Month: February 2019

The point of this article is to give you a full understanding of the role fuses, overloads and circuit breakers play in the protection of HVAC/R equipment. If you skim read or jump to conclusions you will be tempted to argue. Be patient, if you want to understand you will need to read all the way through and possibly even watch the videos at the end. This topic is WIDELY misunderstood so the odds are when you first read it you may think I’m crazy. Do your own detailed research once you get to the end if you still dispute what is contained here.

There are a few topics in HVAC/R that get widely confused and result in a lot of misinformation because of the similarity of the concepts. If you have two terms that have SIMILAR meaning but get used interchangeably you can come to completely logical sounding (though totally incorrect) conclusions.

For example, a tech could say a particular circuit is reading “no Ohms to ground” and by that he could mean zero Ohms or he could mean the meter is reading OL which means infinite Ohms.

In the same way, I often hear people say something is “shorted” and what they really mean is it’s not working, or something inexplicable is happening. So let’s define some terms starting with one of the most often confused. Here is the dictionary definition.

Short Circuit

In a device, an electrical circuit of lower resistance than that of a normal circuit, typically resulting from the unintended contact of components and consequent accidental diversion of the current.

When a professional uses the term short or short circuit, they can mean an electrical path with lower than designed resistance or they can also mean any unintended path.
For example, if two conductors in a cable are compromised and touching one another a tech will often say they are shorted even if there is not a low resistance overall in the circuit.
Because of this the term “Short” has become a broad term and must be used carefully.


Place too much a load on

Pretty simple, when you put too much load in the bed of your truck it bottoms out, when you place too much load on an electrical circuit or device, it fails. In the case of a conductor, this load is in the form of amperage, more amperage than designed and the conductor will fail due to overheating.

In the case of a motor, this same thing holds true but actual load (opposing force) on the motor results in increased amperage load which causes increased amperage and overheating.

This is why a compressor with failing bearings will draw higher amperage, the motor slips due to the additional mechanical load, this drops the impedance (resistance) in the motor windings, resulting in higher amperage.

Ground Fault

The momentary, usually accidental, connection of a current carrying conductor to ground or other point of differing potential

A ground fault occurs when an electrical conductor or device that is electrically charged comes in direct contact to ground, a grounded assembly or substance, usually resulting in large current spikes until either a protection device opens the circuit or the circuit itself fails open (breaks) due to heat.

I say USUALLY  because there are cases when a ground fault may exist with no spike in amperage, such as when you are using an ungrounded, two-prong appliance like a hair dryer or an old drill (or a drill that you cut off the ground plug in order to use on a two prong cord). If the internal windings on the device short to the casing there will be no path from the casing to the ground unless something else makes a path, like say , YOUR BODY. Then when your hand touches the drill casing and connects to ground, some current will leak to ground through the very high resistance load that is your flesh and organs. The circuit will not “overload” because it will not be drawing abnormally high amps but you may still die from the incident. This is why ground fault circuit interrupters (GFCI) are used in some high-risk applications, to break the circuit when a ground fault exists, even if that ground fault does not result in an overcurrent condition.

Overcurrent Protection

A form of protection in an electrical circuit that prevents excessive current usually at a predetermined value – Usually refers to a type of protection designed to deal with instantaneous spikes in current

Over-current protection can be used as a broad term that can include circuit breakers, fuses, etc… basically anything that prevents a current from rising above a predetermined value. It CAN be a pretty broad term in some circles, HOWEVER, in the electrical community when overcurrent protection is used it is generally referring to short circuit or ground fault conditions.

Any condition that results in quick, massive spikes in current is addressed by overcurrent protection. If you want to argue read THIS from Siemens.

Overload protection

Overload protection is a protection against a running overcurrent that would cause overheating of the protected equipment

Overload protection deals with higher current resultant from too much current being pulled by a load. When the compressor goes out on overload after one second because it is locked, that is an example of overload.

When a condenser fan goes off after running with a blade that it has too steep of a pitch, that is an example of overload. Overload in a motor is dealt with by the overload in the motor, not by the overcurrent protection/circuit breaker/fuses.  In the case of motor loads specifically, if the overload were to fail, the overcurrent protection would usually break the circuit eventually, but that is not it’s primary design function in most cases. Again, read THIS from Siemens if you are getting riled up at this point.

