∫ HVAC Basics

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.

6 Responses to ∫ HVAC Basics

  1. Nice article that concisely describes the essentials of the HVAC system. One key thing I learned was that it is not pronounced “H-vac” among other key things. Often we tend to forget that humidity impact and the role of ventilation in feeling comfortable.

    • I heard keeping the filters clean has a big impact on performance. I would love to hear more about this concept. :) This was a great blog! Very detailed!

      • Raphael says:

        Good question about filters. Filters are important for many reasons, not least of which is ensuring indoor air quality so that we can breathe air free of unwanted particulates.

        Air filters and energy are actually quite complicated. How much energy is used for ventilation depends upon the power draw of a fan. The relationship between fan power and filters depends upon whether a system is CAV (constant air volume) or VAV (variable air volume). In a CAV system, flow will usually drop as the air filters get dirtier which can save energy. In VAV systems, the energy impact is more complicated but the fan may ramp up as the filters become dirty, increasing energy usage. In each of those cases, however, total energy used must factor in heating and cooling loads as well as flow. Lower flow may save energy but may not be good for equipment or comfort.

        In many cases, changing filters is not an energy saving measure. In others it is. In all cases, it is important for ensuring indoor air quality.

  2. Great explanation of HVAC systems. What would you recommend as the #1 action people can take to insure they have an efficiently running HVAC system?

    • An oldie but goodie: turn it off at night. Even if a building thinks it is doing this and/or has mechanical time-clocks to ensure the HVAC turns off at night, we frequently find equipment that runs 24/7 because a time-clock is broken or there is a disconnect between what the building management system thinks it is doing and what is actually happening in a building.

      For example, we recently worked in an office building where cooling tower fans ran 24/7 because the BMS was turning off exhaust fans that were labeled as cooling tower fans. We also worked in another building with 9 mechanical time clocks controlling equipment, 5 of which were not functioning. Unless you have real data logged from the piece of equipment you are considering (or a fanatically dedicated building chief willing to spend Saturday at 2am on the roof), you can’t be sure equipment is actually turning off. And 50% savings are exciting.



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