Air Conditioning and HVAC Basics



This explanation of air conditioning and HVAC basics will expand a little bit on the air conditioning cooling process.


The first topic to discuss about air conditioning and HVAC basics will be the laws of thermodynamics that describe how heat moves.

The laws of thermodynamics state that:

1. Heat is a form of energy, and
2. Heat energy will move from a place of high intensity towards a place of lower intensity.

Kind of like water flowing down-hill, heat energy will naturally and automatically move from a hot place to a cooler place.

You don't have to do anything to make it happen.

Just like you don't have to do anything to a natural magnet to make it work according to the laws of magnetism; you don't have to do anything to heat to make it move from hot to cold.


The second topic of discussion about air conditioning and HVAC basics will be about the types of heat: sensible heat and latent heat.


Sensible heat describes how hot something feels; and you measure it with a thermometer.

Sensible heat is measured in degrees C, F, or K.

Latent heat measures the quantity of heat energy in a substance.

With regards to air conditioning and refrigeration, we need to understand that latent heat is the heat that a substance absorbs or releases as it warms, cools, and changes state (evaporates and condenses).

Latent heat is measured in British Thermal Units, or btu.

When latent heat energy is added to a substance, or when a substance absorbs latent heat, the substance will feel "warmer".

When latent heat energy is removed from a substance, or when a substance releases latent heat, the substance will feel "cooler".


1 lb. of water is a standard reference point for latent heat.

It takes 1 btu of latent heat to raise or lower the temperature of 1 lb of water 1 degree F.

So if 1 lb of water is at 45 degrees F, and warms to 50 degrees F, it will have absorbed 5 btu of latent heat.

And if that 1 lb of water is warmed to 212 degrees F, it will have absorbed 162 more btu of latent heat.


And now we're at 212 degrees F, the boiling point of water, and something significant will happen that is an extremely important aspect of air conditioning and HVAC basics.


By the time the 1 lb of water has totally evaporated, it will have absorbed 970 btu, which is almost 1,000 times more heat energy than it absorbed warming from 211 to 212 degrees.

970 btu is the latent heat of evaporation of water, or, the amount of heat energy required to evaporate 1 lb of water.


And when the water vapor, or steam, is cooled back down, it will release that 970 btu of latent heat as it condenses from steam to water.


Latent heat moves the same way in an air conditioning and refrigeration system, which is why we have evaporators and condensers in the systems,

So you can see that it is an essential topic when discussing air conditioning and HVAC basics.


The evaporator is the place in the system where the refrigerant will absorb immense quantities of latent heat as it evaporates.

It's the "cooling" coil, and absorbs latent heat from the air or water that flows over it or through it.

The condenser is the place in the system where the refrigerant will release that latent heat as it cools and condenses.

It will release the latent heat to the air or water that flows over it or through it.


The third topic of air conditioning and HVAC basics will be the relationship of pressure to temperature in a sealed system; a sealed container, or refrigerant piping circuit.


In a sealed system, pressure and temperature follow each other.

If pressure rises, temperature rises. If pressure falls, temperature falls.

If temperature rises, pressure rises, and if temperature falls, pressure falls.


Think of pressure as a lady named Priscilla Pressure, and temperature as a man named Tommy Temperature.

They're madly in love, and wherever one goes, the other follows.

If Priscilla climbs a hill, Tommy follows.

And if Tommy skis in Aspen, Priscilla is skiing down the slope side by side with him.


In a sealed system, Tommy Temperature and Priscilla Pressure walk side by side, hand in hand; and wherever one goes, the other follows.

I emphasize the pressure temperature relationship because it is another crucial point to comprehend when discussing air conditioning and HVAC basics.


Because of this relationship, a technician can tell what's happening inside the refrigerant piping by attatching a set of gauges to the system.

One gauge reads the low side, or suction pressure; and the other gauge reads the high side, or discharge pressure.


The pressures and temperatures that the gauges indicate are another important topic when discussing air conditioning and HVAC basics.

The low side gauge is actually reading the pressure of the entire low side of the system, which is from the outlet of the metering device back to the suction valve of the compressor.

The way I think of this is; I imagine the evaporator being a glass container, with the metering device feeding refrigerant liquid in at the bottom, and the suction line returning refrigerant vapor from the top of the container back to the compressor.


When the unit is running, and conditions are normal, refrigerant will be feeding into this container, and will fill it to about 1/2 full, so I imagine it as about 1/2 full and boiling away, with the boiled off vapor returning to the compressor.


And I imagine that the low side gauge is reading the pressure in the glass container exactly at the top surface of the boiling refrigerant.

So the temperature that the gauge or a pressure/temperature chart will indicate is the temperature right there at the boiling top surface of the refrigerant.


