Lockout/Tagout Basics

DISCLAIMER: HVAC School is NOT an official OSHA safety training resource! Although we provide safety tips in good faith, our website is not a substitute for safety training from an authorized OSHA training source.


Locking out and tagging equipment is one of the most basic safety procedures in general industry and maintenance work, especially in larger buildings. The HVAC and refrigeration industries have lots of equipment with moving parts, electrical circuits, and other hazards. These machines can severely injure or kill workers if someone energizes them while workers operate on them. 

Some technicians may think that the lockout/tag-out procedure is overkill, but it can save many lives. On the employers’ side of things, OSHA can also slap heavy fines on organizations that lack proper lockout/tagout programs. In fact, according to OSHA’s official data and statistics, lockout/tag-out violations were the 4th most common source of citations in the 2019 fiscal year.

Lockout/tagout, also known as LOTO, is a set of procedures that either forbids the equipment from being energized without authorization or warns other people that someone is working on the system. If used correctly, these methods may be the difference between life and death.


Lockout vs. tagout

Although lockout and tagout are very similar and fall under the same safety umbrella, there is a critical distinction between them. 

Lockout occurs when a worker completely locks a machine out of its power source. Unless someone breaks the lock that the worker has applied, the machine cannot be energized while it is locked out. In most cases, lockout devices may only be removed by the person who applied the lockout device. That is because these devices usually require a key, though a person may break them through extraordinary force in rare cases. (You would need heavy-duty bolt cutters and an immense amount of pressure to break these locks. It’s not like any ordinary cutting tool can break a lockout device.)

Many lockout kits have a hasp with several holes. These hasps allow multiple employees to lock a piece of equipment. This feature comes in handy when several people must repair or maintain a machine. Nobody can energize the equipment until all workers have removed their locks. A worker who continues working after everyone leaves will not be in danger because the machine cannot be energized until he removes his lockout device.

Tagout occurs when a worker fastens a set of tags near the power source. The purpose of tagout is to warn other workers that somebody is working on the system. It exists to deter other workers from powering the machine on, but it doesn’t physically stop them. The tagout method doesn’t lock power out and relies on other workers’ observational skills (and good intentions). It can only offer similar protection to a lockout program if everyone complies with the tagout program’s safety standards. As such, it is not as ideal as the lockout method. Tagout materials must also have text with clear instructions, such as “Do Not Start” or “Do Not Open.”

If a lockout isn’t possible, it’s still better to use the tagout method than nothing at all. OSHA recognizes the lockout method’s superiority, but it also requires employers to set forth a tagout procedure when locking out is infeasible.


LOTO procedure

According to OSHA, the LOTO procedure has six main steps, regardless if you use the lockout or tagout method:

1. Prepare for shutdown.

2. Shut down the system.

3. Disconnect or isolate the machine from the energy source(s).

4. Apply the lockout or tagout device(s) to the energy-isolating device(s).

5. Release, restrain, or otherwise render safe all potentially hazardous stored or residual energy. If a possibility exists for reaccumulation of hazardous energy, regularly verify during the service and maintenance that such energy has not reaccumulated to hazardous levels.

6. Verify the isolation and de-energization of the machine.


LOTO programs and products

OSHA requires employers to have a LOTO program in place. One of the main components of a LOTO program is proper training. Before any employees conduct work on equipment that must be isolated from its energy source, they need to undergo lockout/tagout training. Employers are also required to issue retraining whenever they modify their LOTO program, and employees must review the company’s LOTO programs at least annually.

You can also purchase LOTO kits with lockout materials (locks, keys, and hasps), writable and waterproof tagout devices, and non-reusable tagout device attachments (such as nylon zip-ties). These can be a bit expensive, but you’ll certainly be a lot safer if you carry one on your truck and use them in compliance with your company’s LOTO program.


Maybe you’re just an employee who wants to walk away from a job site with all your limbs intact. Perhaps you’re an employer who doesn’t want OSHA to slap you with a fine. Regardless of your involvement in the industry, lockout/tagout is a basic but effective safety measure that you should know. Its goal—and OSHA’s mission as a whole—is to protect human life on the job. 

If you would like to learn more about OSHA’s standards or your rights as a worker, please visit OSHA’s website. Some of the links below are OSHA resources specific to lockout/tagout.

Link to OSHA’s Control of Hazardous Energy Lockout/Tagout publication

Link to OSHA’s overview of lockout/tagout and directory resources

Prevent Refrigerant Migration

Refrigerant migration is a natural process that occurs during the off-cycle. The refrigerants have an affinity for oil and seek out the lowest-pressure areas, so it only makes sense that some refrigerant would be drawn to the compressor crankcase and may condense there. When the refrigerants condense, they saturate the oil.

As a result, refrigerant migration is also the culprit behind slugging and flood back. These can both be fatal to your compressor. Even if you don’t get the worst possible outcomes when your refrigerant migrates, the migration will cause you to lose some oil. When the liquid refrigerant boils on startup, the reaction may rattle the compressor quite violently, which leads it to eject some of the oil from the compressor. When you lose oil, the compressor can’t lubricate all of its moving parts effectively.

This article will go over some strategies to help you prevent refrigerant migration from wreaking havoc on compressors in the off cycle. 


Crankcase heaters

Many of us are probably already familiar with crankcase heaters. They do exactly what their name suggests: they heat the crankcase.