When a manufacturer writes their system specs and prints their equipment labels they use guidelines provided by the National Electrical Code (NEC) and they refer to articles 430 and 440 of the code to calculate the required minimum conductor size and maximum overcurrent size.  This is how they come up with the MOCP, or Max Breaker / Fuse size and the MCA or minimum circuit ampacity/conductor size required.

Here is some manufacturer electrical data from a Carrier 25HCC condensing unit –

Notice the maximum breaker or fuse is 40 amps on the 4 ton and the MCA is 26.1 with an allowable wire size of  #10 on an assembly rated at 75°C and a 60°C circuit


On this system it is perfectly acceptable by the National Electrical Code and the manufacturer to run #10 wire and a 40 amp breaker so long as the wire run is a properly rated copper conductor under 70 feet (on this specific unit because of spec shown above). Wire length has an impact on voltage drop which is only addressed as a suggestion in the code but is clearly laid out by the manufacturer either directly or based on the minimum voltage if you do a voltage drop calculation. In this case, the voltage must by 197v or above

This is because the circuit breaker or fuse is providing the overcurrent protection (as well as some backup overload protection) and the motor overloads are providing the overload protection.

Now.. If you would like, you are allowed to put in a lower rated breaker than the max so long as you don’t go below the MCA rating because the breaker itself also needs to be able to handle the rated capacity. Just be aware that the lower you go the more likely you will be to have nuisance tripping.

You can also install larger wire if you like, just be aware that in some cases the equipment lugs may not be rated to hold/connect the larger wire. Also, keep in mind that you must also upsize the grounding conductor when arbitrarily upsizing the current carrying conductors.

If you would like some more discussion on the topic you can see the three videos I did on this HERE, HERE , HERE and HERE

Finally, before you start leaving comments about what is written, please see the two videos below. If you watch these videos, read the reference material and STILL think that what I’m saying is false in some way, feel free to join in the conversation.

— Bryan

P.S.- Mike Holt (shown below) is a friend of mine and considered the authority on NEC and electrical training in the US.

This tech tip is written by experienced tech and VRF / VRV specialist Ryan Findley. Thanks Ryan! (Note: Ryan refers to VRV rather than VRF because he specializes in Daikin and these articles are written from a Daikin VRV perspective)

In this tech tip, I’ll be going over a few things related to the install of VRV systems.  



Any questions should be directed to your rep sales engineer or the manufacturers specific instructions. It’s better to ask and double check than have to do something twice.

The install is the most critical part of a successful system, without a quality install you will not get desirable results.  There are many issues that can come arise from install issues, but here are a few of the most common ones.

Leaks and Tightness Test 

Flares…..I know, everyone hates them but a properly made and installed flare is an effective mechanical joint.  Be sure to ream (deburr) the tubing, use your flare sizing guide tool to assure the flare is a good flare. I have made my fair share of flares that looked good but didn’t pass the test with the gauge.  If you don’t have one, you need one. The link to the one I use will be at the bottom. Also, be sure to use a torque wrench and torque all flares to the appropriate torque specs. I have come across a lot of them that leak after the change of season with the corresponding expansion and contraction.  Often, they’ll last 3-6 months before they start to leak.

Be sure to complete the tightness test in accordance with the install procedure, finishing pressure being 550 psi (450 for the FXTQ) for 24 hours or more. Be sure to record your test pressure and temperature at the beginning and end and compensate for pressure change based on changes in ambient temperature 

Notes from Bryan: The HVAC School app is free and has a nitrogen pressure change calculator. Also, In my contracting business, we find that using a good, quality, modern flaring tool with a depth gauge and clutch as well as some assembly lubricant such a refrigerant oil or Nylog can really help make a great, tight-fitting flare with less galling. 


Be sure to mount your refnets within the allowable angle which is 15 degrees for outdoor refnets and 30 degrees for indoor refnets.  If you’re interested in why this matters, see this video.

Finishing Vacuum

Be sure to get below 500 microns and hold.  A new and tight system should easily be able to get down under 300 and hold there, if not then you want to investigate further.  

Pipe Insulation

Make sure you use insulation with the outside diameter of at least ¾”.  If line sets run through an attic or unconditioned space, this is especially important.  Be sure to seal the joints with appropriate glues or tapes designed for the purpose. 

