Month: August 2017

I was about 13 years old the first time I bent EMT with my uncle. We were doing a renovation at a church, and watching him bend EMT and then getting to do it MYSELF was a truly religious experience.

There are a few things in the trade where workmanship really comes into play such as copper pipework, making up a panel or fabricating ductwork… bending EMT belongs on that list. While most commercial electricians do it every day, HVAC techs and installers only run into an application where we do it on a rare occasion. When that does happen its good to have a basic understanding of how it works. In this video Juan from the The Air Conditioning Guy channel goes over some quick basics on bending EMT

For more info you can read this great guide on bending from Klein

— Bryan

For those of you who follow the podcast you know how excited I am about the new MeasureQuick app and what it will do to help technicians make better measurements and diagnosis. The app is NOW AVAILABLE on Android and will be available within hours on IOS (Apple)

To find out more and to download just go to

Here is a video from Jim on the launch

And a video from Brad Hicks with HVAC in SC showing his system at home on the app

and you can hear the MeasureQuick launch podcast episode HERE

Big thanks to Jim Bergmann for bringing this excellent technology to the HVAC industry.

As a technician gains skill they will learn that regularly testing your tools is a huge part of success. It isn’t long in the field before techs find out that just because a meter or gauge gives a particular reading it doesn’t ALWAYS mean it is correct. Vacuum is one of these areas.

Everything in an air conditioning and refrigeration system leaks to some extent, our job isn’t to eliminate all leaks, our job is is to reduce the leakage rate to as low as possible. When using a sensitive micron gauge we find that isolating an assembly and checking the “decay” or standing leak rate is a great way to test and ensure that a system has minimal leaks and moisture. The challenge is that all of the connections in your rig leak and even the vacuum gauge itself leaks.

Some techs attempt to test the leak rate on micron gauge by connecting it to a core tool and then straight to the pump, evacuating the gauge down to very low level and then valving off. If you do this, you will find that every commercially available vacuum (micron) gauge shoots up pretty quickly. This is because the VOLUME of the gauge and coupler are so low that ANY leakage whatsoever has an enormous effect.

In this video Ulises Palacios shows us how to use an an empty recovery tank to better test the leak rate of a vacuum gauge rig.

It is certainly important to test all of your vacuum rig components, just remember that volume makes a huge difference when decay (standing vacuum) leak testing.

— Bryan

The piston (fixed orifice) and TXV (Thermostatic Expansion Valve) are the two most common metering devices in use today, with some modern systems utilizing an electronically controlled metering device called an EEV (Electronic Expansion Valve).  It should at least be noted that there are other types of fixed orifice metering devices like capillary tubes, but their use is not common on most modern A/C systems though you will see them in refrigeration.

While the compressor creates the pressure differential to get the refrigerant moving, by decreasing the pressure on the suction and increasing the pressure on the discharge side, the purpose of the metering device is to create a pressure drop between the liquid line and the evaporator coil or expansion line (the line between the metering device and the evaporator when there is one). When the high-pressure liquid refrigerant is fed into the metering device on the inlet the refrigerant flows out the other side and the immediate pressure drop results in an expansion of a percentage of the liquid directly to vapor known as “flashing”. The amount of refrigerant that “flashes” depends on the difference in temperature between the liquid entering the metering device and the boiling temperature of the refrigerant in the evaporator. If the difference is greater, more refrigerant will be “flashed” immediately and if the difference is less than less refrigerant will be flashed.


A piston is a replaceable metering device with a fixed “bore”. It is essentially a piece of brass with a hole in the center, the smaller the bore the less refrigerant flows through the piston and vice versa. The advantage of a piston is that it is simple and it can still be removed, the bore size changed and cleaned if required.


Some piston systems also allow the reverse flow of refrigerant as shown in the diagram to the above. In a heat pump system when the reversing valve is energized (cool mode), the unit will run in cool mode and the refrigerant will follow the path indicated on the bottom.  This seats the piston so refrigerant must pass through the orifice.  With the reversing valve de-energized the flow reverses.  This unseats the piston and allows the free flow of refrigerant.  In this case, there is a metering device in the condensing unit (outside unit) that meters the flow of refrigerant in heat mode and one inside that meters in cooling mode.


