Superheat, Subcooling, and the Lessons Techs Learn Too Late

My HVAC career began with me flipping through map books, trying to find my way to my next maintenance call. At the same time, I was calling in to the base on a bulky Nextel radio to inform the dispatcher that I was en route to the next call. I started off learning air conditioning, like many other technicians, by being thrown to the wolves. 

My first manifold was a hand-me-down analog three-hose setup. Come to think of it, I think most of my tools were hand-me-downs. It was nothing fancy, but enough to get me into trouble. And believe me, I was up to my neck in trouble. 

I have learned many lessons while wandering through this trade, and one of my first lessons was how to check the refrigerant charge using a manifold.

Introduction to Refrigerant Charge

Speaking of refrigerant charge, from the beginning, I learned to compare the condensing saturation temperature to the outdoor ambient air temperature. I was told newer units would have a condensing temperature around 22 degrees above the outdoor ambient temperature, while older R-22 systems would usually run about 30 degrees above the ambient temperature. 

As time passed, I thought this had to be just an old rule of thumb. Years later, I learned this was a metric called Condensing Temperature Over Ambient (CTOA). At NCI, we call it Condensing Temperature Difference (CTD). 

There are cool things you can do with CTOA/CTD, like perform a non-invasive cooling system check by calculating the target liquid line temperature. This is a detail I didn't grasp at the time, but wish I had known. 

While CTOA/CTD are extremely useful in determining how well a system is operating, you should consider other measurements as well. The problem was that I didn't understand which other measurements to take.

The Era of 10 SEER Equipment

A year or two in, I took an air conditioning class at my local distributor. The instructor handed out slide charts with a superheat calculator on one side and subcooling on the other. The class was a great introduction to air conditioning theory, but it gave me a false sense of confidence. There was so much more that I didn't know. 

The class was back in the 10-11 SEER days, so most of the evaporator coils still had piston metering devices. I remember purchasing a Fieldpiece meter that came in two parts, which snapped together. The meter could calculate target superheat by connecting two K-type wired thermocouples. 

One of them had what looked like a shoestring at the end, which you were supposed to dip in distilled water to measure the return air wet bulb. However, I was guilty of never using distilled water, but let's keep that between us. This was a big upgrade from the old slide charts, even though I used normal tap water.

Years later, I stumbled across a YouTube video of a technician sitting at his kitchen table explaining the formula for target superheat. He rattled off:

Target Superheat = Indoor wet bulb x 3 – 80 – outdoor ambient / 2.

I was amazed by this formula and compared it to my meter and slide chart: the math worked! From that point on, I used my calculator to determine target superheat instead of a tool or chart.

I thought I was on top of my game. Fixed-orifice coils needed to be charged by superheat, and I knew the formula to calculate the target superheat. What else was there to know? 

For starters, the difference between evaporator and total superheat.

Evaporator Superheat is the amount of heat measured above the evaporator's saturated temperature at the evaporator coil outlet. This is an important measurement for understanding the volume of liquid and vapor present in the coil. The evaporator superheat will also indicate whether liquid refrigerant is entering the suction line.

Total Superheat is the amount of heat measured above the evaporator's saturated temperature at the condenser inlet. Total superheat is what most techs measure in the field, while ignoring evaporator superheat. The total superheat and the evaporator superheat should be close to each other. You can use total superheat to diagnose missing or inadequate suction-line insulation. 

Saturation Temperature is the temperature at which both liquid and vapor exist. It is the temperature at which a liquid can become a vapor and the temperature at which a vapor can become a liquid. The amounts of liquid and vapor present at the saturation point depend on the amount of heat added or removed from the refrigerant. Pressure temperature (P/T) charts show the saturated temperatures at different refrigerant pressures. 

All the pieces were in front of me, but I was unaware that I still couldn't see the big picture.

The TXV Curveball

Just when my confidence was at an all-time high, the government threw me a curveball and mandated a new minimum equipment efficiency. Most of the new 13 SEER units required an evaporator coil with a Thermal Expansion Valve (TXV) metering device to meet the minimum efficiency. 

Shortly after, I took another class that focused on charging by subcooling. Around the time I bought my first Testo digital manifold. It made measuring superheat and subcooling so much easier. No more math! 

Just like superheat, I assumed there was only one type of subcooling. I always measured subcooling at a single location, at the liquid line near the condenser discharge. 

Condenser Subcooling is the amount of heat removed from the refrigerant below the condenser's saturated temperature, measured at the condenser discharge.

Total Subcooling is the amount of heat removed from the refrigerant below the condenser's saturated temperature, measured at the evaporator coil inlet. 

You can use total subcooling to confirm liquid refrigerant isn't boiling off before entering the metering device. The goal is to ensure that you have a solid column of liquid at the inlet of the evaporator coil.

What Delta T Doesn't Tell You

While we are on the topic of indoor coils, a common measurement I think way too many technicians hang their hats on is the evaporator temperature change, also called Delta T (ΔT). Looking back, I thought every correctly charged system should have an 18-22 degree Delta T. 

Boy, was I wrong. I learned that many techs share this same misconception.

When I am teaching a class, I ask the students, what is a normal Delta T for an evaporator coil? They yell out a variety of numbers, so I encourage them to keep guessing. I usually surprise them by saying that there isn't a correct number for normal operation.

Evaporator Delta T is a variable that fluctuates with operating conditions. The Delta T is affected by the coil entering dry-bulb temperature, the wet-bulb temperature, airflow, and the outdoor ambient temperature. 

A 20-degree Delta T could be compared to the Easter Bunny or Santa Claus. It is a fabricated tale, like those we tell our children to help them fall asleep at night. Realistically, the normal range is larger than most technicians would think.

The Value in ETD

A metric we don't pay enough attention to is the Evaporator Temperature Difference (ETD). ETD is the difference between the return air temperature and the evaporator's saturated temperature. When you set airflow correctly, you can use ETD to predict evaporator/saturation temperature. 

The ETD can be used to calculate the target suction line temperature for non-invasive testing.

Both the CTOA/CTD and the ETD can be used to determine if a system is operating normally. Outdoor temperature, coil condition, cleanliness, and charge impact the CTOA/CTD. Return temperature, airflow, and charge impact the ETD.

Learning Should Never Stop

A few weeks ago, I read the book "Pressure Enthalpy Without Tears" written by a friend of mine, Eugene Sillberstien. That was the second time that I have read his book. I think I learned more the second time around. 

Enthalpy relates to heat, and there are two types of enthalpies that we should pay attention to in an air conditioning system. 

National Comfort Institute (NCI) teaches technicians to measure the change in enthalpy or Delta H across the evaporator coil, while Eugene's book talks about heat transfer through refrigerant. Both measurements can provide valuable insights into the system's performance.

When I look back at my career and remember all the different things I learned along the way, I realize I am still learning to this day. However, instead of wandering through a string of facts, I have become more purposeful in what I learn, trying to understand why it would be a beneficial topic to learn.

One lesson you can take from all of this is to stop thinking of your HVAC knowledge as a destination or a finish line. The fact is, it's impossible to know everything. Think of this trade like a lifelong marathon: never limit your education or curiosity. 

This cooling season is just one more lap in the race. Once you prioritize your knowledge, each lap becomes easier.