Purge & Flow Nitrogen

It’s very important that you displace all air with nitrogen by purging first, then flow nitrogen at a very low rate anytime you are brazing.  There are filters/strainers everywhere in the system that are fine mesh that can be clogged up very easily.  See pictures below.


Moisture can be a major issue with the use of PVE oil.  PVE is more hygroscopic than POE is but rather than hydrolysis occurring it changes into a sludge.  This sludge can gum up mechanical components in the refrigeration system which can cause premature failures. Be sure to complete a decay test after you have reached your finishing vacuum, it’ll tell the tale if there’s moisture in the system or not.   

Communication wiring

Be sure to daisy chain your wiring.  Also, make sure to use non-shielded 18/2 stranded wire (or whatever your particular product requires) and install it according to the submittals from your sales engineer.  

Setting The Units

Verify that the units are set in the proper order, the largest unit goes closest to the indoor units on down to the smallest unit (VRV 4).  VRV 3 has a cross over line that goes between the modules so this is not a concern.

20/40 Rule

To reduce refrigerant noise, it’s recommended to keep the first elbow after a refnet at least 20” away and 40” away for branch boxes.

Line Length Measurements

Please, please, please keep track of and measure the lineset!

Send it to your sales engineer so that the correct additional charge can be calculated. These are critical charge machines so every pound matters.

Oil Traps

Only inverted traps are allowed in the VRV piping.  Oil traps are a major concern for these machines as they aren’t able to overcome them in oil return mode. The more oil that gets trapped out in the system, eventually you’ll start losing bearings in the compressors.  I have changed many compressors that have nearly no oil left in them. Be mindful of keeping piping on the same level. There are specific rules about oil traps in your install class.

Expansion Joints

Install expansion joints per your sales engineer requirements.  This is important because of the possibility of a large change in temperature that the pipe is under and needs to be allowed to expand and contract.  If not, there’s a possibility of blowing out the end of a fitting.

Pipe Clamps

Do NOT tighten your pipe clamps down with your impact driver. The pipe needs the ability to move, otherwise there’s a chance you can blow out a fitting.

Unit Placement

Be sure to have the units mounted on stands above the highest average snowfall.  Having snow pile up or water from defrost freeze in the pans or on the bottom of the coils can be very problematic.  Be sure the bottom of the units are clean as that can cause the water not to drain out of the pan also.

— Ryan

PS- Here is a good Flare Gauge available from TruTech Tools 

It’s always good to reaffirm some of the basics of energy, work and matter and these five rules are some of the most basic and important

The First Law of Thermodynamics

States that energy cannot be created or destroyed. It can only change form or be transferred from one object to another.

The Second Law of Thermodynamics 

Energy cannot be created or destroyed, but it can change from more-useful forms into less-useful forms. As it turns out, in every real-world energy transfer or transformation, some amount of energy is converted to a form that’s unusable (unavailable to do work). In most cases, this unusable energy takes the form of heat.

Newton’s 1st Law

An object in motion will remain in motion unless acted upon by an external force.

  • An object that is at rest will stay at rest unless a force acts upon it.
  • An object that is in motion will not change its velocity unless a force acts upon it.

Newton’s 2nd Law

Acceleration is produced when a force acts on a mass and that the change varies by the mass of the two objects. In layman’s terms, if I hit you in the head with a bowling ball it would hurt a lot more than a bag of feathers even if they were going the same speed. This rule establishes a relationship between acceleration and mass.

Newton’s 3rd Law 

For every action, there is an equal and opposite reaction.

In practice, these rules teach us things like

  1. It requires a transfer of energy to move things
  2. It requires a transfer of energy to stop things
  3. It requires more energy to move heavier things than lighter things
  4. Every time something moves in one direction there is an equal force pushing in the other direction

These are all fundamentals that help us to be really clear on things like

  • There is no magic energy that we can “create” from nothing. Most of the energy we experience on earth started from the sun and then gets stored in plants, animals and in the very “stuff” that makes up the earth itself.
  • To get something moving that isn’t moving or to redirect motion we need to input energy from somewhere
  • The mass of an object impacts the amount of energy it takes to move it and the amount of energy it takes to stop it from moving

These ideas apply to big objects like a boulder rolling down a hill as well as to really small objects like molecules moving around inside a glass of water or the air moving around in the room you are in.

The 4 HVAC/R Rules 

For the sake of practicality, let’s look at 4 simplified rules that can help you envision and diagnose the movement of heat and matter but first at one big idea.

