Why Checking Gas Valve Leakage is Hard

This article is an extension of HVAC School’s “How to Measure Gas Pressure” video. Additional input was provided by Jim Bergmann. Bubble test procedures and standards were provided by Honeywell. Links to source materials are available at the end of this article.

If there’s one complaint that HVAC techs should never take lightly, it’s a report that a customer smells gas in their home. Even here in Florida, we receive a fair volume of calls from customers who smell natural gas. The customers may not know what is leaking, but they’re sure they have a leak on their hands. For many of us, our minds will immediately jump to the gas valve. 

However, many gas valves are replaced unnecessarily. Needless valve replacement can waste your time and the customer’s money, so everybody loses. We’re here to help reduce some of that heartache. 


A note about gas leaks

All gas valves leak. You’re not going to get a 100% leak-proof system. The question really isn’t about if there is or isn’t a gas valve leak.

The real question about gas valves is if they leak significantly or not. Some residual gas will always remain in the manifold and slowly dissipate, even when the unit isn’t running. It has nowhere else to go but out. Still, the presence of residual gas alone does not indicate a significant leak.

Even though we recommend using an electronic leak detector in our video, Jim Bergmann advises against it for the manifold (outlet of the valve). Depending on the electronic leak detector’s sensitivity, it may pick up residual gas or minor leaks that do not affect the customer’s air quality or safety. Instead, Bergmann recommends using a bubble test to determine the extent of leakage (not its presence). 

A bubble test measures the gas bubbles that escape from a valve over 10 seconds. Manufacturers have standards for allowable leakage, and a bubble test reveals if the leakage is within normal limits or excessive. Gas valves only need to be replaced if the leakage is above the maximum standard value.

The bubble test has two parts: setting up testing conditions and restoring the system to normal. We will walk you through the steps of each part.

Bubble test: part 1

The first part of the bubble test sets up the testing conditions to produce measurable bubbles. We need to divert the gas flow to a water source. When we redirect the gas, we can physically see the leak if we watch the gas move through a fluid in a different state of matter.

For a visual aid, you may want to consult the diagram below to see the positioning of the valves you will be opening and closing throughout the test:

1. De-energize the control system.

In plainer terms, “shut off the control system.” When you de-energize, you will want to make sure that no power runs from the gas supply source. Ensure that the safety shutoff valve (SSOV) does not have power running to it. You will perform the test just beyond the SSOV.

2. Close the upstream manual gas cock.

The upstream manual gas cock lies between the gas source and the pressure regulating valve (PRV).

3. Make sure the manual test petcock is closed in the leak test tap assembly.

Depending on your gas piping system, this could also be a permanent petcock. Regardless, you will want to ensure that the petcock is closed.

4. Remove the leak test tap plug and connect the testing apparatus to the leak test tap.

The leak test tap plug is on the petcock. Remove it and fasten the testing apparatus. The testing apparatus you’re going to use is ¼” wide flexible tubing attached to ¼” wide aluminum or copper pilot tubing.

5. Close the downstream manual gas cock.

The downstream manual gas cock is located between the petcock and the burner itself. Make sure it is closed.

6. Open the upstream manual gas cock.

Again, the upstream manual gas cock is between the gas source and the PRV. We closed it during Step 2. You are going to want to open it this time.

7. Run the safety shutoff valve (SSOV) to its fully open position. Then, immediately de-energize the system to close the valve.

You will want to make sure the SSOV is fully open, but you don’t want to let it stay open for too long. You have to be quick to shut it down right after you completely open the valve.

8. Immerse your testing apparatus (tube system) ½” deep into a glass of water.

Dipping the testing apparatus in water will allow you to see the gas against a liquid. The tube will act like a straw, and a gas leak will act like a person blowing bubbles out of the straw and into a drink. Make sure the end of the tube that enters the water is cut to a 45-degree angle.

9. Slowly open the test petcock.

Take your time with this one, and don’t be too surprised if you see a short, quick burst of bubbles, even if you open it pretty slowly. The bubbles will stabilize. Remember, you don’t want to test the immediate release of gas. You want to test the constant flow of leaking gas.

10. When the rate of bubbles coming through the water stabilizes, count the number of bubbles that appear during a 10-second interval.