When the system is off, the crankcase gets cold, which may make the migrated refrigerant condense back to its liquid state. The condensation is especially prevalent and problematic in colder ambient conditions. I’m not saying that crankcase heaters aren’t important in Florida (because we still use them here), but they’re a lot less critical here than a heat pump in Massachusetts. 

Crankcase heaters use a simple electrical circuit to supply extra heat to the crankcase during the off-cycle. The added heat prevents the refrigerant from liquefying and causing wear or putting the compressor at risk of slugging.

Crankcase heaters come in several shapes and sizes. You’ll probably see belly band crankcase heaters most frequently, which look like belts that fit around the compressor exterior. There are also insertion-type crankcase heaters that go inside the crankcase. We also used to use the compressor windings to heat the crankcase, but you won’t see these very often nowadays.

Crankcase heaters are good for preventing the refrigerant from migrating to the compressor and condensing inside it, but it isn’t a comprehensive solution. It doesn’t stop the refrigerant from migrating into the evaporator coil or flooded starts.


Liquid line solenoid valve

In essence, the liquid line solenoid valve closes off the liquid line. It prevents the refrigerant in the condenser from migrating to the evaporator coil and compressor during the off-cycle.

The liquid line solenoid valve remains open while the system is running, and it closes when the system is off. That way, it stops refrigerant from entering the evaporator coil during the off-cycle, but it doesn’t impede normal operation.

Like the crankcase heater, they work well for their isolated section of the system, which is the liquid line in this case. Liquid line solenoids don’t do anything to prevent condensation of refrigerant that’s already in the compressor. If you want to use a crankcase heater with a liquid line solenoid valve, check the manufacturer’s literature to make sure it’s advised.


Pump down solenoid system

A pump-down system operates quite similarly to a liquid line solenoid valve, and it’s easy to confuse the two. Like the liquid line solenoid, a pump-down system stops refrigerant from migrating to the evaporator coil.

The pump-down solenoid system causes the liquid line solenoid to de-energize, but the compressor keeps running. That way, the system cycles off on a low-pressure switch. You can essentially cycle out any remaining liquid refrigerant in the evaporator and store it in the condenser coil.

In some cases, you may also benefit from using a crankcase heater. Before you make that call, it’s best to refer to the manufacturer’s specifications to see what they recommend for the model you’re working on.  

You’ll have to prevent short-cycling whenever you use pump-down solenoids. To do this, you can install a pump-out control, which allows the pump down to occur once without repeating an endless loop of short-cycling.


Hard shutoff TXV

Also called non-bleed TXVs, hard shutoff thermostatic expansion valves work similarly to the liquid line solenoid. These hard shutoff TXVs close whenever the compressor shuts off,  to keep the majority of the refrigerant in the condenser coil during the off-cycle.

Depending on the system, you may need a hard start kit to help the compressor restart at times. Hard shutoff TXV valves work to confine the liquid refrigerant to the system’s condenser side, which forces the compressor to start with an unbalanced load. Of course, it will depend on the system, but that’s something you should be aware of and look out for when you use a hard shutoff TXV to prevent refrigerant migration.

To learn more about hard shutoff TXVs, check out this article and this video.


As always, read your manufacturer guidelines before implementing these strategies. They’re there to guide you through the ins and outs of their systems so that you can make an informed decision about which control strategies to use.

Zonal Pressure Diagnostics—The Backtalk Series (Pt. 1)

This article was submitted by Genry Garcia of Comfort Dynamics, Inc. It is the first part of his series on zonal pressure diagnostics. Thanks, Genry!


Zonal pressure diagnostics have been around for a while. Here is a brief description of ZPD from the website redcalc.com

Formerly known as the “Blasnik methods” in a nod to their main inventor, “Zone pressure diagnostics (ZPD) has become an established tool for diagnosing indirect air leakage paths in houses since its introduction to the U.S. weatherization community around 1990. In cases where air must pass through at least two barriers to leak into or out of a house, ZPD is a way to use measured pressures to infer the location and size of air leakage paths.”

There is also this description from the operation manual of an older blower door from The Energy Conservatory: 

“Diagnostic procedures have been developed over the past decade to analyze series leakage. These procedures, called zone pressure diagnostics (ZPD), are widely used by weatherization professionals to prioritize air sealing efforts in houses by estimating the amount of air leakage from attached zones (e.g., attics, crawlspaces, garages, and basements). ZPD techniques typically combine Blower Door airtightness test results with zone pressure measurements made both before and after an opening or hole has been added to one surface of the zone being tested.”

So, with that, we can deduce that a good place to start with ZPD is:

1. To measure the connection that exists between outdoors and the attached or intermediate zones. 

2. Determine the amount of air leakage between the house and the attached or intermediate zones that are not part of the conditioned space. 

3. And determine the portion of the whole house leakage that can be attributed to an attached or intermediate zone. 

As mentioned above, these zones could be a vented attic, a garage, an unfinished basement, or a crawl space. These are zones that are structurally connected to the house but are often open to the outside through intended ventilation paths, intermediate between the house and the outdoors. In some climates, zones like garages may not have any ventilation. Even in these cases, ZPD can still be useful. 

In addition to possibly leaky windows and exterior doors in a bedroom (for example), when we measure how much this bedroom is connected to outside, we essentially measure how much is the attic/crawlspace/garage connected to that bedroom.  


