As air conditioning season gets underway, it’s a good time to review how to measure superheat and subcooling. These two measurements are two of the most important parameters needed to understand what’s happening in an air conditioning system when either charging or troubleshooting. In simple terms, and in the context of air conditioning, superheat is above the boiling point of a substance – in our case, refrigerant.
The function of an evaporator is to boil liquid refrigerant by absorbing heat from the warmer air going over the coil. This is the same concept as a pot of water absorbing heat from the element on your stove to boil, except that in the air conditioning coil, it’s happening a lot colder, around 40 F. Once the refrigerant has completely boiled and is 100 per cent vapour, it is still colder than the air going over the coil; this means that any more heat we add to it will start to increase the temperature of the refrigerant. This increase in temperature is what we call superheat.
It’s very difficult to measure the actual temperature of the coil since it’s inside ductwork with air moving over it; instead, we use the relationship between pressure and temperature to determine the temperature of the coil. A pure fluid boils at a constant temperature for a given pressure. For example, water boils at 100 C at sea level, but at the top of Mount Everest it boils at 68 C because the air pressure is so much lower. Similarly at sea level, R410A boils at -51 C and on the top of Mount Everest it would boil at -71 C. Explained in the opposite way, as we increase the pressure on a fluid, the boiling temperature goes up. It turns out that for R410A, if we measured a pressure of 118.4 psig in our coil, then the temperature at the evaporator coil would be 40 F.
Figure 1 shows this relationship on a screenshot from a refrigerant pressure-temperature (PT) app and schematically with a pressure measured on the suction line determining the temperature in the evaporator coil.
Measuring on the suction line
Superheat is heat that is added above the boiling temperature. On split systems, this is often measured on the suction line near the compressor, as shown in Figure 2. In this case, you can see that the temperature in the coil is 40 F and the temperature on the suction line is 55 F. The superheat is the difference between these temperatures, which would be 15 F.
There are two potential pitfalls with these measurements to pay very close attention to; suction line pressure drop and suction line temperature gain. Consider a suction line that has a pressure drop of five psig. In this case you would see what is shown in Figure 3. The pressure at the condensing unit would indicate 113.4 psig, which would lead you to believe the evaporator temperature is 38 F. With the same temperature measurement of 55 F as the previous example, the superheat would now seem to be 17 F.
Similarly, there is a potential problem with measuring the temperature at the condensing unit. Since the suction line is most often colder than the ambient air temperature around it, the refrigerant gets additional superheat as it travels from the evaporator to the compressor. This temperature rise is particularly noticeable if the suction line is long or poorly insulated. Figure 4 shows how this could cause an overestimation of superheat. In this case, the actual evaporator superheat is 5 F and there is 10 F of suction line superheat.
The distinction between evaporator superheat and suction line superheat — often called useful and non-useful superheat, respectively — may or not be important to you. If you are just checking the superheat to ensure that the compressor is protected from liquid flood-back, a degree or two may not matter at all. However, if you are setting up a new air conditioner or troubleshooting, these two potential measurement errors can have significant consequences. One of the reasons these concepts are important to grasp is that many types of electronic gauges do all of these calculations for you. This can lead to a reliance on the information the gauges provide without a solid understanding of the limitations associated with where you hooked them up. In a perfect world, you would be able to measure the superheat at the evaporator and eliminate the error caused by pressure drop and temperature rise. Some tools use Bluetooth to be able to do a remote temperature measurement, but a pressure measurement is not possible unless there is an access valve added at the evaporator outlet.
Subcooling is the temperature below the condensing temperature of a substance. The condenser in an air conditioner is designed to reject the heat absorbed in the evaporator and added by the compressor. In the condenser, the refrigerant is condensed from vapour to liquid. The process is essentially the reverse of what is happening in the evaporator but it’s happening at a warmer temperature because we’ve increased the pressure.
An R410A air conditioner condensing at 110 F would have a condensing pressure of 365.5 psig. Figure 5 shows this relationship with a screenshot of the PT app and a schematic of the air conditioning system. One of the trip-ups that I see regularly is caused by the fact that subcooling is happening in the warm part of the system where superheat is usually discussed in relation to the cold part of the system. One way that sometimes helps get these straight is to realize that your hot cup of coffee is subcooled since it is below the boiling point of coffee — hot things can be subcooled.
Once the refrigerant in the condenser has completely condensed, it is still warmer than the air outside.
If there is enough refrigerant in the system for liquid to back up at the condenser outlet, then the refrigerant will have a chance to cool off more. This additional change in temperature is the subcooling. Figure 6 shows the relationship between the pressure measured at the condensing unit outlet being used to determine the condensing temperature and the liquid outlet temperature. The difference between these temperatures, 10 F in our case, is the subcooling.
Subcooling measurements on residential air conditioning systems are not prone to the same errors as superheat measurements are because they are being done right at the condenser outlet.
Both adding refrigerant and troubleshooting air conditioning systems properly is highly dependent on being able to accurately measure and understand both superheat and subcooling. This article has outlined the process for obtaining superheat and subcooling measurements and explained some of the errors we can see when doing them.
However, it has so far ignored the additional challenge of blended refrigerants. In the case of 400 series refrigerants (R449A, R410A, etc.), it is important to remember to use the dew point temperature from the PT chart or app to determine superheat and the bubble point temperature when calculating subcooling. If you take a look at Figure 6, you will see that the PT app I used has a slider to switch between the two. Happy troubleshooting!