Unless you’re either uncomfortable, or a mechanical engineer, odds are you don’t think about the temperature of the building you’re in. When you walk into a pizza place for lunch on a hot summer day, you expect to be greeted by a rush of cold air as you pass through the doors and would be disappointed if you weren’t. In winter, if you leave your seat in a cozy coffee shop, you would go from lounging in a t-shirt to needing three layers on before walking out the door. If you go see a movie, you would be slightly upset (and possibly grossed out) if you actually felt the humidity of a couple hundred breaths filling the air.

The comfort you expect is the product of something called HVAC, pronounced H.V.A.C. (not “H-Vac”). It stands for Heating, Ventilation, and Air Conditioning. The goal of this post is to give a quick and dirty overview of why HVAC is needed, how it works, and the equipment used to provide it.

Without HVAC, where you are sitting right now would be either too hot, too cold, or too stuffy to sit long enough to read this article. As the name implies, HVAC systems provide heat in the winter, cooling in the summer, and fresh air throughout the year. The ideal HVAC system will make it so that the only thing someone focuses on when they enter a building is their business, not their temperature. If the HVAC system is doing its job, no one will notice it is there.

While explaining a typical HVAC system, it’s helpful to use some industry lingo. So we’ll start doing that right about… now. In order to make the “occupant”, comfortable in the room, or “space” they are in, an HVAC system will bring in fresh air (with lots of good oxygen) from the outdoors, also known as “outdoor air.” Next, the system mixes the outdoor air with some “return” air from the room that has been made too hot or cold by people, computers, and heat gain or loss through the windows and walls. The HVAC system then heats or cools this “mixed” air to a comfortable temperature (hopefully), and “supplies” that air back into the space.

The supply air is distributed throughout the building through sheet metal tubes called ductwork, and provided to each space from the ductwork by diffusers, those funny looking round or square things generally located next to the lights in the ceiling. At the same time, the system will take the return air, filled with carbon dioxide from everyone breathing, and “exhaust” it out of the building. Getting that mixture of outside air and return air perfect before conditioning it to become supply air is the key to minimizing energy losses while still removing enough CO2 to meet an occupant’s comfort level (and government regulations). Engineers are less-than-happy when people are sleepy from lack of oxygen.

The last part of the HVAC equation is the equipment itself. There are several different types of equipment that can be used to provide HVAC. However, at least for commercial buildings, there are many common components between systems. An Air Handling Unit (AHU) of some sort will use a fan to draw air across a heating coil (gas, electric, or hot water) to heat the air if it is too cold, or a cooling coil (refrigerant or chilled water) to cool the air if it is too hot. This air is then blown through the ducts to diffusers which distribute the air to the space. Another fan draws the now stuffy return air out of the space and mixes it with outdoor air from, well, the outdoors. The HVAC system continues this process of putting conditioned air into the building and taking the stuffy air out of the building until the occupant, or really the thermostat in the space, is satisfied.

That’s it for now. Look for future posts that go through specific components of HVAC systems. At least now you know the terminology that keeps you breathing.