How we are misrepresenting ZPD

We are, by and large, misusing ZPD to estimate how leaky a room can be. When we use a blower door, the pressure in the envelope can be driven and maintained at an industry-standard -50 Pascals (Pa). That is a safe pressure differential between indoors and outside that we can use to cancel out the effects of wind and stack effect on a structure.   

Once the envelope pressure is maintained at -50 Pa, the practice is to isolate a given room inside the conditioned space, generally through the use of a door. At this point, with the ‘Reference or –’ port left open to factor the house pressure, a tube connected to the ‘Input or +’ port on a precision manometer is slid under the door. After that, we can read the pressure of said room WRT to the rest of the house. We know that the pressure in the main body of the space is -50 Pa. If we read a room pressure of 10 Pa, then it is said that this room is 20% connected to outside (10/50=20%). It makes sense, right? Not quite.

It would make sense if the room did not have any ductwork—or any other penetrations, for that matter—connecting it to the rest of the house. That’s only true when the only connection between the room and the rest of the space is the door under which we slid the tubing. We are accounting for the only path through which pressure from the room can travel to the rest of the house en route to the blower door fan.

However, rooms have ducts. Some have supply ducts only; others have both supply and return air paths. Once you isolate the room by closing the door, its pressure differential induced draft is also traveling into the main body of the envelope through said ducts on its way to the blower door fan. 

Let’s take, for example, two leak-free, identical 10’x10’x9’ rooms. They’re next to each other, but one is a corner bedroom with two exterior walls. Let’s call that one “room A.” Let’s say it has a window on each wall. That’s one more wall and one more window than its otherwise adjacent sibling, which is a middle room. We’ll call it “room B.” Invariably, this will mean that the corner room has a greater need for cooling capacity. As such, its design air volume is, let’s say, 150 CFM. The room with the same dimensions right next to it only needs 80 CFM since it has less heat gain. 

That may result in the corner bedroom having a larger diameter duct to feed the conditioned air to it. If, say, room A had an 8” round duct and room B a 6” round duct, then A would have around 50 square inches of duct area instead of the 28 square inches room B would have. These rooms will also need returns. If the returns are sized accordingly, room A will end up with something like 78 square inches of return duct area. Room B will only have 50 square inches.

That is a total of 128 square inches for room A and 78 square inches for room B. 

If we open a hole of the same diameter from each room to outside and proceed to perform ZPD using the procedure outlined above, we would invariably find that room A would show a lesser pressure WRT to the rest of the house than room B. How could that be? We have two rooms of identical volume and equivalent leakage area, so how could that happen?

Hypothetically speaking, if, with the exception of the door, these rooms were completely isolated from each other and the rest of the house (and neither room had any ducts), then both of their pressures WRT to the house would be identical during ZPD. But because room A has 40% more duct area connecting it to the main body of the envelope than B, B’s pressure will be higher than the pressure in A, even though they both have the same floor area, volume, and equivalent leakage area to outside. In other words, we would mistakenly conclude that B is leakier than A when, in reality, the leakage is exactly the same. The only difference is that A has a larger path for pressure equalization between it and the rest of the house. 

Bigger hole (A), lower pressure (A). Smaller hole (B), higher pressure (B). 

Here is an example of the point being made. This example shows one of many homes I’ve tested where I see this trend over and over again. These first two photos are of ZPD readings taken in an isolated room with both a supply and a return vent. With the blower door running, keeping the house at -50 Pa:

These two next ones are of the very same house. I took these pictures a couple of minutes later, which is all it took to cover the vents with a duct mask:

The pressure reading more than tripled! If we had accepted the first reading at its face value, we would have grossly underplayed the large amount of attic air leakage coming in through the poor drywall finish around the bath exhaust can. Heck, some might’ve even missed it altogether. The original ZPD reading was less than 10%, so it would be hard to fault a tester for shrugging it off.   

Can you see where the process is flawed?

“But what if the ducts are inside?!”

That's a very popular argument. The answer is that there is no difference.

Think of a room with a properly sized return duct. If the HVAC system is running and said room is isolated by closing the door to it, the air pressure being created in the room by the supply air feed is also being simultaneously relieved by the return air path. That allows the air to travel back to the HVAC fan through a return duct or through a central return. That happens because of the pressure differential between the isolated room and the adjacent space. The pressure differential is being driven by the indoor motor of the HVAC unit. Following so far? Read it again.

Now, substitute the HVAC unit for the blower door as the pressure differential driving force. Close the same door to the same isolated room. Under those conditions, the forces pressurizing the room are the envelope leaks between the room and the adjacent zones (attic/crawlspace/garage) and any direct-to-outside leaks through window/door gaskets, etc. Now the duct(s) in this room act as an air pressure relief path between the room and the adjacent space, the main body of the house, where the blower door fan is. It works just like a return air duct.  

For ZPD purposes, it does not matter if the ducts are inside or outside the envelope. There is still a pressure relief path between the isolated room and the house. It will relieve the pressure in the room if the duct runs through a finished basement, a completely sealed soffit, or exposed inside the envelope. It will relieve the pressure the exact same way it would if it were to run through an unconditioned attic. 

The only difference would be that if there is any duct leakage, the duct running through a vented attic will add to the total CFM50 of the envelope. If it were to run inside the conditioned space, this duct leakage would not have any effect on the total leakage of the envelope.