As I said during Step 9, the bubbles may flow irregularly at first, but they will stabilize. When they even out a bit, count the number you see within 10 seconds. The acceptable number of bubbles will vary depending on the gas piping’s size and the manufacturer’s recommendations. The chart below shows the appropriate amount of bubbles for each pipe size with a Honeywell V4730C, V8730C, or V4734C gas valve. It also presents rough conversions (and formulas) to measure leakage in cubic centimeters per hour.

Bubble test: part 2

The second part of the bubble test deals with restoring the system to normal and making sure that there are no leaks at the test tap (not the valve!).

1. Close the upstream manual gas cock.

Once more, this is the valve between the source and the PRV. We closed this valve during Step 2 of the first part of the test and reopened it during Step 6. We are going to close it again.

2. Close the test petcock and remove the testing apparatus. Replace the leak test tap plug.

Remember, the leak test tap is on the petcock. You will want to make sure the petcock is closed and the apparatus is removed before you try to plug the tap again.

3. Open the upstream manual gas cock and energize the safety shutoff valve (SSOV).

Didn’t we just close the upstream manual gas cock? Yes, we did, but we need to reopen it for this step. Now is also the time to re-energize the SSOV.

4. Test with soap bubbles to assure that there is no leak at the test tap.

Liquid soap is a suitable medium for testing gas leakage in a single small area because it’s thick, and the bubbles tend to stick for a while. You’ll want to apply soap bubbles to the test tap to make sure the area you just tested is secure.

5. De-energize the safety shutoff valve (SSOV).

You should have powered the SSOV in Step 3. You will now remove power from the SSOV.

6. Open the downstream manual gas cock.

We have finally reached the downstream manual gas cock again. Remember, this is located between the burner and the test tap location. You will want to open it.

7. Restore the system to normal operation.

This is the end of the test procedure. You are going to restore the system to its usual operating mode. Be sure to double-check the SSOV closure’s tightness, especially if the unit has more than one SSOV.


After repeatedly opening and closing many valves, you have completed the bubble test. There is no way you’re going to spot a gas leak if the gas dissipates into the air, so it’s a good idea to analyze gas movement against a liquid medium. This test method also allows you to apply and sharpen your visual inspection skills.

Even though it may seem simple (or silly), counting bubbles is a more reliable way to detect significant valve leaks than an electronic leak detector. Don’t get us wrong, we still love our electronic leak detectors for finding gas leaks elsewhere, but they can be a bit too sensitive for valve testing. 

Is this process practical for normal field testing? I will leave that up to you and your own company operating procedures to decide. 


For more information on trace leakage and Bubble-O-Meter tests, check out this Honeywell article on the topic (provided by Gray Cooling Man).

For more information on V48 and V88 gas valves, check out Honeywell's PDF on their product data.


3 Bad Techs That Don’t Know It

First, let's state the obvious and clear the air a bit. The photo above is SUPER CHEESY! But this story is about three bad techs who don't know it so three models clearly posing in clean clothes makes as good of a proxy for a bad tech as anything else.

First off, I'm not being negative about the trade or making fun of people, the point of this story is to identify some traits that many of us may exhibit or see in others techs and it can be hard to identify our own issues or issues within your organization. See if any of these techs sound a LITTLE TOO FAMILIAR and maybe we can learn something. Before you ask… No, these are not real people…. probably… maybe

Randy the Drama King

Randy, like most dramatic people who work in the trades, doesn't see himself as being dramatic. He just thinks he is being constantly disrespected by management and co-workers and customers are crazy and the dispatcher is out to get him and it's always about to rain and that ladder (and every ladder) looks unsafe.  These things aren't DRAMA they are FACTS in Randy's world and if you question this reality you get added to the long list of people who are disrespecting him.

Randy starts conversations with customers with phrases like “you aren't going to want to hear this” or “Do you want the good news or the bad news”. He also tends to pass blame to his coworkers or his company because they are just clueless and he knows what's REALLY going on.

Randy is actually a good tech, but he get's in a lot of conflicts with coworkers, customers don't always like his negativity (or as he calls it “being honest”) and he is inefficient and largely unpopular with other techs and management. Randy knows people think poorly of him because everyone is conspiring against him with that blankety-blank dispatcher Donna!

