Transferring Heat Efficienctly with HVAC Systems

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"What does an efficiency rating mean?"

When it comes to air conditioning and heating, energy efficiency ratings can come in a wide variety of forms and definitions, often making the concept of HVAC energy efficiency difficult for us to wrap our minds around, but it's important to remember the main goal of any efficiency rating is to describe how well a machine can do it's job. And, when it comes to air conditioning and heating equipment, that mean's how well a machine, like an air conditioner, or heater, can add or remove heat to and from our home's and offices, in turn making them warmer or colder. Consequently, since there are so many different ways to heat up or cool down your home, there are equally as many different energy efficiency ratings.

So many different ways to cool or heat your home!
So let's keep in mind that energy efficiency can certainly be confusing at first when it comes to HVAC, but that's just because there are so many different efficiency ratings in the realm of HVAC, and once you start looking at HVAC from an "energy perspective", the "efficiency" part will all become much clearer. Until then, the subject will definitely feel overwhelming, and you're going to ask yourself, "Why couldn't they have just used one single efficiency rating for everything, like my car and it's fuel economy rating?", and instead you're going to find that some HVAC efficiency ratings are being used exclusively to describe how well a machine like a heater can do it's job, while others are being used exclusively to describe how well a machine like an air conditioner can do it's job. Again, don't let all the different terms confuse you, all an efficiency rating is, is a simple comparison of a machine's input energy to it's output energy, and how a machine being given a specific amount input of energy, can take that energy, and can output that energy into peforming specific a job.

Afterall, that's what machines do right? A machine performs a specific job. That is to say, a machine is given an input of energy, does something with it, and outputs energy into some form of work. Take a car for example, which takes in fuel as an energy source, does something with it, and outputs power into physically rotating wheels that propel us down the highway. Speaking of cars, have you ever wondered how much fuel it takes for you to travel to your place of work and back? Or, how efficient your car is when it comes to using fuel? We call this topic of vehicle fuel efficiency, "Fuel Economy", and it's simply a ratio of how much work the car can do, or how much distance the car can travel when given a specific amount of fuel. HVAC energy efficiency ratings work the same way, and again, are simply ratios that compare the input energy to the output energy of HVAC equipment.

Now, in order to understand just what all these efficiency ratings mean, we first need to understand a few things about how HVAC systems work and what an HVAC system's job is, in the same way you understand what your car's job is. Let's start by discussing what exactly it is all of these different energy efficiency ratings are comparing, which is, well, energy!

Energy Efficiency = Energy Input / Energy Output
Through out these articles we're going to be talking a lot about energy, and we're going to be discussing many different HVAC efficiency ratings that describe energy savings for many different forms of energy that HVAC systems make use of. In fact, this entire series of articles is pretty much one big talk about the different ways heating and cooling machines can convert and make use of energy in one way or another. Which is why there are so many different efficiency ratings - Because there are so many different forms of energy. And more importantly, so many different ways for machines like air conditioners and heaters to make use of those various forms of energy to heat or cool our homes.

Air conditioners and heaters are machines that transfer heat, but did you know that heat is actually a form of energy known as, Thermal Energy? Heat is a form of energy, and it's the primary goal of every HVAC system to either add or remove heat to or from our homes in order to keep us comfortable within them. Therefore, the more efficiently an HVAC system can add or remove heat to and from our homes and do its job, the less we have to pay for our energy bills at the end of the month, kind of like fuel economy when it comes to your vehicle, so it's kind of important.

Energy In / Energy Out
So, let's talk about that for a minute and how these HVAC machines like heaters and air conditioners can simultaneously take in energy in the form of electricity and then somehow output energy in the form of heat, in turn making our homes warmer or colder, and how this heating and cooling process is all just a matter of looking at it from an "energy perspective".

HVAC systems are complex machines that deal with energy in two distinct ways. First, like most machines we use in our everyday lives, HVAC systems require energy in order to operate and do their job, which includes heating or cooling our homes and offices, and the source of this input energy generally (but not always!) comes from electricity. Secondly, the very act of heating or cooling a home or a building, or anything for the matter, is nothing more than adding or removing energy in the form of heat to or from that home or building, or whatever else it is we are heating or cooling. In other words, HVAC systems use energy in the form of electricity to transfer energy in the form of heat, aka thermal energy, in or out of a volume of space, which is what allows that volume of space to become either warmer or colder, and it all has to do with energy and moving it from one place to another.