In the next part of this series, we will explore other methods that take a different approach to ZPD. Stay tuned!


—Genry Garcia

Comfort Dynamics, Inc.


Boyle’s Law

“Robert Boyle” by Stifts- och landsbiblioteket i Skara is licensed under CC BY 2.0

Robert Boyle was an Anglo-Irish philosopher and scientist born in 1627. He had a privileged upbringing, as he was a son of the first earl of Cork, and his social standing gave him access to a first-class education at Eton College. He studied abroad with his brother as a teenager, traveling all over Europe and learning from his experiences. When the Irish rebellion broke out in the early 1640s, Boyle stayed in Vienna to continue his studies away from the political turmoil.

Instead of returning to Ireland, he started his career in England. There, Boyle wrote about ethics and authored some devotional works informed by his Christian faith. 

Boyle eventually returned to Ireland and began studying natural philosophy in the 1650s. He joined a few other notable physicists and philosophers and formed the “Experimental Philosophy Club.” Shortly after that, he partnered with Robert Hooke to study pneumatics in 1659, and most of his contributions to society came from his studies of air. 

Boyle’s most profound discovery was derived from a report published in 1662. Boyle measured the volume of constant quantities of air compressed by different mercury weights, and he discovered an inverse relationship between the pressure and volume of a gas. This relationship later became known as Boyle’s law.


The law

Boyle’s law states that the volume of a gas varies inversely with the absolute pressure, provided the temperature stays the same. To put that in simpler terms, if you increase the pressure, the volume should decrease. The opposite is also true. 

You can use the following equation to represent Boyle’s law:

P1 × V1 = P2 × V2

In short, the product of the original absolute pressure and volume equals the product of the new pressure and volume. This relationship indicates that a pressure increase will cause the volume to decrease and vice versa.

“Boyle's Law, Volume, and Pressure Inverse Relationship” by myers_pictures is licensed under CC BY-NC-SA 2.0 

Let’s say you have an initial absolute pressure of 50 PSIA and an absolute volume of 64 in3. The new pressure is lower: 32 PSIA. You can find the new volume by multiplying 50 PSIA by 64 in3 (3200) and then dividing that product by 32.

The volume would be 100 in3, which is higher than the initial absolute volume (64 in3). Even though the new pressure is lower than the original pressure, the new volume is higher than the initial volume. That’s the core of Boyle’s law.


When might we use Boyle’s Law in HVAC/R?

When we think about pressure and volume changes, our minds may jump to the compressor. After all, the compressor’s primary function is to apply pressure to reduce the refrigerant’s volume.

However, the gaseous refrigerant will not obey Boyle’s law alone in the compressor because the temperature doesn’t remain constant. The broad idea of Boyle’s law remains true (volume decreases as pressure increases), but the temperature change throws off the exact proportionality. 

As you can see, Boyle’s law is impractical for compression as we know it because it requires a constant temperature. However, Boyle’s law works with Charles’s law (or Gay-Lussac’s law) to describe a typical HVAC/R compressor’s function. 

Charles’s law establishes an ideal gas’s absolute temperature proportionality to its volume under constant pressure. Charles’s law gives Boyle’s law a more practical basis because it defines temperature’s relationship with the other variables: volume and pressure. They can combine to form the general gas law or law of a perfect gas.

So, although we don’t see Boyle’s law alone in HVAC/R, we see it at work with Charles’s law in a greater equation: the general gas law. Boyle’s law is a mere component that establishes the proportional relationships between volume, pressure, and temperature.





Special thanks

Modern Refrigeration and Air Conditioning, 21st edition (Andrew Althouse, Carl Turnquist, Alfred Bracciano, Daniel Bracciano, and Gloria Bracciano)

Refrigeration & Air Conditioning Technology, 9th edition (Eugene Silberstein, Jason Obrzut, John Tomczyk, Bill Whitman, and Bill Johnson)

These truly are the reference guides for the industry and deserve attribution for all such articles.

Third Law of Thermodynamics

When someone says there is no such thing as “cold” only the absence of heat you can point out that while cold is the absence of heat… absolute zero is the definition of COLD.

The third law of thermodynamics addresses the absence of heat and what that means for entropy. It implies that absolute zero (0 kelvin, about -460° F) is theoretically possible but impossible to achieve naturally. The law officially states that a perfect crystal’s entropy is zero when a crystal’s temperature is equal to absolute zero.

We cannot reach absolute zero because of the principles in all the other laws of thermodynamics: heat naturally moves towards colder temperatures. Even though scientists have created laboratory conditions where they’ve cooled metal molecules within nanokelvins (10-9) of absolute zero, they cannot reach absolute zero because heat from the environment still tries to transfer to the metal molecules. As such, the heat results in some entropy, which leads to the formation of imperfect crystals.

A perfect crystal is an object where all the atoms are identical and positioned symmetrically. Any form of entropy indicates thermal motion, which causes those atoms to bounce around and leads to imperfections.


Absolute zero

Like reversible processes under the second law of thermodynamics, absolute zero is possible only in theory. Nevertheless, it is still a crucial benchmark that we use when measuring real temperatures and processes in our world. 