The TXV can vary the amount of refrigerant flow through the evaporator by opening and closing in response to evaporator heat load.  compared to a fixed orifice a TXV operates more efficiently in varying environmental conditions (theoretically at least).

To operate, the TXV has a needle and seat that restricts the flow of refrigerant and acts as the orifice.  This needle, when opened, allows more refrigerant to flow and, when closed, restricts refrigerant flow.  There are three factors that affect the flow of refrigerant flow through a TXV.  A sensing bulb filled with refrigerant exerts force to open the TXV.  Since gas pressure increases with a rise in temperature, the bulb, which is attached to the suction line after the evaporator coil, “senses” the temperature of the suction line.  If the suction line becomes too warm, the additional pressure created by the heated refrigerant opens the TXV more to allow additional refrigerant flow.  A spring inside the bottom of the TXV exerts pressure to close the valve.  An external equalizer senses pressure in the suction line after the evaporator, and also works to close the valve. In essence, the TXV is a constant superheat device, it sets a (relatively) constant superheat at the evaporator outlet by balancing bulb, spring and equalizer pressures.

The primary method of charging system changes based on the type of metering device. A piston system uses the superheat method of charging and the TXV uses the subcooling method of charging.

No matter what primary method of charging you use it is still important to monitor suction pressure (Evap temperature) head (condensing temperature), Superheat, subcool and delta t (or some other method of air flow verification).

While a TXV and a piston function differently the end result is a pressure drop and boiling refrigerant in the evaporator.

— Bryan

First off, the correct acronym for a GFI (Ground Fault Interrupter) is a GFCI (Ground Fault Circuit Interrupter) and the purpose is to act as a safety device to protect from electrical shock.

GFCIs can be built into outlets, circuit breakers and even extension cords and are generally used for safety in wet environments like bathrooms, kitchens and outside.

A GFCI measures the difference in current between the line (hot) and the neutral. When even a small difference exists between neutral and hot the GFCI trips. This happens because a difference between neutral and hot means that some of the current is “leaking” to ground instead of being carried properly on neutral.

An example would be an electric drill plugged into an outlet outside and the cord plug falls into a mud puddle. If there is no GFCI some of the current will go out of the plug to ground through the puddle, causing hot to carry more current than neutral and making the puddle a potential shock hazard. If the circuit were protected with a GFCI it would trip immediately when the imbalance was detected.

Another nice thing about a GFCI is that it can help protect a circuit that does not have an equipment ground such as tools and appliances with two prong cords or two conductor outlets.

— Bryan

In this 60-second tech tip video by Brad Hicks with HVAC in SC. he shows us how and why to remove the weep port plugs on a condensing fan motor. I know from experience that motors can fail prematurely when this practice isn’t followed. Remember that motor orientation dictates which are removed. It (generally) the ports facing down that need to be removed and the ones face up stay in place.


What’s going on guys here is a quick 60-second tech tip is on changing condenser fan motors. Whenever you’re changing them, most all condenser fan motors have plugs that are supposed to be removed depending on the orientation of the motor. Since this shaft is facing down into the unit these need to be removed and basically what they do is, they open the weep holes so any condensation or moisture that can get into the motor doesn’t stay in there to corrode the windings and in turn prematurely make the motor fail. So make sure you take those plugs out, if you don’t, like that motor over there you’ll be back within a couple years to replace it again. Just a quick tip make sure you take those plugs out like I said this motor is oriented this way so you want to take the plugs out of the bottom like I just did and your motor will last much longer. There you go thanks for watching.
— Brad Hicks


This is a basic overview of the refrigeration circuit and how it works. It isn’t a COMPLETE description by any means, but it is designed to assist a new technician or HVAC/R apprentice in understanding the fundamentals.

