Why Can’t You Just Measure Pressures to Check a Refrigerant Charge?

Most technicians in the HVAC field know the normal range of operation for the low-pressure side of an air conditioning system. This tends to be around 60 PSI to 85 PSI for R-22 and 105 PSI to 143 PSI for R-410A and is dependent upon operating conditions. On the high-pressure side of the system, there is a wide variation in pressure due to the wide outdoor temperature swing and due to the actual SEER (Seasonal Energy Efficiency Ratio) rating of the system being worked on. Because there is a wide variation on the high-pressure side of a system, technicians seem to focus on patterns seen on the low-pressure side of the system and unfortunately some use this as a short cut instead of checking the charge in a proper way. One such pattern is when the outdoor temperature is higher, the pressure measured on the low-pressure side of the system is usually higher. Because this pattern is noticed, it is confused with a charging method and unfortunately, there are technicians out there setting the refrigerant charge level in a system based on a guess of what pressure they think it should be at, during a given outdoor temperature. This couldn’t be further from a real method and doing this will eventually cost the technician as well as the system and system owner.

If a system is charged in this manner, at the best, the system may work, and the technician may get lucky and be able to get by for a while. At the worst, the system’s compressor may fail, the system may run for a long time at a low capacity, the electrical cost may be higher than it should be to run the unit, the lifespan of the system will be reduced, and after its all said and done, the technician still doesn’t know how the system is really supposed to function. If you don’t understand how the system really functions, then you have very little chance in troubleshooting a problem when one inevitably occurs. For a technician to grow in the HVAC field, one must know proper charging and troubleshooting methods to quickly and confidently service systems. In this article, we will be focusing on the charging methods. Check out some of our other articles for troubleshooting. Let’s start off with what is really going on with the refrigerant in a running system in order to know how to measure the refrigerant level correctly.

Up until this point, we focused on pressures. However, we only check pressure in order to convert the pressure to saturated temperature. Remember that refrigerants have a known pressure/temperature correlation when the refrigerant is saturated (Saturated means both liquid and vapor refrigerant are present). While the system is running, the refrigerant is saturated in only two places, the evaporator coil and the condenser coil. We can measure the saturated temperature of the refrigerant in the evaporator (indoor) coil when we measure pressure on the low-pressure side of the system. This is done on the large vapor line. We can measure the saturated temperature of the refrigerant in the condenser (outdoor) coil when we measure pressure on the high-pressure side of the system. This is done on the small liquid line. All we need to do is to convert the low and high side pressures to saturated temperatures using a P/T chart, a P/T chart overlaid on a gauge face, a P/T app, or with a digital manifold gauge set. Below is an example of a P/T chart.

Let’s look at the pressure/temperature correlation of R-410A on the P/T chart above. At 118 PSI, the saturated temperature of R-410A is 40°F. At 318.5 PSI, the saturated temperature is 100°F. If you measure 118 PSI on the low-pressure side of the system, then you know that the saturated temperature of the refrigerant flowing through the middle of the evaporator coil is at 40°F. This is a useful number when combined with the temperature of the tubing near the pressure port. The temperature on this tube will give you the temperature of the refrigerant running through the tube. This actual temp on the large vapor tube will be higher than the saturated temperature. The actual line (tube) temperature minus the saturated temperature will give you what’s called Total Superheat. This is not only a charging method but also a measurement of how safely the refrigerant is entering the compressor. Below is an example of the total superheat on a running air conditioning system.

In the above picture, we see a total superheat of 15°F.

Actual Line Temp – Sat Temp = Total Superheat

55°F - 40°F = 15°F

In order for the refrigerant to rise in temperature like this, the refrigerant must finish changing from a saturated refrigerant to a completely vapor state. The saturated refrigerant in the evaporator absorbs heat, changes to a completely vapor state, and then rises in temperature (superheats), all while in the evaporator coil. We can measure all of this when we read the pressure and line temperature at the outdoor unit service port on the large vapor tube. The low-pressure side of the system is measured at the vapor tube as shown in the picture below.

To measure the total superheat with a manifold gauge set, take a pressure measurement on the vapor line where the refrigerant enters the outdoor unit. This is done with a manifold gauge set with the blue, low pressure gauge and hose connected to the pressure port on the outdoor unit’s large vapor line service valve. Measure the pressure and convert this pressure to saturated temperature (sat temp) using the gauge face. After you find the sat temp, measure the temperature on the vapor line within 3 inches of the service valve. This will give you the vapor line temp and therefore the actual temperature of the refrigerant running through the line.

Calculate the total superheat based on the picture:

Actual Vapor Line Temp – Saturated Temp = Total Superheat

55°F - 40°F = 15°F of Total Superheat

If you are looking for basic tools to check the charge, check these out and remember that an EPA 608 license is needed in the US to work with refrigerants!

Three port manifold gauge set: http://amzn.to/2aenwTq

Hoses with low loss fittings: http://amzn.to/2aBumVI

Dual temp meter with bead temp sensors: http://amzn.to/2wc1ME3

For air conditioning systems with a piston or capillary tube (otherwise known as a fixed orifice) the refrigerant charge level can be determined using the Total Superheat method. In the example above, we measured a Total Superheat of 15°F. This total superheat must be compared to the target superheat to know if we are undercharged, correctly charged, or overcharged. The target superheat is determined by measuring the Indoor Wet Bulb (WB) and Outdoor Dry Bulb (DB) temperature and inputting both into a target superheat chart, app, calculation or digital manifold gauge set. To get a deeper look at the Total Superheat and Target Superheat, click here to go to the article on the Total Superheat Charging Method.