So, not only do HVAC systems make use of energy in order to work and perform their jobs, but the jobs these systems perform by definition are nothing more than a process of transferring energy in the form of heat from one place to another - A machine powered by energy, that also transfers energy. This means we can take this whole concept of a heater, or and air conditioner (or any machine), and boil it all down to a simple ratio of energy coming into the machine versus the energy coming out of the machine.

Energy In vs Energy Out

Right now, this brings us to an important point, which is, energy can come in many different forms. When it comes to HVAC systems, energy that powers the mechanical and electrical components that make up these machines is typically converted out of electricity. Take an electric heater for example, a machine that we plug into the wall and it heats up our home: The instant the machine is given a specific input of electrical energy, the machine begins to output a specific amount of energy in the form of heat, and slowly warm up our home, that is to say, it simply converted one form of energy (Electricity), into another (Heat).

Since HVAC systems deal with lots and lots of energy, you'll often find that people have come up with different ways to cool or heat our homes, some ways being more efficient than others. This consequently means that there has to be different efficiency ratios in order to describe each of these different efficiency ratings. For example, the electric heater we described above may have one efficiency rating that describes how efficient the machine is at converting electricity into heat, while another machine that utilizes another energy source, such as natural gas heater, will have another efficiency rating which describes how efficient the machine is at converting natural gas into heat, and an air conditioner another, and so on. It's kind of like how a vehicle that runs on fuel has a different efficiency rating when compared to an all-electric vehicle.

So the topic of concern in this article then, is definitely energy, and more importantly, thermal energy, or heat energy, and how we move it from place to place through the use of an HVAC system. It's true, energy is everywhere and it comes in many different forms, all of which can be converted from one to the other. Some energy is free, while other forms of energy can turn out to be pretty expensive to make use of. Take the human body for example (because our bodies happen to use energy too), when we eat food, that food is broken down and converted into a form of energy known as the calorie, and this energy is then used to power all the cells that make up our entire body. Motor vehicles convert energy out of gasoline, which is what allows their engines to operate and their wheels to spin so that we can travel vast distances. The Sun puts out vast amounts of many different types of energy, including light, which is then converted into energy by plants in order for them to grow and produce fruit, and the sun also emits vast amounts of energy as heat, which bathes the Earth and keeps our planet warm and inhabitable (and often our electric bills really high from running an inefficient AC system!).

Since HVAC systems not only use energy in order to operate, but they also transfer energy in the form of heat in order to warm or cool our homes as a function of their operation, a ratio that compares the machine's energy input to it's energy output can be expressed, and that's exactly what an HVAC energy efficiency rating happens to be. Since HVAC systems output energy in the form of heat, or transfer energy in the form of heat, let's first start off by talking about heat energy, or thermal energy, since that's primarily what we're going to be dealing with when it comes to HVAC efficiency ratings. So other than describing something as "Hotter" or "Colder" than something else, just how do we measure this thermal heat energy stuff? What is heat?

A British Thermal Unit
Introducing the British Thermal Unit (BTU) - When we talk about HVAC equipment, we are of course talking about machines that make something hotter or colder, that is to say, we are talking about machines that transfer heat from one thing to another, or machines that add or remove energy in the form of heat from one place to another, and in essense a machine that is adding or removing BTUs of heat from one place to another.

In cooling mode, HVAC systems remove heat from our homes.

So, when an HVAC system transfers heat away from the air in a home, and removes heat from a home's air (such as when the air conditioner is operating), heat is being removed from the home as the air conditioner transfers heat out of the air within the home, and in to the air outside of the home. Since during this process, heat, or BTUs of heat energy, are being taken out of and removed from the the home's air and transferred outside of the home by the air conditioner, the air inside the home soon lacks thermal energy and consequently feels "Colder". When enough BTUs have been removed from the home's air, you may begin to see a thermometer drop degree by degree as the air in your home continuously loses thermal energy, and finally drops in temperature.

In heating mode, HVAC systems add heat to our homes.