First, let’s address some areas of possible confusion 

  1. The Word “Condenser” Can Mean two Different Things Many in the industry will refer to the outside unit on a split air conditioner, heat pump or refrigeration unit as a “condenser” even though it will often contain the condenser, compressor, and other parts. It’s better to call the outside component the “condensing unit” or simply the “outside unit” to reduce confusion.
  2. Cold and Hot are Relative terms Cold and Hot are both an experience, a description, a comparison or an emotion. Cold is a way to describe the absence of heat in the same way that dark describes the absence of light. We will often use the words cold and hot to compare two things “Today is colder than yesterday” or to communicate comfort “It feels hot in here”. These are useful communication tools, but they are comparisons not measurements.
  3. Heat and Absolute Zero Can be Measured We can measure heat in BTUs and light in lumens, we cannot measure cold or dark. Absolute cold is the absence of all heat.  -460°F (cold) is known as absolute zero, -460°F is the temperature at which all molecular movement stops. Any temperature above that has a measurable level of heat. While this is a known point at which all molecular movement stops, it has not (and likely cannot) be achieved.
  4. Boiling Isn’t Always Hot When we say it’s “boiling outside” we mean it’s hot outside. This is because when we think of boiling we immediately think of water boiling in a pot at 212°F (100°C) at atmospheric pressure, which is 14.7 PSI (Pounds Per Square Inch) at sea level. Boiling is actually just a change of state from liquid to vapor, and the temperature that occurs varies greatly based on the substance being boiled and the pressure around the substance. In an air conditioner or a refrigeration system, refrigerant is designed to boil at a low temperature that corresponds to the design of the system. On an average air conditioning system running under normal conditions with a 75° indoor temperature, the evaporator coil will contain refrigerant boiling at around 40°F. In air conditioning and refrigeration when we refer to “boiling”, “flashing” or “evaporating of refrigerant” we are talking about the process of absorbing heat, otherwise known as cooling.
  5. Cooling and Heating Cannot be “Created” We are not in the business of making heat or creating cool; it cannot be done. We simply move heat from one place to another or change it from one form to another. When we “cool” a room with an air conditioner, we are simply absorbing heat from the air into an evaporator and then moving that heat outside to the condenser where it is “rejected” or moved to the outdoors.
  6. Heat and Temperature Aren’t the Same  Imagine a shot glass of water boiling away at 212°F. Now imagine an entire lake sitting at 50°F. Which has a higher (hotter) temperature? That answer is obvious-I just told you the shot glass had 212°F water in it so it is CLEARLY hotter. But, which contains more heat?  The answer is the lake. You see, heat is simply energy and energy at its basic form is movement. When we measure heat we are measuring molecular movement; the movement of molecules–atoms stuck together to make water or oxygen or nitrogen. When molecules move FASTER they have a HIGHER temperature and when they move SLOWER they have a LOWER temperature. Temperature is the average speed (velocity) of molecules in a substance, while heat is the total amount of molecular movement in a substance. The lake has more heat because the lake has more water (molecules).
  7. Compressing Something Makes it Get Hotter (Rise in Temperature) When you take something and put pressure on it, it will begin to get hotter. As you pack those molecules that make up whatever you are compressing, they get closer together and they start moving faster. If you drop the pressure the molecules will have more space and will move slower causing the temperature to go down.
  8. Changing the State of Matter Moves Heat Without Changing Temperature  When you boil pure water at atmospheric pressure it will always boil at  212°F. You can add more heat by turning up the burner, but as long as it is changing state (boiling), it will stay at 212°F. The energy is changing the water from liquid (water) to vapor (steam) and the temperature remains the same. This pressure and temperature at which a substance changes state instead of changing temperature is called its “boiling point”, “condensing temperature” or more generally “saturation” point.
  9. Superheat, Subcool, Boiling, and Saturation Aren’t Complicated  If water is boiling at sea level it will be 212°F. If water is 211°F at sea level we know it is fully liquid and it is 1°F subcooled. If water is 213°F at sea level we know it is vapor and superheated. If something is fully liquid it will be subcooled, if it is fully vapor it will superheated, and if it is in the process of change (boiling or condensing) it is at saturation.

Where to start 

Take a look at the diagram at the top of this piece and start at the bottom left. Are you looking at the part at the bottom left? OK, now read this next line OUT LOUD:

Compressor > Discharge line > Condenser > Liquid Line > Metering Device > Expansion Line > Evaporator > Suction line and then back to the Compressor

When I first started in HVAC/R trade school this was the first thing my instructor forced me to LITERALLY memorize forward and backwards before he would allow me to proceed.

While I am not always a huge fan of rote memorization as a learning technique, in this case, I agree with committing this to memory in the proper order.

These four refrigerant components and four lines listed above make up the basic circuit that every compression refrigeration system follows. Many more parts and controls may be added, but these basics are the cornerstone on which everything else you will learn is based. Once you have these memorized we can move on to describing each.