As said earlier, absolute zero is equal to 0 kelvin. Kelvin is the only temperature scale with perfectly proportional values. (For example, 25° F is NOT half the heat of 50° F, but 25 kelvin is half of 50 kelvin.) 

Having a scale with absolute zero means that there aren’t any negative numbers that may complicate the heat content. The Celsius scale has negative numbers because it indicates degrees below the freezing point of water; there is still heat energy at -30° C, but it is not enough to melt solid ice under standard atmospheric pressure conditions. There is NO heat energy at absolute zero, and negative numbers are impossible because you cannot have less than no heat.

The coldest natural spot in our universe (that we know of) is the Boomerang Nebula, which has a temperature of 1 kelvin. It is still above absolute zero.

“Boomerang Nebula (NASA, Hubble, 09/13/05)” by NASA's Marshall Space Flight Center is licensed under CC BY-NC 2.0

To reach absolute zero (by natural or lab-created means), we would have to break all the other laws of thermodynamics. The heat removal required to reach absolute zero is infinite, and you can’t have a heat sink at absolute zero. A heat sink at absolute zero would indicate 100% efficiency, which is theoretically possible but naturally impossible.

This means that since heat moves from hotter to cooler and we can't get anything cooler than absolute zero it's essentially impossible to get to absolute zero. 

We clearly don’t see absolute zero in our work or daily lives. However, engineers who design the parts for the HVAC/R products tend to use absolute temperature scales, like Kelvin or the Fahrenheit-equivalent Rankine scale.

Nevertheless, the third law of thermodynamics’ core implication is that all matter contains heat energy and entropy. It establishes the limits of the other thermodynamic laws in our universe.






Special thanks

Modern Refrigeration and Air Conditioning, 21st edition (Andrew Althouse, Carl Turnquist, Alfred Bracciano, Daniel Bracciano, and Gloria Bracciano)

This is one of the reference guides for the industry and deserves attribution for all such articles.

Second Law of Thermodynamics


The second law of thermodynamics puts the first law into context and establishes entropy as a concept. We can simply define entropy as a state of disorder that can only increase or remain the same in natural processes. Processes in which entropy remains the same are reversible, but ones that result in an entropy increase are irreversible.

Although the second law states that entropy can remain the same, it doesn’t happen naturally. Every process results in a net increase of entropy, no matter how small it may seem. We cannot account for some negligible factors like friction, which still add entropy to a system and make processes irreversible. However, reversible processes still exist theoretically and are included in the second law of thermodynamics. 

The second law also describes heat movement in nature. Heat only transfers from warmer objects to cooler ones until both objects reach a state of equilibrium; hot objects cannot “gain” coldness from cooler objects. When you get into a cold bathtub, you might feel very cold, but your body doesn’t absorb coldness from the bathwater. Instead, heat leaves your body and transfers to the bathwater.

When you step out of the bathtub and leave the water alone, it won’t return it its original cold temperature. Your body heat brought it closer to equilibrium, and that process is irreversible.


The law’s mathematical basis 

The second law of thermodynamics states that a change in entropy in the system and environment (ΔS) equals the heat transfer into a system (ΔQ) divided by temperature (T). We can represent the law with the following equation:

ΔS = ΔQ / T

The second law also allows you to determine if a process is reversible or irreversible by comparing the initial entropy of the system and environment (Si) with the final entropy of those (Sf). The first equation below is a reversible or isentropic process, and the second equation is an irreversible process.

Si = Sf           or           Si < Sf

Again, you will never see the left equation in any real process. It is impossible to convert heat to mechanical energy with 100% efficiency. We do, however, sometimes compare natural processes to theoretical ones to determine their efficiency. 


Where do we see the second law of thermodynamics in HVAC/R?

The entire function of our HVAC/R systems operates on the principle that higher temperature matter transfers heat to lower temperature matter, which is why heat from the air gets absorbed by the colder refrigerant in the evaporator coil. We wouldn’t use the cold refrigerant as a vehicle to move heat if the heat didn’t naturally transfer from warmer to cooler.

As we said earlier, we also use theoretical reversible processes as benchmarks for our real systems’ efficiencies. Entropy indicates that there is some inefficiency, so it stands to reason that a process without any net entropy gains is 100% efficient.

We do that with HVAC/R compressors. Compression is theoretically isentropic because the entropy changes from the added temperature and pressure cancel each other out, but there are still negligible processes that increase the entropy. However, we compare actual compressor work to isentropic compressor work. We call the resulting value a compressor’s isentropic efficiency.





First Law of Thermodynamics

The first law of thermodynamics is an extension of the law of energy conservation. The latter states that energy can be neither created nor destroyed but converts from one form to another. Thermodynamics is the study of heat’s relationship with mechanical work, and it establishes heat as a form of energy that can be neither created nor destroyed. 

Unlike the energy conservation law, the first law of thermodynamics makes a distinction and a connection between mechanical energy and heat and it states that both are subject to the same laws of energy conservation.

That distinction between mechanical processes and heat is critical, as heat is not necessarily useful. The second law of thermodynamics covers the role of heat in greater detail, but the first law still requires us to understand that distinction.


The law’s mathematical basis

The first law of thermodynamics analyzes a variable called change of internal energy (ΔU), which is the difference between heat transferred into the system (Q) and work done by the system (W). The internal energy measures the amount of heat in a system that does not do work between phases of equilibrium, and the following equation can represent it: 

ΔU = Q W

When energy enters or leaves a system, not all of that energy performs work. Instead, it gets stored as heat. The equation above represents the internal energy as the amount of new heat stored in the system.