Randy always feels persecuted by the people around him and usually has something negative or conspiratorial to share about every topic. Politics, The weather, customers, co-workers, spouses… you name it.

Here is a test you can take to see if you might be a bit of a Randy

  • You have more than 5 people you are ticked off with or avoiding at any given time
  • You consistently see “danger” around you that nobody else sees
  • During work hours you have multiple conversations over 5 minutes with others about things that are “wrong”
  • You use a lot of negative and fear-inducing language with customers

If you find you are allowing negativity and drama get to you the best practice is to give yourself a break from negative speech. Like your grandma used to say, “If you don't have anything nice to say”. Negativity is a hard habit to break but the best time to start is now and the best antidote for negativity is gratefulness.

Bob the Excuseful  

Yes, excuseful is a word… I made it up and I like it.

Bob is confident that he would be able to do his job if he was just given the proper training and tools and enough time to do the job and enough sleep and wasn't forced to work these ridiculous hours. Bob often wonders if he should go back to school and get his degree in …. something and all the courses he would take if his cheapskate boss would just invest in him.

Sure he was given a book and sent to a seminar last month but that was all THEORY, he is a hands-on learner and he CANNOT learn from books or videos or seminars or anything unless he can get his hands on it.  Once he DOES get his hands on it he can't be held responsible for any mistakes he makes because he has to be SHOWN what to do and how to do it and if he isn't SHOWN how can he be held responsible? Now, when he is shown, he is a hands-on learner so he can't learn things by being shown… he needs to do it himself.

His truck may be a mess but he would clean it if he ever had time with these ridiculous hours but in the slow season that is his one time of the year to relax, you can't expect him to take his own time during the slow season to clean his van can you?

Here are some indications you may be struggling with a bit of Bobish excusefullness

  • You feel jealous when others succeed  and immediately give some reason why they have an advantage over you
  • You read fewer than 5 books last year but still feel like your lack of education is someone else's fault
  • You find yourself using “hands-on learner” as a reason for failing to understand something
  • When you don't understand something you call or text someone rather than looking up an answer yourself
  • You have a sense that your lack of progress is due to a lack of “opportunity”

The best way to stop making excuses is to begin living and working with what old-timers called “grit” or “gumption”. This means doing whatever it takes to solve problems, making excellence a goal and going after it no matter the barriers. Start by reading and learning on your own, don't wait for someone to show you or tell you, go get it yourself.

Todd the Careless

Todd knows he is just forgetful, he TRIES to remember to tie down his ladder and put the caps back on and close his back doors on his van but he just forgets sometimes OK!

Sometimes Todd get's defensive when other techs call him out for leaving the panel off or “forgetting” to clean the drain, but usually Todd just apologizes and says he will do better next time, but he knows he won't because he didn't do it on purpose, it just …… happened.

Some of the “Grouchy” old techs have told him that doesn't seem to care about his job, but they are WRONG! (in Todd's mind) he does care, he just has other things going on in his life and in his mind and sometimes accidents happen… like the time he stepped through the attic ceiling, or the time he slipped on the ladder, and that one time he rear-ended that car in the parking lot… oops

You may be a Todd if ….

  • You regularly make mistakes where you “just forgot”
  • You find yourself looking at your phone, texting and using social media during the workday
  • Your mind is preoccupied with personal matters during work and while driving

We have entered a new era of carelessness due to the advent of smartphones, social media, and texting. Many of us find our minds constantly distracted by things other than work during the work day and it leads to poor outcomes, mistakes and safety hazards. everything from climbing a ladder, to driving, to filling out a service call requires ATTENTION and distraction can lead to costly and dangerous mistakes. The best advice is to put the distractions way during the work day… unless it is reading this article. Just remember to put the panels back on and run test the equipment when you are done.

— Bryan






Impacts of Compression on Temperature

Many in our industry can misunderstand the differences between temperature and heat, although these are related. 

Substances are composed of many moving molecules that change their speed as they release or gain heat. When we heat a glass of water in a microwave, its molecules start to move more quickly. The velocity increase results in a temperature increase. Molecules can have different velocities, but the temperature is the average value of all their speeds.