The exact opposite is true with a furnace or the heating function of an HVAC system. For example, a machine called a natural gas furnace, adds heat, or BTUs of heat, to a homes air by combusting natural gas, in turn converting the natural gas into heat, and adding this heat into the home's air. Since BTUs of heat are being added to the home's air by the furnace, the air inside of the home soon begins to feel "Warmer" because it contains more thermal energy, and once enough BTUs of heat are added to your home's air, you may begin to see a thermomter rise degree by degree, as your home slowly rises in temperature.

This means that whenever we are adding or removing heat from the air within our home's by use of the HVAC system, we are simply adding or removing British Thermal Units of heat energy, or thermal energy, to or from the air, and if we knew exactly how many BTUs of heat we were adding or removing at any given moment, we could then describe this Heating or Cooling process in great detail and beyond that of simply saying the home is getting "Warmer" or "Colder". So, from now on, instead of simply thinking in terms of "Hotter" and "Colder", when we talk about something that is getting warmer, such as the air within our homes when the heater is operating, we should think in terms of BTUs increasing, or being added to that object (in this case, the home's air), and when something is getting colder, we should think in terms of BTUs decreasing, or being removed from that object. Simple, right?

Cold is simply the lack of thermal energy.

Adding thermal energy to air will increase it's temperature.

Removing thermal energy from air will decrease it's temperature.

"So, if Heat energy can be measured in units called British Thermal Units (BTUs), and adding BTUs to something makes that something "Hotter", and removing BTUs from that something then makes that something "Colder", then what exactly is a BTU? Why can't we just stick to what we're all used to and describe temperature in terms of degrees fahrenheit or celsius?"

There's actually a big difference between temperature and thermal energy (heat). Temperature level, which is measured in degrees fahrenheit, tells us how "Hot" or "Cold" something is, but it does not tell us how much thermal energy that object contains. The best way to visualize the difference between the concepts of thermal energy and temperature are with an example. So, for example, the boiling temperature of water is 212°F and this fact will remain the same whether we boil 1 gallon of water or 1000 gallons of water. But, the amount of heat, or thermal energy required to boil 1 gallon of water is much less when compared to the amount of thermal energy required to boil 1000 gallons of water, which is much greater.

"Alright, that makes sense about temperature, but what exactly is a BTU again?"

So, as previously stated, when it comes to HVAC equipment and since we're constantly dealing with a machine that can move heat energy from one space to another, we need a basic unit of measurment that we can all agree on as 1 unit of heat energy, and in the field of HVAC in the US a single unit of heat happens to be called a British Thermal Unit (BTU).

The BTU is a single unit measurment of heat energy, AKA thermal energy, and the BTU is actually defined as the amount of heat energy that is required to raise the temperature of 1 pound of pure water by 1 degree fahrenheit. A BTU can also inversely be defined as the amount of heat that is required to be removed from 1 pound of pure water to lower it's temperature by 1 degree fahrenheit. It's simply our basic measurement of heat energy, like a mile is a basic measurment of distance for the vehicles we travel within, and although the BTU has been replaced by the metric system in most parts of the world (which now measures heat in metric units called "Calories") it's still standard in the United States' imperial measurment system, and therefore still used within the US field of HVAC.

So, just like the cities we inhabit, which may span several miles wide, and states that span many more miles wide than those cities; A single room within your home will have a much smaller volume of air we could heat up or cool down when compared to the entire volume of air contained within your entire home. The hotter the air in your home, the more BTUs of heat energy we could count inside the volume of your home's air. And, for example, if you have two rooms inside your home that are the same size, and one of those rooms happens to feel warmer than the other, then we know there are more BTUs of heat inside the room that feels warm versus the room that feels cool. It's an air conditioner's job to remove these BTUs of heat from your home and make your home colder, and it's a heater's job to add more BTUs of heat to your home, and make your home hotter - It all really is just a matter of transferring energy, which consequently takes time. Just like a vehicle takes time as it travels mile after mile, burning gallon after gallon of gasoline, the HVAC system performs it's heating and cooling process over a period of time, except it uses energy in one form or another to transfer heat over time where a vehicle uses energy in one form or another to transfer people over time.