Suppose you add 50 Joules of heat to a system, and only 30 of those Joules perform useful work. In that case, the internal energy change is 20 Joules. Those 20 Joules are stored as heat, as they don’t perform any work but must still enter the system because they cannot be destroyed.


When do we see the first law of thermodynamics in HVAC/R?

The first law of thermodynamics helps us think about heat as it enters and leaves a system. It pushes us away from the mindset that we’re adding “cold” to buildings. We’re actually removing heat from those environments and adding it to the systems we install and maintain. 

The A/C system evaporator coil is an excellent example of where we see the first law of thermodynamics in action. The refrigerant enters the coil as a liquid, and warm air passes over that coil. The refrigerant absorbs heat and removes it from the air, raising the refrigerant’s temperature to the boiling point in the process.

A good portion of heat from the environment enters the A/C system. Some of that causes the refrigerant to boil. However, not all of the heat performs that “latent” work. Some of the heat increases the gas molecules’ speed as it raises the overall gas temperature in the superheat phase. Both the latent boiling and sensible superheat contribute to the rise in internal energy that the first law of thermodynamics describes.

In the end, the heat that causes boiling and superheating has an origin, and we don’t eliminate the heat that our units remove from the environment. HVAC units merely convert the environmental heat to a different form of energy within their respective systems.






Special thanks

Modern Refrigeration and Air Conditioning, 21st edition (Andrew Althouse, Carl Turnquist, Alfred Bracciano, Daniel Bracciano, and Gloria Bracciano)

Refrigeration & Air Conditioning Technology, 9th edition (Eugene Silberstein, Jason Obrzut, John Tomczyk, Bill Whitman, and Bill Johnson)

These truly are the reference guides for the industry and deserve attribution for all such articles.

Zeroth Law of Thermodynamics

Zeroth? ZEROTH? is that even a word?

Apparently so because…. SCIENCE

The zeroth law of thermodynamics establishes the relationship between multiple objects and a common state of balance between them. It is sometimes called the law of thermal equilibrium.

Heat always tries to find a state of balance. “Hotter” is a relative term, but an object with higher molecular energy will have more heat than an object with lower molecular energy. When they are in contact, the heat from the item with higher energy will transfer to the lower-energy one until the energy content is equal. Once the energy content is equal, no more heat transfer will occur.

In short, the law states that if object A is in contact with object B and reaches equilibrium, and object B touches and reaches equilibrium with object C, then object A must be in equilibrium with object C.

This law was formulated after the other three laws of thermodynamics. However, it’s called the zeroth law instead of the fourth law because it describes the fundamental heat behavior that influences all the other laws.


The law’s mathematical basis

Rather than requiring a unique formula, the zeroth law of thermodynamics is a mere transitive relation. We see these in algebra and logic, and the zeroth law’s transitive relation looks like this:

If A = B and B = C, then A = C

For example, let’s say you dip two hot steel rods into a single container of cold water, but they don’t touch each other. As time goes on, each steel rod will lose some heat to the water, which approaches a balanced temperature. In time, each rod will be at equilibrium with the water. If they are both at equilibrium with the water, the rods must be at equilibrium with each other because the equilibrium conditions are the same.


When do we see the zeroth law of thermodynamics in HVAC/R?

In HVAC/R, we don’t necessarily see three objects at equilibrium with each other. However, we see evidence that objects with higher molecular energy transfer heat to lower molecular energy objects.

The condenser coil of an A/C system is an example of that. The outdoors will have a higher overall molecular energy because so many more molecules are contained in a larger space than the condenser coil. However, the molecules move so quickly (higher temperature) in the condenser coil that they transfer heat from the refrigerant in the coil to the air outdoors, which has a lower average molecular velocity. That’s why HVAC systems reject heat to the outdoors.

“IMG_8910 – Technology” by ArturoYee is licensed under CC BY 2.0

Indoor coils of heat pumps work the same way. The outdoor coil allows the refrigerant to absorb outdoor heat, and the molecular energy increases quite heavily during compression. The gaseous refrigerant gets a lot hotter as a result. When that refrigerant reaches the cold home’s indoor coil, it transfers its heat to the frigid building interior. 

In all, the zeroth law of thermodynamics establishes how heat attempts to reach equilibrium between multiple objects. It sets the tone for heat and energy behavior in the other laws of thermodynamics.





Newton’s Laws of Motion

“Isaac Newton” by paukrus is licensed under CC BY-SA 2.0

Sir Isaac Newton was an English physicist and mathematician born in a small Lincolnshire village called Woolsthorpe in 1642. His father died before he was born, and his mother remarried when Newton was three years old. He stayed with his maternal grandparents when his mother remarried, and his grandmother taught him how to read and write.

Newton grew up in a tumultuous time for the English government, as the English Civil War began around the same time he was born. By the time Newton reached university age, England had gone through three different monarchs and separated its higher education systems from the Anglican Catholic Church’s grip. Charles II rose to power in 1660 and restored religion to the universities. Still, the educational landscape had already changed quite a bit from Newton’s youth to his university years.