Two substances have the same temperature if their average molecular speed is equal. If we dip a 50°F glass of water into a 50°F pool, we do not transfer heat between them because there is thermal equilibrium. In other words, the substances have the same temperature. However, the glass of water has a much lower volume than the swimming pool. That means that the glass has much less heat content, even though they have the same temperatures. 

Usually, in HVAC systems, the amount of heat is measured in BTUs, which corresponds to the amount of energy it takes to change 1 pound of water by 1°F. In air conditioners, BTUs measure the quantity of heat that an air conditioning unit can remove from a space per hour. So, if you want to cool down a room where many people work or where equipment heats up, you need to consider this heat when selecting the system in addition to the heat entering through the walls, floor, ceiling windows, etc…Those are heat sources, so they will affect the quantity of heat to remove from the environment. We often call these heat gains, and these gains always occur based on a temperature differential. 

Most air conditioning and refrigeration systems use compression refrigeration to manipulate temperatures and therefore move heat. The refrigeration cycle consists of four stages: compression, condensation, expansion, and evaporation, as shown in Figure 1. All of these stages are essential to accomplish the job of moving heat. However, in this article, the spotlight is the compression stage. That stage is considered the heart of the refrigeration circuit.

Figure 1 – Refrigeration Circuit

There are many different types of compressors used in HVAC systems. Reciprocating, rotary, scroll, screw, and centrifugal compressors are the five standard types technicians see. Regardless of their different sizes and applications, their function is the same: to reduce vapor refrigerant’s volume resulting in a pressure increase. This process makes the molecules get closer to each other, increasing their velocity and raising the vapor temperature. Imagine this process as a room full of ping-pong balls bouncing around, and the balls represent molecules. If the walls of the room start moving in, and the ping-pong balls begin to speed up, bouncing against one another and the room walls. This example illustrates an increase in the average molecular velocity, temperature. The same thing essentially happens inside a compressor. Referring to physics, Boyle’s and Gay Lussacs’s Laws explain the effect of compression on temperature. 

In the refrigeration cycle, the refrigerant vapor flows through the suction line from the evaporator to the compressor. At this point, the vapor temperature is often about 55°F, and its pressure is low. When it enters the compressor, the vapor’s volume rapidly decreases. The piston, powered by an electrical motor, compresses the vapor within the cylinder. The reduction in volume results in an increase in pressure and temperature. In this process, similar to our room full of ping-pong balls, the velocity of refrigerant molecules increases, which results in a high-temperature rise, usually from around 55°F to about 165°F in typical comfort cooling systems under typical operating conditions. There is an additional heat gain due to the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. That heat gain occurs because the refrigerant cools the compressor down when it flows over it.

It’s essential to keep the compressor lubricated and ensure that the refrigerant flowing in the suction line is completely vapor to have a stable operation. Liquid refrigerant can damage a compressor quickly. Also, if a liquid refrigerant dilutes the oil in the compressor crankcase and creates foam, it can significantly reduce the compressor’s lifespan. We call this problem bearing washout or “flooding.”

To better understand vapor compression in HVAC systems, it’s also important to look at the Pressure vs. Enthalpy diagram (P-H) of the refrigerant. Figure 2 shows an example of this diagram.

Figure 2 – Pressure vs. Enthalpy Graph.

In this diagram, the y-axis shows the pressure, which is non-linear, and the x-axis shows the enthalpy. The bottom part of the graph is low pressures, while the top is high pressures, and from left to right, the enthalpy increases. Enthalpy is the total heat content of a system, represented here in BTUs per lb. Thinking of the compressor, when it increases the vapor temperature, it also increases the system’s enthalpy because the heat content in the refrigerant now is much higher than before. From Figure 2, it’s possible to notice that when the vapor refrigerant is compressed, its enthalpy increases. Another example of enthalpy is when we boil water on the stove. In this case, the water molecules gain heat from the stovetop, so the energy (heat content) in this system increases, which results in a boost in enthalpy.

You can also see the effects of vapor compression on temperature in the Temperature vs. Entropy (T-S) diagram presented in Figure 3. Entropy is how much energy has flowed from being localized to becoming more widely spread out. It also measures the level of order in a system. For example, ice is in a more ordered state because the atoms are locked in an ordered form. On the other hand, liquid water is more disordered because the molecules have spread out. Therefore, when we put an ice cube into a glass of water at ambient temperature, the disorder level increases, so the entropy also increases. When you melt ice, dissolve salt or sugar, or boil water in your kitchen, you increase entropy. The molecules spread out in all those processes. However, compressing a vapor is an isentropic process, which means constant entropy. Since the compressor does not allow for any heat exchange with the surroundings, the entropy level is unaffected.

