What will happen to the evaporator if the superheat of the evaporator is above the normal superheat?

Most service technicians can sustain a conversation about evaporator superheat and total superheat, but are far less comfortable delving into the emerging conversation about superheat through the duct system. However, measuring superheat in the duct system is just as important as measuring it through the equipment.

Start With the Basics

By way of background, let’s start with a quick review of the cooling cycle in the most common residential cooling system: direct expansion (DX), using refrigerant and forced air.

Refrigerant leaves the condensing unit as a liquid and travels through the liquid line to enter the evaporator. The liquid moves through a metering device and its pressure is reduced. Heat from the house is drawn through the return air and added to the refrigerant as its conversion to a gas begins. As heat is removed from the house, the refrigerant in the coil gains heat. It becomes superheated when all liquid refrigerant is turned to a gas.

Cool air now exits the coil and is distributed throughout the house through the supply duct system. Sensible heat is pulled by the compressor back to the condensing unit through the suction line, where it is discharged outside through the condensing unit. Any moisture removed from the evaporator (latent Btus) is discarded down the condensate drain.

Problems begin when operating conditions of the system fall outside of the published design parameters. HVAC systems are dynamic; when one temperature or pressure changes, every other temperature and pressure reacts to that change. HVAC systems are built to change. When operating, everything is constantly changing. There is really no such thing as “steady state” in a heating or cooling system. Keep this in mind when testing and diagnosing every system.

Diagnostics and Manufacturer’s Specifications

When evaluating superheat issues, start by knowing the manufacturer’s specifications. When stating equipment capacity, the operating conditions are always attached to the rated Btus of the equipment.

For example, when three-ton equipment is rated at 33,600 Btu, it is at 95F outdoor and an entering (entering the equipment) air temperature of 63F wet bulb and 75F dry bulb at 400 cfm per ton. (63F wet bulb/75F dry bulb indicates air that is around 50% relative humidity.) When the outdoor temperature changes, the temperature or humidity of the entering indoor increases or decreases, or the system airflow changes, equipment capacity will change as well.

Refrigerant temperature and pressure charts are also invaluable when testing and diagnosing systems because they also express the relationship changes under varying conditions.

Measuring Three Types of Superheat

Just as crimes are often solved by “following the money,” performance issues in an air conditioning system can often be solved by “following the heat.” Follow heat as it moves through the system to see if the system is or is not performing as it should. That means understanding and measuring all three types of superheat: evaporator, total, and duct system.

1. Evaporator superheat. To measure evaporator (indoor coil) superheat, first measure the suction line temperature at the evaporator outlet. Next, measure the refrigerant pressure at the suction line of the indoor coil. In order to do this you may have to install a Schrader valve tee into the external equalizer fitting on the suction header of the coil, or externally through a brazed-in fitting. Purge your hoses, connect your gauges, and start the system. Let it run for approximately 10 minutes, and then read the evaporator outlet pressure. Using the appropriate refrigerant pressure chart, convert the pressure reading to saturation temperature.

Evaporator coil superheat may then be calculated by subtracting the saturation temperature from the suction line temperature. Compare the evaporator superheat to the manufacturer’s specifications based on the type of metering device the system uses. Remember, the amount of superheat is based on the volume of refrigerant allowed to enter the metering device and the return air temperature at the time of the test.

2. Total superheat. As the refrigerant exits the evaporator through the suction line and travels to the outdoor condensing unit, it typically picks up additional heat. Ideally the suction line is well-insulated, and the refrigerant’s trip through a hot attic (in summer months) is quite short. However, this is not always the case, and total superheat must be measured and calculated to ensure the longevity of the compressor.

The procedure and test instruments required for measuring and calculating total superheat are the same for measuring and calculating evaporator superheat. The only change is the location where the measurements are taken.

Measure the suction line temperature and suction pressure at the suction side service valve. Ensure the temperature probe is insulated from any external influences. Convert the gauge pressure to saturation temperature and subtract this temperature from the suction line temperature. This is the total superheat.

Compare the total superheat to manufacturer’s specifications and ensure it is within range for the conditions at which you’re testing.
If the readings are unsatisfactory, the system may have an equipment airflow issue or a problem with the refrigerant charge.

3. Duct system superheat. While technically not normal refrigerant side superheat, one of the most overlooked performance issues missed by service techs is the superheat of the duct system. The duct system plays a critical role in the performance of the system and deserves equal testing and diagnostics.

Generally we assume that equipment capacity and system capacity are the same thing. Rarely is this the case in the field. The equipment may be performing perfectly, but as the air passes through the duct system it may be superheated or subcooled before or after it leaves the equipment. If a technician is too focused on the equipment and the refrigeration cycle and fails to look beyond the box, the real source of a poorly performing system can be missed.

To measure duct system superheat, start on the return side. Measure and record the air temperature entering the return grille. Then measure and record the air temperature entering the equipment. Subtract these two temperatures. If the temperature loss or gain through the duct system is more than 5% of the temperature change through the equipment, it warrants a repair such as eliminating or reducing duct leakage or adding additional duct insulation.

On the supply side, measure the air temperature leaving the equipment and the air temperature at the farthest register. Subtract the two temperatures and if the temperature change through the duct system exceeds more than 5% of the equipment temperature change, duct tightening or added insulation can be recommended.

Finally, don’t forget to always check a system’s static pressure. On the pressure side of the duct system, measure operating total external static pressure by using a low-pressure manometer and a static pressure tip. Measure the pressure entering and exiting the air-moving equipment and add the two pressures together. Compare this pressure to the published maximum total external static pressure listed on the equipment nameplate. If the pressure exceeds the manufacturer’s rated pressure, modify the duct system, clean the indoor coil, or replace the air filter with a less restrictive filter.   