The compressor is the heart of the refrigerant circuit. It is the only mechanical component in a basic refrigeration system. The compressor is like the heart that pumps the blood in the body or like the sun that provides the earth its energy. Without the compressor to move the refrigerant through compression, no work would be done and no heat would be moved.

The compressor creates a pressure differential, resulting in high pressure on the high side (discharge line, condenser & liquid line) and low pressure on the low side (suction line, evaporator and expansion line).

There are many different types of compressors, but you will most likely see Scroll and Reciprocating type compressors most often. A reciprocating type compressor uses pistons, valves, and a crankshaft. Reciprocating compressors operate much like car engines; pulling in suction vapor on the down-stroke and compressing that vapor on the up-stroke. A scroll compressor does not have any up-down motion like a reciprocating compressor. A scroll compressor uses an oscillating motion to compress the low-pressure vapor into high-pressure vapor.

The compressor pressurizes low-pressure vapor into high-pressure vapor, but it also causes the temperature of the gas to increase. As stated in the gas laws, an increase in pressure causes an increase in temperature and a decrease in volume. In the case of refrigerant cooled compressors, heat is also added to the refrigerant off of the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. Compressors require lubrication; this is accomplished through oil that is in the compressor crankcase, as well as oil that is carried with the refrigerant. Liquid entering the compressor through the suction line is a very serious problem. It can cause liquid slugging, which is liquid refrigerant entering the compression portion of the compressor. Liquid slugging will most likely cause damage to the compressor instantly. Another problem is bearing washout or “flooding”. This occurs when liquid refrigerant dilutes the oil in the compressor crankcase and creates foaming, and it will greatly reduce the life of the compressor because it will not receive proper lubrication and too much oil will be carried out of the compressor and into other parts of the system. The compressor also (generally) relies on the cool suction gas from the evaporator to cool the compressor properly, so it’s a delicate balance to keep a compressor from being flooded and also keep it cool.



Condensers come in all different types, shapes, and sizes. Regardless, they all perform the same function: rejecting heat from the refrigerant. The refrigerant entering the condenser was just compressed by the compressor, and this process increased the temperature by packing the molecules together which added heat to the vapor refrigerant due to the motor and mechanical workings of the compressor. This process in the compressor also greatly increased the pressure from a low-pressure in the suction line entering the compressor, to a high-pressure vapor leaving the compressor.

The condenser has three jobs:

  1. Desuperheat the refrigerant (Drop the temperature down to the condensing temperature)
  2. Condense (saturate) the refrigerant (Reject heat until all the refrigerant turns to liquid)
  3. Subcool the refrigerant (Drop the temperature of the refrigerant below the condensing / saturation temperature)

The condenser’s job is to reject heat (drop the temperature) of the refrigerant to its condensing (saturation) temperature, then to further reject heat until the refrigerant fully turns to liquid. The reason it must fully turn to liquid is that, in order for the refrigerant to boil in the evaporator, it must first have liquid to boil.

The way in which the condenser removes the heat from the refrigerant varies. Most modern condensers flow air over the tubing where the refrigerant is flowing. The heat transfers out of the refrigerant and into the air. The cooling medium can also be water. In the case of a water source system, water is circulated across the refrigerant in a heat exchanger.

In either case, the condenser relies on the removal of heat. For instance, if you turned off the condenser fan so that no air was flowing over the condenser coil, the condenser would get hotter and hotter. This would cause the pressures to get higher and higher. If it kept going that way it would eventually ruin the system.

The hot vapor from the compressor enters the condenser and the superheat  (temperature above condensing temperature) is then removed. The refrigerant then begins to change state from vapor to liquid. The refrigerant maintains a constant temperature until every molecule of vapor is condensed. The temperature of the liquid again starts to fall. This is known as subcooling. When we measure subcooling we are measuring degrees of temperature rejected once the refrigerant has turned completely to liquid.

Temperature above the saturation temperature is called superheat. Temperature below the saturation temperature is called subcool or subcooling. So when something is fully vapor (like the air around us) it will be superheated, and when it is fully liquid (like the water in a lake) it is subcooled. 

Metering Device

The metering device is a pressure differential device that creates a pressure drop to facilitate refrigerant boiling in the evaporator coil.