Figure 3 – Temperature vs. Entropy (T-S) 

After being compressed and reaching a high-temperature level, the refrigerant now flows through the discharge line towards the condenser. In this stage, the superheat will drop off until the refrigerant reaches its saturation temperature. The refrigerant then condenses, subcools, expands, and evaporates. This refrigeration cycle continues until the system turns off.

Therefore, it’s essential to understand basic thermodynamic concepts such as heat, temperature, pressure, and the impacts of vapor compression on temperature. These concepts are fundamental when working with HVAC. 




[1] “Refrigerant Compression and Temperature,” HVAC School, 22 July 2019. [Online]. Available: https://www.youtube.com/watch?v=Y2ex2OxIXT0.
[2] “The Basic Refrigeration Circuit, Pressure & Enthalpy w/ Carter Stanfield,” HVAC School, 04 September 2017. [Online]. Available: https://www.youtube.com/watch?v=siV5xUPTRas.
[3] “Pressure / Enthalpy Diagram Example,” HVAC School, 13 December 2018. [Online]. Available: https://www.hvacrschool.com/pressure-enthalpy-diagram-example/.
[4] “What is Temperature?,” HVAC School, 28 February 2019. [Online]. Available: https://www.youtube.com/watch?v=RDIIpkVH_Jc.
[5] “HVAC/R Refrigerant Cycle Basics,” HVAC School, 01 March 2019. [Online]. Available: https://www.hvacrschool.com/hvacr-refrigerant-cycle-basics/.
[6] “Air Conditioning Compressor Basics,” HVAC School, 20 February 2019. [Online]. Available: https://www.youtube.com/watch?v=0lfa9rm8_x8&pbjreload=101.

Refrigeration Without Refrigerant

We just wrote about rejecting heat to the atmosphere via radiant cooling. That’s one example of cooling without refrigerants, but there are quite a few others out there. 

In this article, we’ll look at some other cooling methods that don’t use refrigerants.

Vortex tubes

Vortex tubes swirl gas in a chamber, separating it into hot and cold streams. 

After hot gas gets deposited into the vortex tube at an angle, it spins along the tube’s sides and travels up it in a wide spiral. At the end of the tube, the hot air outlet is interrupted by a conical nozzle. This nozzle limits the hot gas that passes through, and it sends the cooler gas backward through a countercurrent in the center of the tube. The countercurrent deposits the cooler gas out of the other end, and that’s where cold air flows out. No refrigerants or moving parts are used in this process.

Georges Ranque invented the vortex tube in 1931, but it didn’t become popular until a German physicist named Rudolf Hilsch published a paper on it in 1947. An engineer named Charles Darby Fulton acquired patents to develop the vortex tube from 1952 to 1963. In 1961, he founded Fulton Cryogenics and began manufacturing the vortex tube. Fulton Cryogenics became Vortec Corporation in 1968 and focused almost exclusively on the development and manufacturing of vortex tubes. Vortec continues to exist and produce vortex tubes today.

Vortex tubes have also been repurposed for a few different separation purposes. For example, English physicist Paul Dirac found out that it can separate isotopes and gas mixtures. (This process is called Helikon vortex separation and isn’t relevant to HVAC, but this example shows that vortex tubes have uses beyond our field.)


What do we use vortex tubes for?

It would be great if vortex tubes could cool houses, but they’re far too small to cool an entire building. I spoke with Ellen Chittester at Vortec, and she told me a little bit more about vortex tube applications, benefits, and limitations.

She said that vortex tubes are less efficient than typical compression-refrigeration A/C units, but they have a unique set of benefits. For example, vortex tubes don’t rely on ambient air temperature, and they can cool effectively in ambient conditions up to 200° Fahrenheit. Vortex tubes are also inexpensive to purchase, easy to install, have a small footprint, and are easy to maintain.