Nature Seeks Balance. The Greater the Imbalance the Greater the Forces Nature Will Apply to Return to a Balanced State.

While that may not be a scientific way of way of saying it, this big idea explains everything from flowing rivers to hurricanes to avalanches to forest fires. It also applies to simple things like why an ice cube melts in your hand or why more sweat soaks your clothes in a humid environment than in the deserts of Arizona.

So here are the four simplified Rules –

  1. Hot goes to cold (Heat flows from higher temperature objects to lower temperature objects) 
  2. High pressure goes to low pressure (Fluids move from higher pressure areas to areas of lower pressure) 
  3. High voltage goes to low voltage (electrical current moves from areas of high charge concentration to low charge concentration also known as potential) 
  4. High humidity goe to low humidity (water vapor in the air moves from higher concentration to lower concentration) 

We see all of these forces at play every day in HVAC/R work and we manipulate and control them to accomplish the movement of fluids (air/refrigerant/water), the movement of heat from one place to another and ultimately control over the spaces we are trying to make comfortable or the products we are attempting to cool, heat or control humidity in.

— Bryan


If you went to school and learned the “Gas Laws” early on and it seemed boring a LAME but then later things like mass flow rates and air flow conversions and compression ratios seemed HARD.


It’s because HVAC is hard if you don’t understand the concepts behind the gas laws, even if you never learn to do the math.

First, we need to make sure and define some words so we are all on the same page. Don’t skim over this part unless you are really sure you understand what these words mean.

Matter – Matter is “stuff”, anything that has weight and takes up space

Mass – You can think of it as weight for most purposes, it is a measurement of how much “stuff” there is.

Volume – How much space the stuff takes up.

I always think of volume as “boxes” and mass as “the stuff in the box”. The box defines the space but you can’t tell how much stuff is in the box until you try to pick it up or weigh it.

Density – A measure of “compactness” of matter. The more dense, the more mass it will have by volume. If you let a sponge take its natural shape it is less dense then when you ball it up in your fist and it becomes denser.

Temperature – The average intensity of heat energy at that point. It is literally the average molecular velocity of the stuff you are measuring.

Pressure –  The force exerted on, in or by matter.

In HVAC/R we are constantly dealing with concepts contained in the “Ideal Gas Law” which informs us of how matter and contained environments respond to changes in temperature, volume, pressure, and mass. There are several gas law(s) that are often taught but they are all combined in the ideal gas law to bring it all together. We will get back to the ideal gas law shortly, but first we need to understand a bit about systems.

Whenever you are thinking about matter and energy it is important to consider whether the situation you are observing or testing is an open, closed or isolated system.


Open System

An open system allows both matter (mass) and energy to move in and out of a system. You can think of this like an open pot of water where water molecules can leave or enter through the top and energy can also enter and leave through the top and through the walls of the pot.

Closed System

A closed system can allow energy to enter and leave but the mass of the matter remains fixed. This is like a pot of water with a lid tightly sealed on the top or for our purposed the refrigerant sealed inside an air conditioning system.

Isolated System 

An isolated system means a system that can neither transfer matter (mass) or energy in and out. This would be like a cooler full of cold beer with a top tightly sealed and with perfect insulation that would never let any heat in or out. In practice this is impossible but if it were the beer would never change temperature once the temperature stabilized inside.

In an open system like an open pot of water or a lake and we heat it, the water molecules on the surface begin to break free and move away as they evaporate. This leaves the surface below a little cooler as that high energy molecules leave. The lake or pot don’t pressurize when the molecules leave becasue the system is open to the atmosphere.

Now, if we add heat in a closed system like a pressure cooker the entire system begins to pressurize as the water evaporates and then boils. Energy is being added but the mass can’t change, this leads to a pressure and temperature increase inside the pot. If we keep adding more and more heat without a pressure relief the pot will either explode or melt.

This quick clip from Mythbusters shows what can happen in a closed system when energy is added to a water heater until it fails

What is an ideal gas? 

An ideal gas is a gas that obeys the ideal gas law, it’s ideal because it’s good at following rules. These ideal gasses walk in a straight line, they don’t run on the playground and they never fish without a proper permit. More like an ideal gas behaves in a predictable way with changes in volume, pressure, temperature, and mass.

The problem is, a truly “ideal” gas really doesn’t exist.