The metering device is located between the liquid line and the evaporator. The liquid line is full of high-pressure liquid refrigerant. When the high-pressure liquid hits the small restrictor in the metering device, the pressure is immediately reduced. This drops the pressure of the refrigerant to such a degree, that the saturation temperature is lower than the temperature of the air surrounding the tubing that the refrigerant is in. This causes the refrigerant to start changing from liquid to vapor. This is called “boiling” or “flashing”. This “flashing” brings the refrigerant down from the liquid line temperature to the boiling (saturation) temperature in the evaporator, and in this process a percentage of the refrigerant is immediately changed from liquid to vapor. The percentage of the refrigerant that changes during flashing depends on how great the difference is. A larger difference between the liquid line temperature and the evaporator boiling temperature results in more liquid lost to flashing and reduces the efficiency of operation.


There are a few different types of metering devices. The most common ones being the Thermostatic Expansion Valve (TXV/ TEV) and the Fixed Orifice (often called a piston)– as well as electronic expansion valves, capillary tubes, and others.



The evaporator is also known as the cooling coil, because the purpose of the evaporator is to absorb heat. It accomplishes this through the refrigerant changing from liquid to vapor (boiling). This boiling process begins as soon as the refrigerant leaves the metering device, and it continues until the refrigerant has absorbed enough heat to completely finish the change from liquid to vapor. As long as the refrigerant is boiling it will remain at a constant temperature; this temperature is referred to as saturation temperature or evaporator temperature. As soon as the refrigerant is done boiling, the temperature starts to rise. This temperature increase is known as superheat.

When the indoor air temperature or the air flow going over the coil is higher, the evaporator pressure and temperature will also be higher because more heat is being absorbed into the coil. When the air temperature or airflow over the coil is lower, it will have lower pressure and temperature in the coil due to less heat being absorbed in the coil.

The refrigerant leaves the evaporator, travels down the suction line and heads back to the compressor where the cycle starts all over again.

Refrigerant Lines 

Suction Line = Line Between the Evaporator and the Compressor

The suction line should contain low-pressure superheated suction vapor. Cool to the touch on an air conditioning system, and cold to the touch in refrigeration.

Discharge Line = Line Between the Compressor and the Condenser 

The discharge line should contain high temperature, high pressure superheated vapor

Liquid Line = Line between the Condenser and the Metering Device

The liquid line should be high pressure, slightly above outdoor temperature subcooled liquid

Expansion Line (When applicable) = Line Between the Metering Device and the Evaporator

On most systems, the metering device will be mounted directly to the evaporator making the expansion line a non-factor. Some ductless mini-split units will mount the metering device in the outside unit making the second, smaller line and expansion line. The expansion line is full of mixed vapor/liquid flash gas.

Yes, this was long, but more than anything else just keep repeating over and over: compressor>discharge line>condenser>liquid line>metering device>expansion line> evaporator>suction line and on and on and on…





Connecting more than one wire on or under a single lug or connection point is called “double lugging” and it is ONLY allowed in line voltage wiring under one condition according to NEC 110.14

If the terminal, lug or connector is specifically rated for more than one wire

In the case of a conductor splice like a wire nut or a split bolt, they are only designed for 2 wires unless they specifically state otherwise on the box on the connector itself or in the instructions / product data.

This means that wiring in a surge protector under the same lugs as the main, or jamming as many wires as you can make fit under a split bolt or wire may be common, but it is not allowable according to NEC 110.14 

HVAC techs and installers will often double lug contactors when making a repair, or they will connect to the closest, easiest point when installing a 120v or 240v accessory like a UV light , humidifier or air purifier. 

In all of these cases is is best to take a few minutes and find an approved and permanent method of making the connection instead of taking the easy way out.

It is also worthwhile to mention that some connections are rated for copper only and will be marked CU while others designed for aluminum will be marked AL or ALR. Some will be marked as CU / AL which means that either copper or aluminum may be used but not necessarily that copper and aluminum may be MIXED.

There are very few connection that allow the mixing of copper and aluminum and if they do they must be specifically listed for that purpose.

— Bryan 

I hear many techs complain about the finicky and ineffective nature of electronic leak detection. So much so that some claim that is is a waste of time altogether. we recently located a leak inside the fins of a ductless evaporator coil, pinpointed to an exact spot using an electronic leak detector. For demonstration purposes, we took that coil and performed a definitive test to locate it in the video below.

