5.9 HVAC Systems and Components. HVAC Systems. Perimeter Outside Air Ventilation Systems.Perimeter ventilation units shall be self-contained DX package units or air-handling units with fan section having variable speed drive, chilled water cooling coil, hot water heating coil, enthalpy heat recovery wheel, or desiccant wheel and supply air filtration. BASIC Technical Description of the BASIC Air Handling System Modular System The BASIC is designed for comfort ventilation applica-tions. Depending on the type of heat exchanger selected, the BASIC units can be utilised in buildings such as office buildings, schools, day nurseries, public buildings, shops, residential buildings, etc.

Our theme for this month’s blogs is “Things Techs Wish People Knew About HVAC,” and that starts with a very basic understanding about how air conditioning works.

Why should you know or care how AC works, as long as it keeps your space cool?

The short answer is:

• You should know how AC works so you understand what’s needed to keep it working efficiently and reliably
• You’re more likely to notice sooner when something’s not working right, and call for help. In many cases, that awareness can turn small problems from turning into bigger (and more expensive) problems.

So, here’s a basic primer that explains how the most common air-cooled systems & how air conditioning works.

What does air-cooled mean?

Let’s start our primer on how AC works with what we mean by “air cooled.” This means the air conditioning system uses a chemical substance known as a refrigerant to remove heat from the air. (You might have heard of Freon, an older type of refrigerant that’s actually being phased out, but still used in many older systems.) Most homes and light commercial spaces have air-cooled air conditioning systems.

The other type, water-cooled air conditioning systems, are central systems found primarily in large buildings. The difference is, instead of using refrigerant to remove heat, the system uses water.

In this article, we’ll focus on the more common air-cooled system and how that AC works.

How AC works: What refrigerant does

In an air-cooled AC system, refrigerant is central to the process. Refrigerants are chemicals that have an interesting and useful property: the can be converted from a gas to a liquid (and back again) relatively easily. As the refrigerant changes form (as well as temperature and pressure) it is capable of both absorbing heat and releasing it. Refrigerant is added when your system is installed, and cycles continuously (in various forms) through the components of your AC unit in a closed loop throughout the life of your system.

It’s a bit complex, but here’s the part you should know about how AC works.

Your air conditioner is designed to take advantage of refrigerant’s properties to absorb excess heat and moisture from the air inside your space, and then release it outside the building. That’s why your air conditioner probably has both indoor and outdoor components. The compressor and condenser are outside, and the evaporator/air handler is located inside.

(We say probably because if you’re in a big city like NYC, you may not have access to outside space. In that case, your “outdoor” unit may be housed in a mechanical room that has louvers to provide access to outside air, which is an important part of how AC works.)

How AC works: The process and most important components

The process of cooling the air in your space is a continuous cycle (or as we Disney fans among the HVAC community like to say, the “Circle of Cooling Life”). So it’s hard to say exactly where it starts.

So for practical purposes (since this is the part you care most about!) let’s start with this part of how AC works: removing the heat from the air inside your space.

That happens at the evaporator coil that’s part of your indoor unit. It looks like a long, coiled metal tube or hose covered with small fins. The refrigerant is a high-pressure liquid when it reaches the evaporator coil. As it goes travels into the coil, it becomes a gas and removes heat from the surrounding air in the process. The structure of the coil with all those fins provides a lot of surface area for air to move over the coils so that heat can be absorbed. Fans also help the process and are essential to how AC works (which we’ll get to in a minute).

When it leaves the evaporator coil, the refrigerant is now a cool, low-pressure gas, which makes its way to the compressor.

The compressor is often called the “heart” of the air conditioner. It’s a great metaphor, because it’s the compressor’s job to “pump” the refrigerant through the system, and because the compressor is critical to it’s survival and how AC works.

Read more about it here: 8 Preventable Causes of AC Compressor Failure

The compressor literally “compresses” the low-pressure gas refrigerant into a hot, high-pressure gas, which then flows into the condenser.

Your AC has another “coil” in the outdoor unit called the condenser coil. This structure (again, along with the help of fans) releases the absorbed heat, where it dissipates into the air outside.

How AC works: Air flow helps

Those evaporator and condenser coils have important work to do (absorbing and releasing heat), but they need some help to get it done. That’s another important part of how AC works: your indoor and outdoor units have fans. The outdoor fan blows air over the condenser coils to help release heat. The indoor fans (part of the indoor air handling unit), move air over the evaporator coils and blow cooled air into your ducts and ultimately into the rooms of your home or work spaces.

Air flow is one of the most critically important aspects of how your air conditioning works.