This is the saturated suction pressure/temperature, and in an R 22 air conditioning system it is normally around 40 degrees, which is equivalent to 68.5 psi.


The high side gauge is reading the pressure of the entire high side of the system, which is from the discharge valve of the compressor all the way to the metering device.


The way I think of this is; I imagine the condenser being a glass container, with the discharge gas flowing into the top of the container, and the condensed liquid refrigerant flowing out the liquid line at the bottom of the container.


When the unit is running, and conditions are normal, refrigerant vapor will be flowing into this container, cooling and condensing into liquid, and will fill it to about 1/2 full, so I imagine it as about 1/2 full, with the condensed liquid flowing out the liquid line at the bottom.


And I imagine that the high side gauge is reading the pressure in the glass container exactly at the top surface of the condensed refrigerant.

So the temperature that the gauge or a pressure/temperature chart will indicate is the temperature right there at the top surface of the condensed refrigerant.


This is the saturated discharge pressure/temperature, and in an R 22 air conditioning system it should normally be equivalent to ambient outdoor temperature plus 20 to 30 degrees.


In this section of our discussion of air conditioning and HVAC basics, let's describe how heat moves in an air conditioning system.

Lets say that the air to be cooled is at 80 degrees when it starts to flow accross the evaporator.

Since the refrigerant is evaporating at 40 degrees, the heat in the 80 degree air will flow into the 40 degree refrigerant, cool air at about 60 to 65 degrees will flow out of the evaporator, and the refrigerant will totally evaporate.


Lets also say that the outdoor air flowing accross the condenser is 85 degrees.

The refrigerant vapor will be condensing at about 110 to 120 degrees, so the heat in the refrigerant will move into the 80 degree air, the air will leave the condenser at about 105 to 115 degrees, and the refrigerant will have totally condensed into liquid by the time it leaves the condenser.


The fifth topic of air conditioning and HVAC basics will be compressors.

The compressor is the pump that causes the refrigerant to circulate through the system.


The compressor is rated to pump a set volume of vapor, so it will have a set capacity or btu rating, dependng on the refrigerant being used, and the operating temperature in the evaporator.


The compressor may be a reciprocating type, rotary type, screw type, or centrifugal type; depending on the capacity of the unit, and whatever design advantages the engineers might have been trying to optimize for the unit.


The sixth topic of air conditioning and HVAC basics will be the evaporator.

In the evaporator, the refrigerant is evaporating at a relatively low pressure, and as it does, it is absorbing latent heat from the air flowing over the evaporator piping.

The evaporator is the coil that cools the air in an air conditioning system, and it will have a blower to make the air flow.


The seventh topic of air conditioning and HVAC basics will be the condenser.

In the condenser, the refrigerant is condensing at a relatively high pressure, and as it does, the latent heat absorbed in the evaporator is released to the relatively cool air or water that is flowing over the condenser piping.

The condenser is the unit usually placed outside the building, and when running in the cooling mode, relatively hot air will be blowing out of it.


The eighth topic of air conditioning and HVAC basics will be the thermostat.

The thermostat is the switch that turns the unit on to cool or heat the space.


If you are a home or business owner, and your air conditioning isn't running, make sure the thermostat is actually turned on before you call for service.

I have been on service calls, including expensive 1 am emergency overtime calls, where the only problem was that the thermostat was not turned on to "cool".


I have to admit, I have also been on service calls where I opened the unit and started tracing out wiring only to find out that yes, you guessed it, the thermostat was "off", and I hadn't checked it before starting work.


I hope this introduction to air conditioning and HVAC basics will help you understand how the system works.

This is exactly the same information that I discuss in deep detail when teaching air conditioning and HVAC basics in class and on the job.

These are a few of the topics that any air conditioning service technician must understand very clearly to be able to trouble shoot a system efficiently.


You might be interested in our Introduction to Refrigeration and Air Conditioning e book.

It's 2 chapters reproduced from a U S Military training manual; and discusses refrigeration, and air conditioning and HVAC basics.

It's about 100 pages, so it might take several minutes to download, depending on your internet access.


If you're looking for troubleshooting information, you'll find some tips on troubleshooting air conditioning, refrigeration, and chiller systems in our System Evaluation Guide.

It's a PDF file, so you'll need Acrobat Reader to open it.


Otherwise, a good hardcopy HVAC Book will go into all the details about these air conditioning and HVAC basics.


I hope this page has helped, and please, feel free to contact us with any specific HVAC questions you might have, including questions about air conditioning on Guam, and refrigeration on Guam.

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Requesting the information is easy and only takes 2 minutes; and it might be the first step towards changing your whole future.
You'll find a brief review of the course on our HVAC Training page.

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