplish this.  Make sure you select a thermometer that is specifically designed for measuring pipe temperatures and that you can realistically see yourself using it on EVERY service call you make.

Figure 5. Specialized pipe temperature measurement tool. The new Fieldpiece ATC1 taking refrigerant temperature of a suction line.

Try to expose as little of the refrigerant piping as possible to the ambient air to accurately represent how the system will be running when you’re not there.  Place the sensor of the suction line thermometer close to the center line of the pipe to avoid taking the temperature of refrigerant oils or non-condensables which may be present in the line.

Taking the Suction Line Pressure Measurement

The saturation temperature (boiling point) in the suction line is looked up from the type of refrigerant and the pressure measured at the suction line service port.  Many gauges actually have the saturation temperature for one or two refrigerants printed right on the dial of the gauge.  In case you are working on a refrigerant whose saturation temperature does not appear directly on the pressure gauge a P-T chart can be used to lookup the saturation temperature based on the measured pressure and the type of refrigerant.

Automating the Process

There are several instruments for automating the tedious process of getting the measurements for target and actual superheat and then performing the calculations.  Most superheat instruments automate the process of getting the actual superheat.  These instruments can be great time savers for the experienced technician as well as effective training tools for those less comfortable with the measurements.  Whichever instruments you chose make sure the sensors and instrument are specifically designed for your application.

Figure 6. Fieldpiece HVAC Guide (left) and SRH2 (right) will automatically calculate  Target Superheat from direct measuremenents, thereby eliminating tables entirely.  The HVAC Guide can also be used to automatically calculate superheat and subcooling with the most accurate refrigerant tables in the industry, thereby eliminating the need for P-T Charts and hand calculations.  Both have target superheat charts which are compliant to title 24 but also allow for a greater range of preventative maintenance and hotter climate charging.

 There are also a few products that automate the target and actual superheat measurement process and entire superheat diagnosis process as a whole.  Some of these instruments are field upgradeable and contain charts which go beyond the typical target superheat charts allowing for charging air conditioners in preventative maintenance or for very hot climates.  Some of these instruments will even alert you to unusual conditions and possible measurement errors prompting you to retake the measurements.  These instruments are great time saver and also a training tool.

To properly charge a fixed orifice system the indoor and outdoor heat load needs to be taken into account.  Charging to both indoor wet bulb and outdoor dry bulb gives you the flexibility to charge a fixed orifice air conditioner in a much wider variety of conditions and ensures good performance and compressor life in the ranges of operation that the equipment is designed for.


Adolfo Wurts is the Engineering Manager at Fieldpiece Instruments in Anaheim, California. He is also a certified HVAC/R technician.