A leak detector can be tricky to use so here are some of our top tips –

  • Know your detector. Know it’s limitations, it’s sensitivity and what can cause false positives. For example some leak detectors will sound off on certain cleaners or even soap bubbles. My detector sounds off when jostled or when the tip is blocked.
  • Keep a reference bottle so you can check your detector every time before you use it.
  • Maintain your detector and replace the sensor as required. Most heated diode detectors require sensor replacement every 100 hrs or so.
  • Keep it out of moisture. Most detectors will be damaged by almost any amount of moisture.
  • Move slowly and steadily. Don’t jump around or get impatient.
  • Most refrigerant is heavier than air which means that starting from the top and working down is usually a more effective way to pinpoint.
  • Go back to the same point again and again to confirm a leak. Don’t condemn a component bases on one “hit”
  • Find the leak WITH BUBBLES whenever remotely possible, even after pinpointing with a detector.

— Bryan

In my recent classes with my employees at Kalos, we are going over finding target pressures and temperatures for an air conditioning system with the goal being to get techs to have “target” readings in mind before they start connecting tools. This step is an important part of being able to “check a system without gauges” like we have talked about so often. Much of this list makes more sense if you are already familiar with our 5 pillars of diagnosis.

It is important that we start using these terms when speaking to one another, writing notes and diagnosing because these will translate better between systems and between A/C and Refrigeration. Some readings we take in HVAC like static pressure and delta t do not apply to refrigeration, while others like target condensing and evaporator temperature are key in both disciplines.

Keep in mind that when I give a “rule of thumb” you should always consider manufacturer specs, charts, panels, install and service manuals as superior to a rule of thumb. You will be amazed what you might learn reading the service and installation manuals of the systems you install and work on.


Target Evaporator Temperature or DTD (Design Temperature Difference) = The temperature the evaporator coil should be based on the return temp (A/C 35° below ambient) or below box temp in the case of refrigeration (10° below box on a walk in 20° below box on a reach in). on a typical A/C system with a 75° return temperature the DTD would be 35° which means the target eveporator temperature would be 40°. The DTD will vary based on airflow and evaporator coil size.


Measured Evaporator Temperature or TD (Temperature Difference) = The suction saturation temperature (not pressure) as measured on the suction gauge for that particular refrigerant it can then be compared to box or return temperature to calculate your measured or actual TD


Target Condensing Temperature Over Ambient (CTOA) = This is the target temperature the liquid saturation (condensing temperature) measured on the gauge SHOULD be above the outdoor air temperature measured in the shade entering the condenser coil. This will be 30° over ambient on VERY old units, all the down to as low as 15° on new very high-efficiency units. This is LIQUID LINE ONLY the discharge line will be higher pressure.


Target Condensing Temperature – The outdoor temperature + the CTOA = Target condensing temperature Example: Outdoor Temp 95° + 15° for a 16 SEER system = 110° target condensing temp.


Target Superheat – This is the superheat you SHOULD have and it varies based on if the system has a TXV or Piston metering device. If a piston you MUST use a use a superheat chart as well as a thermo-hygrometer / psychrometer to measure the indoor wet bulb and dry bulb because those charts require those readings. If the system is a TXV then set your target at 5-15° if measured inside and 10° – 20° if measured outside


Measured Superheat – The increase of the suction line temperature when compared to the suction saturation.


Target Subcool – The subcool you wish to achieve. Many units will have this marked on the data tag, if not then use 10° subcool on TXV systems and 5° – 15° on piston systems recognizing that on a piston system this rule will not always apply.


Measured Subcool – The measured difference between the liquid line temperature and the condensing temperature (liquid saturation temp) off of the high side gauge. This is liquid line only, not the discharge line.


Outdoor Ambient – the outdoor dry bulb temperature, in the shade entering near the center of the condenser coil


Return DB – Return dry bulb. The temperature of the return air without taking evaporation or humidity into account. Best taken in the return right before the unit and not in the space.


Return WB – Return wet bulb. The temperature of the return air + the evaporative effect. Lower WB, when compared to DB, means lower relative humidity. Wet bulb and dry bulb will be the same at 100% RH. Best taken in the return right before the unit and not in the space.