In the example below, we are using a target superheat chart and have measured an indoor WB temp of 62°F and a DB temp of 85°F. The target superheat is 8°F.

In order to know if we are undercharged, correctly charged, or overcharged, we compare the target superheat to the actual total superheat.

• Actual Total Superheat +/-2° F of Target Superheat = This is Correct

• Actual Total Superheat > Target Superheat = Add Refrigerant

• Actual Total Superheat < Target Superheat = Recover Refrigerant

Our measurement:

15°F Actual Total Superheat > 8°F Target Superheat = Add Refrigerant

If we just set the system to a pressure we think is correct, we will have no idea what the total superheat is at that point. If we leave the system running at 15°F when it should be at 8°F like in the example above, the system would be at a lower capacity and lower electrical efficiency than it should be. We need to add some refrigerant to match the total superheat to the target superheat. If we didn’t measure the total superheat and just added refrigerant in until we thought the system was at the right pressure, we could easily overcharge the system and we may end up with no superheat while the system is running. Earlier we explained that superheat (rise in vapor temperature) occurs in the evaporator after the refrigerant has changed from being saturated (liquid and vapor mix) to a vapor refrigerant. If there is no superheat measured on the large vapor line at the outdoor unit, then saturated refrigerant is entering the vapor compressor which will damage the compressor. The compressor must only have vapor refrigerant entering it. When we check the total superheat, we are making sure that the system is not running with less than 5 degrees of total superheat in order to protect the compressor. (Some systems are equipped with an accumulator which protects the compressor from saturated refrigerant. These are mainly found in heat pumps). Below is an example of a system running with very little to no superheat. This will damage the compressor.

Calculate the total superheat based on the picture:

Actual Vapor Line Temp – Saturated Temp = Total Superheat

49°F - 48°F = 1°F of Total Superheat

If this system has a fixed orifice metering device, the system is overcharged and damage to the compressor will occur.

The thought of setting the system based on vapor pressure is much worse and kind of laughable when it comes to systems with a TXV as the metering device. As refrigerant is added into a running system with a fixed orifice such as a piston, the vapor pressure increases. However, on a system with a TXV, as refrigerant is added to the system, the vapor pressure may not rise at all. In some cases, the vapor pressure may even fall. The TXV’s job is to hold the superheat across the TXV fairly steady even as the heat load in the building changes. This is done for efficiency in order to allow more refrigerant into the evaporator coil during high heat and high humidity and less refrigerant into the evaporator coil during lower temperatures and low humidity. The TXV will modulate the refrigerant flow into the evaporator coil. Because the TXV controls the amount of refrigerant in the evaporator coil and we are checking pressure after the evaporator coil at the outdoor unit, as we add refrigerant into the low pressure side of the system, the TXV may not allow any more refrigerant into the evaporator coil. This results in a steady vapor pressure at the port. In fact, the pressure may fall as the system runs as the heat load at the indoor coil decreases. When the heat load decreases, the vapor pressure on the low side of the system will lower so it doesn’t matter if you are adding refrigerant. What will happen with this extra refrigerant is that it will increase the high side pressure and the subcooling measured on the small liquid line. Someone trying to raise the vapor pressure on a system with a TXV will just overcharge the system, leading to lower electrical efficiency and a lower lifespan for the system.

Even worse is when there is an actual problem with the system such as a liquid line restriction. This will cause the low-pressure side of the system to be very low. If someone is only reading pressure on the low side of the system, they will automatically assume the system is low on refrigerant. All you would need to do is just measure the subcooling on the high-pressure side of the system. If this subcooling is normal to high, then you immediately know the system is not low on refrigerant. An example of normal subcooling on a system with a single or two speed compressor may be around 10°F. A high subcooling may be around 18°F and higher. (Don’t set the subcooling to these numbers as they are just examples.) For a guide of indicators on most troubleshooting problems when checking the charge, check out our quick reference cards available at amazon! The technician should be aware of how to quickly measure subcooling because that is the charging method for air conditioning systems with a TXV metering device and a single or two speed compressor.

If someone thought the system was low on refrigerant by only reading the low-side pressure and were to add refrigerant, all that would happen is that the high-side pressure and the subcooling would increase. The vapor pressure may stay the same or increase only a little. I have personally measured systems with a liquid line restriction and read a subcooling of 45°F and even higher on some units. This is simply because the previous technician did not measure subcooling and they kept trying to increase the vapor pressure by adding more and more refrigerant to the system. They were oblivious to the actual problem which was a liquid line restriction! A liquid line restriction could be a clog in the filter drier, strainer screen, metering device, or it could be a TXV that has lost its bulb pressure. Subcooling is the saturated temperature measured on the liquid line minus the actual liquid line temp.

If you really want to grow in our field and gain knowledge on saturated temperature, superheat, subcooling, and troubleshooting, check out our book which takes you step by step from the beginning of understanding through to a troubleshooting mindset. Also check out our workbook to apply the knowledge you are learning in our book! These are available here on our website and at https://www.amazon.com/shop/acservicetech.

To learn more about the Subcooling, check out our article here.

To learn more about Liquid Line Restrictions, check out our article here.

To learn about diagnosing a Frozen Evaporator Coil, check out our article here.

To see a video on a live unit where I am showing why you shouldn't try to check the charge with pressures only, click here.

Published: 4/22/2020 Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/