How AC works: Removing moisture

It’s not just the heat, it’s the humidity that makes you uncomfortable. The way the refrigerant removes heat is actually by removing moisture from the air, which accumulates on the cold evaporator coil, then flows down through condensation lines to a collection pan and is drained away.

Why is it smart to understand how AC works?

Besides the reasons to know how AC works that we mentioned at the beginning of this article, there’s another important one. Unfortunately, there are unscrupulous and just plain unqualified HVAC vendors out there. You can avoid being taken for a ride when you have a basic understanding of how AC works.

Here’s one example, and it’s a common one. You notice that your AC has not been cooling as well as it used to. It’s not a sudden breakdown; it seems to be happening gradually. You call in someone to take a look, and they give you what seems to be good news: the problem is only a low level of refrigerant. You can just add more; it’s quick and not too expensive.

Read more about refrigerant leaks: AC Troubleshooting: Refrigerant Leak

Wait a second! You just learned that refrigerant cycles through your system in a closed loop. So why is it low? That happens when your system develops a refrigerant leak. If you just add more without fixing the leak, the problem will only happen again. Certain vendors might neglect to mention that.

If it’s happened to you, it’s time to think about switching vendors. That may seem like a hassle, but not if you know the right way to go about it. Learn more from this insightful guide to Contract Confidence: Transitioning to a New HVAC Service Provider.

Here’s what’s up next in our series on what techs wish people knew about air conditioning: 12 reasons your AC is not cooling your NYC business. Don’t miss it!

What are Air Handling Units or Fan Coil Units?

The air handling units are very important parts of the central air conditioning plants, packaged air conditioning plants and also the roof mounted split air conditioning systems. As the name suggests air handling unit is the box type of unit that handles the room air. It comprises of the cooling coil over which the hot return air from the room flows, gets cooled and flows back to the room to cool it. The circulation of the air is carried out by the blower. The filter in the air handling unit enables cleaning of the air. The air handling units are also called as fan coil units in case of roof mounted split AC units, since they comprise of the fan and the cooling coil.

The air handling units are installed at the different places in the building to be air conditioned. They are connected to the cool air supply and return air ducts which are laid in all the rooms to be cooled. In case of the central air conditioning plants the air handling units are installed on the floor, while in case of the split air conditioners, they are mounted on the roof inside the room above the false ceiling. In case of packaged units they can be installed on the floor or the roof.

The various parts of the air handling unit and their working are discussed in details below:

Air Handling Units

1) Box Enclosure:

All the parts of the air handling unit are enclosed in the box type of enclosure. This ensures compactness of the unit and protection of all the components inside it. The whole box is insulated to prevent the loss of heat from the unit.

2) Cooling Coil:

The cooling is one of the most important parts of the air handling units. It is made up of copper tubing of several turns and covered with the fins to increase the heat transfer efficiency of the cooling coil. In direct expansion (DX) type of the central air conditioning plants the refrigerant flows though the cooling coil, which also acts as the evaporator of the plant. In case of the chilled water system the chilled water from the chiller flows through the cooling coil. In packaged and the split air conditioners also the refrigerant flows directly through the cooling coil, thus acting as the evaporator of the system.

The hot return air from the room flows over the cooling and gets cooled. This air is supplied to various rooms via the ducts. The flow of chilled water or the refrigerant to the cooling coil is controlled by the solenoid valve.

3) Ducts:

The air handling is connected to the supply air and return air ducts. The supply air duct supplies the cool air from the air handling unit to various rooms, while the return air supplies hot return air from various rooms back to the air handling unit. There is one main supply duct that bifurcates into various small ducting that are laid in all the rooms that are to be cooled. The return ducts from all the rooms also end into one main duct. All the supply and return air ducts are covered with the insulation material to prevent the loss of the cooling effect. The ducts are designed and laid such that there is even distribution of the cooled air to the rooms as per their heat loads and there is least wastage of the cooling effect.

3) Fan or Blower:

The fan or the blower sucks the hot return air from the room and blows it over cooling coil, cools it and sends it to the room to be air conditioned. There are two possible arrangements of the fans in air handling units: draw though arrangement and blow through arrangement. In the draw through arrangement the fan sucks the return air through the filter and the cooling coil. As the air passes over the cooling coil its gets chilled, it is then passed to the rooms to be cooled. In case of the blow through arrangement the fan absorbs the return air and blows it over the air filter and the cooling coil. The air then flows to the rooms to be air conditioned. The draw through arrangement is used more commonly due to its compactness. The fans are of centrifugal types.