While many gasses behave close to ideal at normal temperatures there is no gas that obeys the ideal gas laws in all conditions.

The ideal gas law is –

P=  Absolute Pressure (gauge pressure + atmospheric pressure)
V = Volume (How much space the gas occupies)
n = Mass measured in “moles” (the number of molecules)
R = The universal gas constant (varies depending on the units of measure being used Example: [lbf ft/(lb mol oR)]= 8.3145 )
T = Absolute Temperature (temperature in a scale that starts at absolute zero like Kelvin or Rankine)

The result is that many gasses that we work with behave in about the same way with changes in mass, volume, temperature & pressure. This is the case because the primary force at play in a nearly ideal gas like nitrogen or CO2 is simply the velocity of the molecules bouncing around in the container and against one another like tiny little ping pong balls.

If the molecules react, or interact with one another through attraction or repulsion due to their intermolecular forces then they can cease to behave as an ideal gas. A perfect example is when a gas is in contact with its liquid form (saturation) it no longer obeys the gas laws. This is why most gasses behave more and more like an ideal gas the hotter they get (within a range) because the hotter they are the greater the force of molecular velocity (temperature) will be relative to the intermolecular interaction of the molecules.

From a practical standpoint understanding the relationships between temperature, pressure, volume and mass in contained (closed) environments like an HVAC refrigerant circuit and uncontained (open) environments like the air outside are really helpful.

The most practical of them is –

A decrease in temperature causes a decrease in pressure or a decrease in pressure causes a drop in temperature. An increase in either results in the opposite effect.

Why Does it “Matter” (Pun Intended) 

In HVAC/R it is really important that we understand the impacts changes in temperature, pressure, volume and mass will have inside the system.

When we add or remove refrigerant we are changing the refrigerant mass which impacts the pressures and temperatures. When one system has a smaller condenser coil than another the decrease in internal volume will increase the pressure and temperature if the refrigerant mass is the same.

When the air passing over an evaporator coil is colder the pressure in the coil will be lower because the gas laws teach us that a decrease in temperature equals a decrease in pressure.

All of these factors are interconnected and if you don’t understand the gas laws you won’t understand how they impact one another.

Here’s one for the nerds!

— Bryan

Every HVAC/R tech needs an electronic leak detector nowadays and with HFC refrigerants getting more and more commonplace a VERY sensitive electronic leak detector. There are three types we often see but let’s toss out the corona discharge leak detector right off the top. It just picks up too many other types of chemicals to be useful and can easily result in false positives. This leaves the heated diode (or Pentode in the case of Tif) and the Infrared.

Heated Diode

The heated diode is the standard for sensitivity and the ability to pinpoint a leak. Some common heated diode leak detectors are the H10G, the Testo 316-3 shown above and the Tif ZX.


  • You can move to the spot of the leak and hover over that point to detect very small leaks. This allows you pinpoint the leak.
  • Tremendous sensitivity on many models
  • Many techs feel they are more accurate because the motion is more intuitive. Place the probe over the leak and the detector goes off.


  • Heated diode can give false positives from some other substances
  • The sensor is prone to fouling from moisture and oil
  • The sensors need to be replaced fairly often (usually at about 100 hours of operation)
  • The Sensor needs to be allowed to heat up before use


Infrared leak detectors are increasing in popularity over recent years. An infrared leak detector draws the sample across an optical sensor that analyses how much IR radiation the sample has absorbed. Some common types are the Fieldpiece SLR8 and the Bacharach TruPointe.


  • The sensors last longer than heated diode
  • They are less susceptible to false positives from other gasses and maintain full accuracy for a greater range of refrigerants
  • Very good sensitivity, though not generally as sensitive as the most sensitive heated diode models


  • Infrared detectors compare samples to one another for detection. This means that the probe generally must be moved continuously to work properly. This can make pinpointing the leak more tricky.
  • It can be more difficult to get a sense of the size of the leak because they are constantly recalibrating (in my experience).

As is true in many things, you get what you pay for, so you should expect to pay $250- $550 for a good quality leak detector. If you stick with a quality brand and read up on the technology and sensitivity specs beforehand you will generally be in good shape with either a heated diode or a infrared leak detector.

— Bryan

Scroll to top
Translate »

Daily Tech Tip

Get the (near) daily Tech Tip email right in your inbox!
Email address