I teamed up with Ty Branaman to design an intuitive way for techs to solve some of the “what should it be?” questions in the HVAC field. 

The topic for this sheet is subcooling.

Subcooling is a measurement of the temperature DECREASE of a liquid below its saturation (mixed liquid/vapor) temperature at a given pressure. —“What Should My Subcooling Be?”

Refrigerant must condense into its liquid state during its journey through the condenser coil. This process is essential because we need a full column of liquid to feed the metering device. However, balance is necessary for the system to run efficiently. Too much sub-cooled liquid or a “flooded condenser”  can result in high head pressures and may indicate that the system is overcharged. While a starving condenser runs the risk of inadequately feeding the metering device the necessary liquid refrigerant.

Visualization Device for Subcooled Liquid

I don’t want to get too deep into the definition of subcooling in this article, as there is already some great information for reference at hvacrschool.com. Instead, I want to focus on the basic principles of how to find a system’s subcooling using these sheets to help you gain confidence as a diagnostician. 

Most of us now have some form of digital equipment, whether it is a digital gauge set or wireless probes that automatically spit out these readings instantaneously. These tools are great, and they’re what I use day in and day out. However, I found that because I started out with digital tools, I only saw the readings as numbers; I wasn’t visualizing what was actually happening in the system, which left me disconnected from the amazing process of the refrigeration cycle.

Wanting to gain a deeper understanding of the practical processes of the refrigeration cycle, I began researching and reaching out to people. In particular, I connected with Ty Branaman @love2hvac, who suggested that we put together these sheets to help people understand how these processes work. 

The images are purposely simple, giving the user a simplistic look at an HVAC system without distraction.

The sheets are meant to exercise oneself in taking essential measurements to understand why you are getting some of the readings you are getting.

Getting Started

As you can see, the sheet is broken down into a few simple sections. At the top half is the skeletal structure of a basic residential split AC unit, followed by a box containing the formula for finding the desired reading. To the right of this box, you’ll find some tech notes giving you some ideas of potential causes for the readings you are getting. It is important to note that this is not an extensive list, as there can often be unique variables that can influence the operation of the refrigeration cycle. Finally, at the bottom, you’ll find a legend that clarifies the labels throughout the sheet.

Solving Subcool 

First, let’s identify the actual liquid line temperature. I have highlighted all the letter Ls on the sheet, as this will be the first measurement we will take. 

By attaching a temperature reading device like a temperature clamp to the liquid line, we can determine the actual liquid line temperature.

For simplicity, we’ll say that our liquid line temperature is 115°F on an R-22 system. We then enter 115 on the L line, as shown below. 

Liquid Saturation temperature

Now, we determine our liquid saturation temperature by attaching a pressure manifold to the service port of the liquid line. 

We find that the liquid line saturation temperature equals 125°F which corresponds to a pressure of 278 PSIG on the PT chart. 

After entering these numbers, we find that we find that we have 10° of subcooling by simply subtracting the actual liquid line temperature from the liquid line saturation temperature.

—JD Kelly