Heat Transfer Diagram
Since we have a standard measurment for a single unit of heat energy, which we call the BTU, and since we have a standard measurement for a single unit of distance, which we call the mile, we need a standard way of measuring the way HVAC equipment moves this unit of heat from one place to another over a period of time. That is to say, we need a way to measure the rate at which a unit of heat (BTUs) can be transfered over a period of time (Hours) by an HVAC system, AKA BTUs per Hour (BTUs/H). Think of this new heat energy per hour (BTU/H) transfer rate in the same way that we have miles, and we have vehicles that travel at a rate of miles per hour, except instead of dealing with miles (M) and miles per hour (MPH) of which vehicles can travel, we're dealing with heat (BTUs) and the rate at which HVAC equipment transfers that heat per hour (BTUs/H).

1 BTU Per Hour

So right now we have BTUs (heat), and we also have BTUs/H (heat per hour), but it's time to take this a step further because the BTU is a measurment of heat that is based off heating up water, and as it turns out, Air Conditioners don't add or remove heat from water now do they? (Actually, sometimes they do, but for now let's assume they don't!) They add or remove heat from Air, which kind of changes things.

A BTU is a very small amount of heat when it comes to the amount of heat we deal with when we are concerning ourselves with air. Infact, the air in our homes contains thousands and thousands of BTUs of heat which makes for some pretty confusing math if we stick to measuring it BTU by BTU, hour by hour. What we need to do now is simplify and create a new measurment and essentially scale BTUs per hour down to a reasonable level that makes more sense when we deal with HVAC equipment that is transferring the thousands and thousands of BTUs of heat that are contained inside of the air within our homes and offices. Think of it in the same way that we have inches, feet, and miles, which we use depending on the length of the distance we are describing. When we are dealing with larger distances, such as the distance across the state of Texas, we don't measure it in inches do we? I guess technically we can, but why measure in smaller units when we can simplify things and spare ourselves the headache? So instead of inches, we need a better and more reasonable unit to measure such large distances, such as the mile, and this same concept applies to when we are measuring large quantities of heat that HVAC equipment transfer from one space to another. Why measure in inches when we can measure in miles? Why measure in BTUs when we can measure in something else? That something else is called: the Ton.

1 Ton Equation
Introducing the Ton - Have you ever heard someone mention the term "Ton" before when referring to an air conditioning system? You've probably have heard this term, "Ton", before from your AC guy when you're both discussing your own HVAC system, but what does it mean? Did you end up asking yourself: "Does this guy really think my air conditioner weighs a ton? Does he have any clue what hes talking about?". Not at all! Infact, what you're actually referring to when you talk about "Tonnage" of air conditioning is how much heat (BTUs) the air conditioning system is capable of moving from inside your home, to outside of your home, within a period of time (BTUs/Hr), it's simply simplified because we are dealing with such large quantities of heat energy.

A speedometer
If we look back at the car and distance analogy, think of Tonnage in the exact same way as we think of Miles Per Hour (MPH) on our speedometers - When you look at the speedometer in your vehicle, you may notice that the engineers who designed your vehicle have chosen to display the rate at which you travel (speed) in terms of "Miles Per Hour" instead of "Inches Per Hour", why? Because it's much more practical. It's the same deal with the term "Tonnage" or "Ton", which is describing a much larger quantity of BTUs Per Hour, 12,000 BTUs Per Hour to be exact.

Just like in the vehicle scenario, where we are usually always traveling at such a fast speed that measuring our speed in units of Inchs Per Hour seems impractical - In an air conditioning in heating scenario, measuring the speed at which these machines transfer heat energy is very impractical if we use BTUs Per Hour, so we coined the term "Tonnage" or "Ton" of air conditioning power.

Technically, a Ton is actually describing a machine that can transfer heat at a rate of 12,000 BTUs per hour. Why do they call it a "Ton"? Well, moving 12,000 BTUs per Hour (1 Ton of Air Conditioning), over the course of 24 hours happens to add up to the same amount of heat required to melt 2000lbs (a Ton) of pure ice at 32 degrees Fahrenheit, for a whopping total of 288,000 BTUs (Hints the term, "Ton")! 288,000 BTUs is not only a big number, it's an ugly number and noone likes it, but it's exactly what a Ton of Air Conditioning adds up to. This is a prime example of why it's much more realistic that we speak in terms Tons of Air Conditioning, and in this case instead of saying: "This air conditioner is capable of transfering 12,000 BTUs per Hour or 288,000 BTUs a Day" we simply say: "This is a 1 Ton Air Conditioner". Simple right? Why measure in inches? The guys in the early ice trade business actually coined the term "Ton" way back in the days when they used to mine, store and transfer tons of ice across the country using horses back in the 1880's.

