Superheat: The Best Way to Tune an Air Conditioning System

As energy costs increase and air conditioning system efficiency becomes more critical, the need for accurate superheat measurements becomes more important.

Unfortunately, many technicians take shortcuts when it comes to performing their superheat calculations. Some technicians, under pressure to hurry to the next job, speed the process by making assumptions without taking measurements, or take quick measurements with inadequate instrumentation.

Bill Brown, HVAC instructor at Brownson Technical School, Anaheim, CA, gives this example: "Instead of measuring suction line temperature, some technicians will wrap their bare hand around the pipe and judge by feel. The theory is that if the line is as cold as good cold beer, the system is charged. Unfortunately, that's just not accurate, and we do everything we can to dis-courage this kind of sloppiness."

The results of these "guesstimates" are overcharged or undercharged systems that burn power and operate below optimum levels. In some cases, poor superheat calculations can lead to compressor burnout.

To determine the best way to measure super-heat, we asked some HVAC instructors how they taught superheat, and what instruments they recommend to make taking superheat measurements faster and easier.

Defining Superheat
Superheat is defined as the difference between the temperature at which the refrigerant boils at the given pressure in the evaporator, and the temperature of the refrigerant gas as it leaves the evaporator. In essence, it's how much extra temperature the refrigerant picks up after it has boiled. In a worst-case scenario with an over-charged system and a low indoor heat load, the refrigerant in the evaporator remains in liquid form in the coil and moves into the compressor as a liquid, quickly destroying it.

A distinction must be made between systems that employ a thermostatic expansion valve (TXV) and those that have a fixed restrictor/flow-rater. In a properly adjusted system, the TXV controls the flow of refrigerant to ensure superheat is always within a certain range regardless of the condition.

On a TXV system that is starved (undercharged), the superheat will go up above the range. But on an overcharged TXV system, the superheat will not go below the range. The TXV will pinch off the flow of refrigerant to keep the superheat within specified conditions, resulting in significantly decreased efficiency. For this reason, to properly charge these systems the technician simply charges to the subcooling.

Most problems with superheat occur on fixed restrictor systems. In a well-tuned system, the actual (or measured) superheat varies with the load. The higher the load, the sooner the refrigerant turns into a gas, the more heat it picks up after it evaporates, and the higher the superheat. The lower the load, the later the refrigerant evaporates in the evaporator coil, the less heat is picked up after evaporation, and the lower the superheat.

Target Superheat vs. Actual Superheat
Using superheat measurements to determine the correct refrigerant charge is not that difficult, and the savings in energy and potential repairs are significant.

There are only four measurements to take: two to determine the target super-heat and two to determine the actual superheat.

For target superheat, the two measurements are outdoor dry bulb temperature and indoor wet bulb temperature.

For actual superheat the measurements are boiling/saturation point and suction line temperature.

First, determine the target superheat. To do this, take the outdoor air temperature from the air that is going into the condenser coil. Then, determine the wet bulb temperature from in front of the indoor return grille, or better yet, just in front of the evaporator coil.

"The dry bulb temperature is obvious," says Les Haddix, an HVAC instructor at Sequoia Institute, Fremont, CA. "However, many technicians think that wet bulb refers only to indoor comfort. In reality, it's very important in determining the right superheat.

"One of the shortcuts a lot of technicians use is to assume a relative humidity of 50% when determining super-heat. But that's just the system's design parameter. If the relative humidity is higher than that and the technician assumes 50%, the tech will overcharge the system. If the humidity is lower, he'll undercharge."

Armed with these two measurements, refer to the manufacturer's display chart supplied with the unit, usually with wet bulb temperature across the top and dry bulb along the side. The target superheat is displayed where the two intersect.

After you've determined the target superheat, you need to determine what the superheat actually is. To determine the actual superheat, you need only two more numbers: The boiling point (saturation point) of the refrigerant in the evaporator (at that pressure), and the suction line temperature.

The boiling point is easy. "The technician can measure the pressure at the condensing unit suction port with a pressure gauge, and in most cases read the boiling point right on the gauge," notes Rob Featherstone, HVAC instructor at Oakland Community College in Auburn Hills, MI. "If the boiling point for the refrigerant you're working with isn't on the gauge, you can look it up on a pressure temperature chart."

To determine the temperature of the refrigerant in the suction line pipe, all you need to do is measure the temperature of the pipe itself within 6-in. of the suction valve.

When you know the boiling point and the suction line temperature, subtract the boiling point located on the gauge or chart from the suction line temperature to get the actual superheat. This is the increase in temperature of the refrigerant gas after it has evaporated.

Once you know the actual superheat and the target superheat, compare them to determine if the system is properly charged. If the actual superheat is lower than the target superheat, recover refrigerant; if it's higher, add refrigerant. Just be sure to always let the system stabilize, and check again after adding or subtracting refrigerant.