Vortex tubes are impractical to use for applications that require more than 5,000 BTUs. However, they can cool small and enclosed spaces, such as cabinets. The most common applications for vortex tubes are sensor and product testing, CNC machine controls, gas sampling, injection molding, and saw blade cooling. 

Many of the applications I just listed are for “spot cooling,” which entails cooling over a small area and may require some degree of portability. That’s why vortex tubes are perfect for cooling machine controls (such as for 3D printers). Vortex tubes are also small enough to be effective in a personal air conditioning system, such as a diffuse-air vest. Vortex tube technology is incorporated into vests that you wear, and cold air diffuses through the vest to cool your body.

When it comes to technological advances, the vortex tube has pretty much reached its full potential as an individual unit. However, it still has the potential to improve lots of other new technologies. It will always provide easy, efficient, and controlled cooling.



Turboexpanders are essentially centrifugal or axial turbines. They rely on high-pressure gas expansion to power a generator or compressor. Turboexpanders can extract liquid from natural gas, generate power, or work with a compressor and electric motor in a refrigeration system.

A vortex tube is a simple form of a turboexpander. They don’t have the rotors that drive many turboexpanders, but the gas movement and energy conservation obey the same rules. Compressed gas enters the vortex tube at an angle, which drives the gas movement without any mechanical help.

Thermoelectric cooling

Apart from vortex tubes, thermoelectric cooling is another means of refrigeration without refrigerant. 

You’re already familiar with thermocouples. These are devices that form an electrical junction between two different types of metal. They generate a small voltage from temperature differences between the metals.

Thermoelectric cooling occurs when heat is removed at a junction between two dissimilar metals. The interaction between metals of two different temperatures is called the thermoelectric effect, and it has a few different extensions.

I’ll touch on the historical significance of the Seebeck effect and the Peltier effect, as they are most relevant to the topic of refrigeration without refrigerant. 

The Seebeck effect is the earliest extension of the thermoelectric effect, discovered by Italian physicist Alessandro Volta in 1794. However, it was named after the person who rediscovered it in 1821, German physicist Thomas Johann Seebeck. The Seebeck effect merely acknowledged the buildup of an electric potential across a temperature gradient. When two metals of different temperatures get connected at a junction, the temperature difference can generate some electrical energy.

We’re mostly interested in a later discovery, the Peltier effect. The Peltier effect is an extension of the Seebeck effect. 

Discovered by French physicist Jean Charles Peltier in 1834, the Peltier effect describes the heating or cooling at a junction of two metals at different temperatures. Instead of merely describing the electric potential, the Peltier effect describes the heat transfer between two different metals of varying temperatures. 

When two metals have different temperatures, one must evolve heat and the other must absorb heat. When this happens, heating or cooling occurs at the electrical junction that connects the heat sources. For this reason, the Peltier effect can be used for refrigeration and heat pumps. Refrigeration is the more common use, so we are going to focus on those applications.


What do we use Peltier cooling for?

Imagine using a thermocouple that generates enough electricity to cool PC towers or lab incubators. That’s essentially what the Peltier effect does.

Like vortex tubes, Peltier cooling is limited to smaller applications. One of its most common uses, as I said, is to regulate temperatures in lab incubators. These are the containers that store lab cultures at controlled temperatures. For example, if certain fungi must grow at a constant temperature, Peltier systems can provide constant conditions for that.

Peltier cooling is not practical for cooling large areas.  In the case of lab incubators, Peltier systems have difficulty maintaining temperatures below 50° Fahrenheit (10° Celsius). Despite that, Peltier coolers can dip well below sweltering ambient temperatures. Many portable camping or car coolers use Peltier cooling for that reason.


Vortex tubes and Peltier cooling rely on natural physics to cool small applications and aren’t affected by high ambient temperatures. Still, they have their practical and efficiency limitations, so they likely won’t ever be used to cool entire buildings. Despite that, they will continue to be useful for cooling small enclosures and overheat-prone technology.

What is Net Refrigeration Effect (NRE)?

Net refrigeration effect (NRE) is the quantity of heat that each pound of the refrigerant absorbs in the refrigerated space to produce useful cooling.

That’s a pretty vague definition. We know that it’s an amount of heat in processes that take place within the evaporator. Still, the phrase “useful cooling” seems rather broad. Even though it may seem a bit undefined right now, “useful cooling” is the key to understanding what NRE is and how it applies to HVAC techs in their everyday operations.