In 1661, Newton began attending Trinity College, a constituent college of Cambridge University. He studied philosophy there, primarily focusing on Aristotlean logic, physics, and ethics. He also studied mathematics beyond his curriculum. However, the plague descended upon England during Newton’s university years, forcing Cambridge University to close and Newton to move back home to Woolsthorpe from 1665 to 1667.

Despite the apparent setback, Newton was highly productive during his years away from school. He performed his initial experiments in the new field of optics, a branch of physics that focuses on light’s properties. He also built upon Kepler’s rule to describe the placement and movement of objects in space, gravitational forces on Earth and outer space, and the tides’ relationship with the moon.

By his mid-twenties, Newton had arguably become the best mathematician in the world, as he expanded upon well-known equations and discovered calculus. He returned to Trinity College in a fellowship program, where he further developed his research and discoveries in optics and calculus. In 1669, he became the Lucasian Professor of Mathematics at Trinity, less than five years after receiving his B.A.

Newton continued his mathematical research as a Lucasian Professor, though many of his findings remained unpublished and caused him to lead a life of isolation. He also studied physics in detail, expanding on optics once again and diving into planetary orbits, conic trajectories, and forces. In 1687, he published these works in his Principia, a series of three books on his discoveries and mathematical concepts. The publication of Principia jolted Newton into the limelight again after many years of isolation.

Newton published multiple editions of Principia throughout his life. It was the main body of work that contained his discoveries, including his three laws of motion.


Newton’s First Law

Newton’s First Law states that an object at a constant velocity will remain at that velocity unless a net force acts upon it. Velocity refers to speed and direction, so an object may either move in a continuous straight line or have zero velocity, meaning that it is at rest. Another term for this law is inertia, or a general tendency to remain unchanged.

A perfect example of this is that a soccer ball lying in the grass will remain motionless until somebody kicks it. When somebody kicks a soccer ball in a straight line, however, it will eventually slow down. That seems to contradict Newton’s First Law, but the grass provides a source of friction, which is a force that acts against the ball’s motion.

Another example deals with centripetal force, which you can generate when you play with a tetherball. The ball rotates around a central point on a string or rope. However, if the rope breaks, the ball will not continue to spin because the centripetal force no longer acts on it. Instead, it will fly off in a straight line, tangentially from where the string broke. You can see this in action below.

AlvaroLopez12, CC BY-SA 4.0 via Wikimedia Commons


Newton’s Second Law

Newton’s Second Law addresses the net force that must act on an object. It states that net force equals an object’s mass multiplied by its acceleration. (You may recognize the equation F = ma.) 

We measure force in units called newtons (N), mass in kilograms (kg), and acceleration in meters per second squared (m/s2). As such, you could rewrite F = ma like this:

Since a newton is equivalent to mass times acceleration, you can represent N values as kilograms times meters per second squared.

Let’s say that someone slides a 10-kilogram box on the floor with 50 newtons of force.  You can find the acceleration by plugging in the N and kg values as such:

The arrow above the a indicates the direction of the motion. Acceleration and force are vectors, meaning that they indicate direction. To solve for the missing acceleration, divide each side by the mass (10 kg) to eliminate the kilograms and leave only the acceleration unit (m/s2).

Newton’s Third Law

Newton’s Third Law states that every force has an equal and opposite force. This law may seem a bit counterintuitive, as you can push a piece of furniture and move it forward. If it were pushing back with an equal and opposite force, wouldn’t it stay in the same place? Wouldn’t every force cancel out, leaving us with no acceleration?

It might seem that way, but that’s NOT the case. Although pairs of objects exert forces on each other, that means the opposite force gets exerted on a different object. For example, when you push a chair, you exert force on the chair. The chair doesn’t exert the opposing force on itself; it exerts the force on your hand (or the body parts that push the chair). That’s why you feel a bit of resistance whenever you push a piece of furniture.

AlvaroLopez12, CC BY-SA 4.0 via Wikimedia Commons

It’s also worth noting that the forces will be equal, but the resulting accelerations may not be. That’s why you can slide a chair four feet on the floor without the chair sending you backward four feet. (Your palms may have imprints from the chair, and your muscles may feel slightly fatigued, but you certainly won’t slide back the same distance you pushed the chair.) The accelerations are nowhere near the same, even if the forces are. 

The Third Law is also why your foot hurts A LOT when you kick a heavy machine out of frustration. The device may not move because of its large mass, but it exerts a painful amount of oppositional force on your foot!


We may not see Newton’s laws directly measured in HVAC, at least not to the same extent as gas laws or laws of thermodynamics. However, Newton’s laws help us understand that nothing can start until something provides the force to make it start. Each exertion of force depends on the mass and acceleration of the exertion, and each application of force has consequences on both objects involved. These laws lay the groundwork for all the motion we see in our everyday work and beyond the HVAC/R field.








Special thanks

Modern Refrigeration and Air Conditioning, 21st edition (Andrew Althouse, Carl Turnquist, Alfred Bracciano, Daniel Bracciano, and Gloria Bracciano)

Refrigeration & Air Conditioning Technology, 9th edition (Eugene Silberstein, Jason Obrzut, John Tomczyk, Bill Whitman, and Bill Johnson)

These truly are the reference guides for the industry and deserve attribution for all such articles.