Return RH% –  The relative humidity of the air in the return. Relative humidity is the percentage of moisture in the air compared to how much moisture is in the air. Hotter air can hold more moisture than the same air at a lower temperature.


Target Supply Air Temperature – Target supply air temperature is calculated using a delta t chart and comparing the return DB and the return WB temperatures. The target supply air temperature is dry bulb and can be compared to the return DB to calculate the target delta t.


Target Delta T (Air Temp Split) – Don’t confuse TD or Evaporator split above with delta t or air temp split. Keep in mind that the 18° – 22° rule that many use only applies to homes with 45% to 55% relative humidity. As RH% goes up the target split will go down and as RH% goes down the split will go up.


Measured Delta T – The measured difference between the supply and return air DB. Keep in mind this should be taken a few feet before and after the unit to allow for air mixing and reduce radiant gains/losses.


Delta H – Delta H is an advanced measurement that calculates the change in enthalpy (heat content) of the air between the return and the supply. You can do this with two digital thermos-hygrometers like the 605i and it takes into account the temperature and humidity of the air entering and leaving the evaporator.


Delivered Capacity – Delivered capacity is the calculation of BTUs of heat being removed from an air stream which combines the Delta H with the CFM of air to give you the total “work” being done across the evaporator coil.


Discharge Temperature – The measured temperature (with a line clamp) of the discharge line leaving the compressor, not the liquid line


Target Liquid Line Temperature  –  When “checking a system without gauges” the target liquid line temperature is the target condensing temperature minus the target subcool. This is usually measured at the condensing unit.


Target Suction Line Temperature –  When “checking a system without gauges” the target suction temp is the target evaporator temperature plus the target superheat. This is most accurate when measured inside but is also valuable when measured outside.


Approach – Approach is just another name for target liquid line temperature and it a reading that Lennox publishes a target for on many of their units in the installation manual and on the back of the panel of the condensing unit. Systems with larger or more efficient condenser coils tend to have a lower approach (cooler liquid line) while those with smaller, less efficient coils ten to have a higher approach (warmer liquid line)


Suction Line TD (Temperature Rise) – The difference between the suction line temperature inside after the evaporator coil and outside by the condensing unit. When a suction TD is more than 10° compressor overheating and oil carbonization can occur under some load conditions.


Liquid Line TD (Temperature Drop) – The difference between the liquid line temperature outside by the service valve and inside before the metering device. ideally, the liquid would have VERY LITTLE temperature drop and any drop of more than a few degrees should be looked into. Long line length, vertical risers, running the liquid line through a low ambient space, contact between the liquid line and the suction line or restrictions can lead to higher than normal LL TD.


Static Pressure – The positive or negative pressure exerted on all surfaces equally within a duct system. Static pressure does not measure flow, it is like the pressure inside a balloon that inflates or deflates. Static pressure is generally measured in inches of water column in the USA (“wc)


Design TESP – This is the total external static pressure, both positive (supply) and negative (return) that a particular furnace or air handler are designed to work under external to the appliance. Most typical residential units are designed for 0.5”wc.


Measured TESP – This is the total external static measured using a manometer or magnahelic gauge and static pressure tips. On a furnace this would be measured inside the furnace before and after the blower but before the coil. In an air handler or fan coil it will generally be measured before and after the unit in the ducts. This is the total difference between the negative and positive reading so if return static was -0.2” and supply static was +0.3” the total would be 0.5”wc TESP.


Static Pressure Drop – This is the measured pressure change across a portion of the air system. For example across a coil, filter, duct etc… This is helpful in diagnosing air flow issues and changes over time.


While this may seem like a long list, most of it is pretty common sense. One thing to mention is the fact that if you do not have a thermos-hygrometer like the 605i and accurate temp clamps you cannot properly check a Delta T on any system or set the superheat on a fixed orifice system. In order to properly set a charge or diagnose a system, you need a way to accurately test line temperatures and measure return / indoor Wet Bulb, Dry Bulb and Relative Humidity.


Step one on diagnosing a refrigerant issue, checking or setting a charge should be to get an accurate return (or box) DB, WB and RH as well as the outdoor ambient temperature and then working from there taking appropriate readings. When calling a senior tech or your manager please be prepared will all relevant readings to make a quick and correct diagnosis.

— Bryan


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