What is “useful cooling,” anyway?

Useful cooling occurs in the evaporator when the refrigerant absorbs heat from the conditioned environment, cooling the space. The total quantity of heat absorbed in the evaporator is the NRE. In essence, the NRE is all the difference between the total energy absorbed in the evaporator and the cooling that occurred anywhere outside of the evaporator (in BTUs/lb). 

When a refrigerant evaporates or condenses, it undergoes a phase change. When it undergoes a phase change from liquid to vapor or vice versa, the temperature stays the same. All of the heat added or removed (in BTUs/lb) contributes to the phase change. The heat energy required to complete the phase change is called the latent heat of vaporization.

In almost all cases, the condensation temperature is higher than the evaporation temperature. That’s because the vapor’s temperature rapidly increases in the compressor before going to the condenser where the heat is rejected to another medium, usually outdoors. 

Some heat must be removed to reduce the refrigerant from the condensing temperature to the evaporation temperature. However, that heat removal occurs in the liquid line and the end of the condenser and does not immediately contribute to refrigeration. We must drop the liquid line temperature to achieve subcooling and limit flash gas in the evaporator inlet. However, the refrigerant doesn’t absorb heat from the conditioned environment in the liquid line, so this process is not useful cooling.  


Enthalpy and NRE

Enthalpy is the total internal energy contained in the refrigerant, including sensible and latent energy.

Changes in enthalpy correspond with the phases of the refrigeration cycle. Enthalpy significantly increases in the evaporator. This occurs because the refrigerant absorbs heat in the evaporator. 

On a pressure-enthalpy diagram of the ideal refrigeration cycle, you’ll notice that the bottom edge of the figure will have a horizontal line. This line represents the evaporation phase. The pressure remains low and constant while the enthalpy rises. The refrigerant’s enthalpy at the end of evaporation is significantly higher than its enthalpy when it entered the evaporator.


How do we determine NRE?

To find the NRE, you subtract the enthalpy of the liquid entering the evaporator (He) from the vapor leaving the evaporator (Hl). All units in the equation will be BTUs/lb.

NRE = Hl – He 

Enthalpy heavily increases during the evaporation phase, so you will have a positive answer. Your answer will tell you how many BTUs per pound are being used to absorb external heat. Thus, the NRE is a system’s ability to cool its environment by extracting heat from it.


Why should HVAC/R techs care about NRE?

A unit’s NRE is its measure of cooling performance in both the HVAC and refrigeration industries. The NRE of an A/C unit will give you an idea of its comfort cooling capabilities. The same idea applies to product cooling in the refrigeration industry. 

How can we improve a cooling system’s NRE?

You can improve a system’s NRE by facilitating greater subcooling in the liquid line. 

Flash gas is reduced at the evaporator inlet when the subcooled refrigerant enters the evaporator at a lower temperature. Flash gas is the percentage of refrigerant that immediately boils upon entering the evaporator. Boiling is necessary for cooling to occur, and some flashing is required to drop the temperature from the liquid line to the evaporator inlet. Too much flash gas in the liquid line hinders the cooling process in the evaporator coil, so you can improve NRE by limiting flash gas at the evaporator inlet and liquid line.

Liquid line/suction line heat exchangers and mechanical subcoolers help promote subcooling when inserted into an HVAC system. You can also reduce flash gas in the evaporator inlet by keeping the liquid line insulated and inspecting it for kinks or inappropriate length.

When you optimize your NRE, you aim to get the most out of your evaporator. A lot of this comes down to keeping your system well-maintained and looking after the suction and liquid lines that surround the evaporator. Another thing you can do to optimize your NRE is keeping your evaporator outlet superheat as low as safely possible. A lower superheat indicates that the saturated refrigerant feeds more of the evaporator coil. Coils that are better-fed with saturated refrigerant are more efficient. 

Superheat can also indicate problems with your metering device at the evaporator inlet or the evaporator load. When you safely limit your superheat by checking and fixing all the causes of high superheat, you can make your evaporator more efficient and increase the system’s NRE. Just don’t overdo the low superheat… You can easily lose control of the superheat and run liquid into the compressor, resulting in flooding or even slugging.


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