Conservation of Energy Law

“Electric Motor energy transformation” by coach_robbo is licensed under CC BY-NC-SA 2.0

Many scientists and philosophers contributed to the discovery of the law of conservation of energy. People have been aware of the concept of energy conservation since ancient Greece. These ancient Greek philosophers, such as Thales of Miletus, believed that all substances on Earth originated from one material.  

Of course, that substance was not a physical one like water, air, earth, or fire, as the ancient people thought. The idea about the “substance” being energy originated around the beginning of the 17th century. Galileo Galilei conceptualized a pendulum in which energy switches back and forth between potential and kinetic energy. However, he did not have the vocabulary or existing knowledge to state his concept in such terms. 

In the later 17th century, Gottfried Wilhelm Leibniz attempted to develop a mathematical formula that described a system’s total motion as its vis viva, or “living force.” Vis viva was a precursor to the total amount of energy, as it accounted for kinetic energy and some instances of potential energy.

In 1837, Karl Friedrich Mohr described the one “agent” of the physical world as energy or work. He claimed that energy could exist as magnetism, light, cohesion, motion, electricity, and chemical affinity. He also contended that energy could transfer between these forms.

The ultimate breakthrough came from discovering that heat energy has a mechanical equivalent, which James Prescott Joule explored in one of his experiments in 1843. He developed the “Joule apparatus,” which used a falling weight to turn a paddle in a tank of water. The paddle (mechanical energy) would raise the water’s temperature. The heat was not spontaneous; mechanical energy transformed into heat energy. You can see a picture of the paddle below.

“File: Joule Apparatus.jpg” by Dr. Mirko Junge is licensed under CC BY-SA 3.0

The public still met Joule’s experiments with some resistance and criticism. However, William Robert Grove drew upon Joule’s experiment when he published theories about magnetism, mechanical energy, light, and heat as forms of a common “force” a year after Joule’s experiments. Modern acceptance of the law of energy conservation came three years after Grove’s publication. That acceptance stemmed from a book written by Hermann von Helmholtz, which he published in 1847.


The law

The conservation of energy law states that energy can neither be created nor destroyed, but it can convert from one form to another.

It may seem like we lose or waste energy every single day. Also, there are instances where it looks like energy is created from nothing. However, energy exists in various forms all around us. Most of the fossil fuels we use in our cars once belonged to decaying animals and plants, so none of the energy we use is “created” or “new.”


The basics: potential and kinetic energy

Potential and kinetic energy are the fundamental forms of energy. Potential energy is the term for stored energy (think about a dam holding water back), and kinetic energy is movement (water flowing over the dam).

In the image below, the water cannot flow downward because a dam is in the way. The trapped water is a source of potential energy.

If the dam were to fail, the water would move by pouring over the side. When the water flows over, it doesn’t create new energy; the potential energy merely converts to kinetic energy. 

Another way to visualize the change from potential to kinetic energy is by analyzing a pendulum in a grandfather clock. The pendulum slows to a stop when it goes all the way to the left or right. When it stops at its highest point, the energy in the pendulum is all potential. When the pendulum swings back into motion, that potential energy converts to kinetic energy. The pendulum moves the fastest at the lowest central point, where all the potential energy has transformed into kinetic energy.


Different types of energy and examples of them

Potential and kinetic energy are elementary concepts. They reveal the general state of energy, but they don’t tell you about where the energy comes from or what type it is. 

In both the archery and pendulum examples, those are forms of mechanical energy. Mechanical energy consists entirely of movement or motion potential. In both examples’ uses of potential and kinetic energy, both cases are forms of mechanical energy.

On the other hand, chemical energy consists entirely of chemical reactions or the potential for chemical reactions. For example, wood is a source of chemical energy in the potential state. When it combusts, the potential energy turns to kinetic energy, and the wood’s chemical energy transforms into thermal and light energy. The forest fire below is a clear example.

Fire contains lots of heat and light energy, but that energy isn’t spontaneously created; the wood merely stores it as potential energy until it meets ideal combustion conditions. Forests are chock-full of potential energy that can convert to deadly kinetic energy in the form of heat and light.

A battery is another source of potential chemical energy. When you charge your cell phone and turn it on, the potential energy within that battery converts to kinetic energy; the chemical energy becomes electrical energy.


Where do we see the law of energy conservation in HVAC/R?

In a business that’s all about heat movement, we see it quite a bit. However, we see it more in the context of thermodynamics, which is a specific branch of physics.

The first law of thermodynamics establishes that energy performs work and exists as heat. When a system obtains heat, such as when warm air flows over an A/C unit’s evaporator coil, some of that heat performs work, and some of it does not. The heat that does not perform work doesn’t disappear from the system or the earth, as it cannot be destroyed. 

We don’t eliminate heat from people’s homes and businesses. We merely move it by allowing it to change forms before our units ultimately reject it outdoors again. In the case of heat pumps, we take the heat from the outdoors and reject it inside the building. We don’t magically create heat or remove it; we rely on natural processes to convert heat as a step in the process of moving it.






Special thanks

Modern Refrigeration and Air Conditioning, 21st edition (Andrew Althouse, Carl Turnquist, Alfred Bracciano, Daniel Bracciano, and Gloria Bracciano)

Refrigeration & Air Conditioning Technology, 9th edition (Eugene Silberstein, Jason Obrzut, John Tomczyk, Bill Whitman, and Bill Johnson)

These truly are the reference guides for the industry and deserve attribution for all such articles.


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