Rob “Doc” Falke, David Richardson, and Scott Johnson serve the industry through National Comfort Institute an HVAC based training company and membership organization. If you’re an HVAC contractor or technician interested in receiving a free procedure to measure duct superheat or total external static pressure, email or call any of the authors at 800/633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.

Superheat and Subcooling are technical readings in an HVAC that measure the Freon (refrigerant) reading. Superheat measures the Freon boiling point in gas form while subcooling measures the Freon in liquid form below the evaporation level.

To calculate superheat and subcooling measurements, a specific Mathematical chart is used, and the process is mostly done by an HVAC technician.  This is usually done when the technician is diagnosing your HVAC for overheating or flooding of the Freon.

You can still learn how to calculate these measurements on your own as part of learning how your air conditioning unit works.

What is Superheat HVAC?

Superheat is a temperature value measured of vapour (steam or refrigerant) above boiling point. The reading will show the amount of refrigerant going through the evaporator and whether it is sufficient.

When the reading is too high, it means that the refrigerant is not sufficient, so the system will be inefficient. On the other hand, when the reading is too low, it means that too much liquid is going to the evaporator. This can damage the compressor as the liquid might cause flooding.

How to Measure Superheat

The first step here is to measure the compressor suction pressure.  With the suction pressure measurement, you can follow the chart to see the corresponding saturation temperature.

NB: Allow the HVAC to run for 15 to 20 minutes so that you can get accurate results. When taking the temperature of the Freon, use a temperature probe.

What is Subcooling

This is when the refrigerant is cooled at a temperature lower than the minimum temperature required. Many reasons can cause subcooling to occur, including underfeeding, overcharging, restricted meter device, or faulty head pressure control. The effect of subcooling is reduced system efficiency and overcharging of the system.

Checking the Subcooling

You will need a temperature probe and gauge to take the measurements.

For accuracy, take measurements near the condenser coil of the liquid line.

Always use the manufacturer’s readings as the guide.

Benefits of Superheat and Subcooling Measurements

Superheat and subcooling are important measurements to determine the performance and efficiency of your HVAC system. It is important to check these measurements during the routine servicing by your technician.

Here are some benefits of regular checking:

Improved Efficiency

Superheat, and subcooling readings are important to help improve the efficiency of your system. Inefficiency is usually caused by inappropriate refrigerant levels, so regular checking might help you determine the right step to take.

If your HVAC has become inefficient, talk to your technician about checking the refrigerant levels, and you will notice a huge improvement.

Easy and Fast Diagnosis

Repairing your HVAC can take a lot of time when trying to pinpoint the issue. Most technicians are not aware of the exact problem just by checking your air conditioning system. They need to check the superheat and subcooling readings to point out the problem accurately.

Evaporator and compressor issues have similar characteristics, so it might be confusing to know the exact issue without checking the measurements.

Avoid Overheating

Overheating can damage your entire system, and it is usually caused by low refrigerant levels. When refrigerant levels are low, the compressor starts overheating, and the first thing that you will notice is efficiency.

Overheating can be quite detrimental, as it can damage other parts of your HVAC, leading to costly repairs.

What causes High Superheat and Low Subcooling

Liquid Line Restriction

The refrigerant passes through a liquid line in the HVAC system. The normal flow of the liquid means that you will be able to maintain optimum levels of the refrigerant. However, if there is a restriction or a blockage that restricts the flow, problems will be inevitable.

Malfunctioning Metering System

The metering system of the HVAC is posed to control the amount of refrigerant passing through the liquid line. If there is a fault and proper control is not happening, you are likely to experience high or low refrigerant levels. This will lead to high superheat or low subcooling.

Excessive air in the Evaporator Coils

The amount of air flowing in the evaporator coils should be regulated to keep the refrigerant levels optimal. Unfortunately, sometimes there is excessive air in the evaporator coils, and this causes the refrigerant to evaporate.

Limited Airflow in the Evaporator

Apart from excessive air in the evaporator coils, limited air can also be a problem. The air levels should be regulated so that the refrigerant remains at the right level.

The Role of Freon in your HVAC

The refrigerant (Freon) is an important part of your HVAC system. Therefore it is crucial to maintain the right levels for the efficiency of your system. Freon is a cooling agent, responsible for keeping your home cool during the summer months.

If you are not a technician, it’s not advisable to attempt to change Freon on your own. This is because it can lead to more damage to your unit.

Here are some benefits of refrigerant:

Protects the HVAC from Overheating

Freon is known as a cooling agent. It is responsible for preventing overheating in the HVAC unit. Without proper levels of Freon, your HVAC unit will undoubtedly overheat, leading to damage.

During the routine servicing, you can check your Freon levels. The technician can drain the fluid and replace it when necessary. Keeping clean fluid and maintaining optimum levels is important for the efficient performance of your HVAC system.

Better Performance and Efficiency

For better performance and efficiency, you need to have enough Freon in your HVAC. It will prevent your HVAC from overworking, giving you the level of efficiency you desire. With clean Freon and the right levels, it is possible to achieve optimal efficiency.

Are Superheat and Subcooling Important?

Superheat and Subcooling are important concepts in HVAC performance. However, if you don’t have the technical knowledge on how to calculate the measurements, it might be stressful to try to figure it out on your own.

Fortunately, consulting a technician regularly will help you monitor your superheat and subcooling. Experts admit that it is impossible to achieve optimum measurements, but you can still keep your HVAC in good condition.