A 1 ton ice cube

A 1 Ton Ice Cube

So if you have a 3 ton air conditioner, that means the air conditioner is capable of moving 36,000 BTUs (12,000 multiplied by 3) of heat per hour, for a total of 864,000 BTUs per day (36,000 multiplied by 24), which is exactly how much heat is required to melt 3 tons of ice! I bet you didn't know that tiny 3 ton air conditioner you may have sitting outside is capable of transfering the same amount of heat that would cause 6000lbs of ice to melt in a single day! That's a ton of energy (3 actually)! And it's no wonder these air conditioning machines make up the majority of our utility bills when they are constantly transfering such large quantities of energy in the form of heat.

So, now we have covered how to measure heat (BTUs), and also how to measure the rate at which air conditioning systems transfer heat per hour (12,000 BTUs per Hour) - AKA the "Ton". This is how HVAC equipment power is measured - Based off a machines ability to transfer the same amount of heat energy that is required to melt 2000lbs of ice in a single day. Now, let's take a step back and think about how this all relates to energy, and more importantly our electric bills. Think about this for a moment: Most homes with central air conditioning and heating contain HVAC systems that are 2-5 Tons. Many homes have several HVAC systems this size. Now, for all the tons of air conditioning power your home has, for each ton, try to imagine a 2000lb block of ice melting - That's a ton of ice melting and that's a ton of energy being used by an HVAC system! Now, since no machine is perfect at doing it's job (No machine is 100% efficient), then air conditioners and heaters are also not perfect, and they too must also consume more input energy when compared to their output energy, and in turn take in more energy in the form of electricity, and output less energy in the form of heat. This is why energy efficiency is so important when it comes to the field of HVAC - Because in the field of HVAC we are literally dealing with moving large quantities of energy from one place to another, and consequently, using more and more energy every single time a new air conditioner is born. Imagine if you will, all of the new homes being built every single day along with all the new air conditioners being installed in these new homes - Now, its easy to see the HUGE demand all these HVAC systems put on the USA's power grid. Think about it: Every single ton of air conditioning power (remember the 2000lb block of ice melting) across the entire United States adds up to a huge demand on the US electrical grid, and it's the main reason we have efficiency laws that are constantly evolving and demanding more and more out of engineers in order to produce better HVAC machines with a better efficiency rating than before - In order to conserve energy.

Final Thoughts on Heat and Energy

So now hopefully we have a good basic understanding of heat and energy and how machines like air conditioners convert that energy into heating and cooling the air within our homes. From here-on-out we're going to be making a bunch of references to this basic understanding of transfering heat with machines using energy as we begin to discuss the differences between the many different HVAC energy efficiency ratings out there and how we can translate those energy efficiency ratings into real life cost savings.

Remember, energy can come in many different forms, and HVAC systems essentially all make use of energy in one form or another (electricity, natural gas, wood, oil, or propane) in order to transfer energy in the form of heat, in and out of our homes. From now on, each of the efficiency ratings we're going to be discussing should be looked at in one way or another as a description of the percentage of energy that is being saved during a piece of HVAC equipment's operation, no matter what form or source that energy comes in or from.

Important things to remember about Energy
  • Remember: Energy comes in many different forms, from electricity, to heat, to natural gas, and even calories in our food.

  • Machines (like air conditioners) all have an energy efficiency rating, or a ratio that compares their input energy to their output energy.

  • HVAC systems use many different forms of energy in order to transfer energy in the form of heat, into or out of our homes.

  • A single unit of heat is called the British Thermal Unit (BTU).

  • HVAC systems transfer heat over time, or BTUs/Hr, in the same way vehicles travel distance over time, or Miles Per Hour.

  • Transfering 12,000 BTUs Per Hour, is called 1 Ton of Air Conditioning.

  • As of today, there is no machine that is perfect or 100% efficient or better - There is no machine that can make something out of nothing, and there is no machine that can convert all of it's input energy into it's output energy, which is why saving energy is important.