What is HVAC?

What is an HVAC system?

Modern HVAC systems combine all 3 functions into 1 high-efficiency air-handling machine. A typical residential furnace can send heated, cooled or simple fresh air through a group of closed plenums (or chambers) and ducts to maintain your desired air temperature in every room of your house.

 

HVAC maintenance and safety tips

• Schedule annual checkups for both heating and air conditioning systems.
• Clean or change air filters at least every 3 months.
• Never block radiators or registers and grilles with clothing or furniture.
• Don't try to service your home HVAC system unless you’re qualified to do so.

Did you know? Look for a certified HVAC technician to work on your system. Certification requirements vary from state to state; there is no national standard. The Environmental Protection Agency (EPA) and North American Technician Excellence (NATE) are among the groups that offer certification programs.

 

Heating systems

Until the 20th century, heating was given far more emphasis than air conditioning. The simplest kind of heat is supplied by wood burning in an open fire pit—an approach still preferred by camping enthusiasts everywhere. Here’s a quick review of heating’s evolution since then.

Hearth and flue damper. When you raise and enclose a wood fire and provide a clear path for the smoke, you gain far more control (and safety) over the process. By the Middle Ages, castles and manor homes used a dozen or more fireplaces for heating. Fireplaces, however, rank very low in efficiency (heat gained versus fuel consumed).

Freestanding wood stove. You might think of Ben Franklin when we talk about wood stoves, but inventors had been tinkering with better fireplace designs at least 100 years before Franklin’s design emerged. The keys to increasing efficiency included using metal enclosures and flue channels or baffles to better hold the heat produced. During the energy crisis of the late 20th century, we saw a resurgence in the popularity of wood stoves for home heating.

Coal furnace. Coal burns longer and makes more heat per pound than wood, so finding ways to use coal for home heating made perfect sense. During the 1800s, the development of coal-fired boilers (such as those used in steam locomotives) and cast-iron radiators led to the first true “central heating” systems. Near the end of the 19th century, Dave Lennox introduced the forerunner of our modern furnaces. Those early coal-fueled boxes, however, used convection (“warm air rises”) to distribute warmth to rooms above.

Forced-air heating. Electric-powered fans changed the whole heating industry in the mid 1930s. The first forced-air furnaces simply used Lennox’s design of firebox and ducting to send warm air directly where it was needed. Homeowners still had to constantly tend to the coal fire, and no method of regulating temperature existed yet.

Thermostatic control. While you can’t just turn off a coal fire, you can turn an electric fan off and on. Warren Johnson’s thermostat (Johnson Controls, still a worldwide leader in the business today) led to a simple system for setting your home’s desired temperature from one location. Once coal had been replaced by natural gas or fuel oil, it was possible to turn the “fire” off and on, too, using pilot lights or electronic sparkers.

Boiler and radiator vs. forced-air furnace. Many homes and commercial buildings still use boilers to create hot water or steam that’s then circulated to radiant heat fixtures. Ranging from the familiar antique cast-iron monstrosities to sleek and unobtrusive baseboard units, radiators provide nearly silent and odor-free heat using regulated zones to even out temperatures. On the other hand, forced-air ducts can be noisy and dusty and require frequent filter replacement or maintenance. However, only ducted systems can also deliver chilled or fresh unheated air. 

Heat pump (heating). In the simplest terms, a heat pump transfers heat from one area to another. And because it is transferring heat rather than burning fuel to create heat, it uses only a small amount of energy. There are many heat pump varieties, but some of the most common take heat out of the air or the ground and move it inside to heat a building. 

Here’s an example of how an air-source heat pump works:

1. A fan pulls outside air over coils containing a liquid refrigerant that then expands into hot vapor.
2. A compressor then increases the temperature and the pressure of the vapor, which then goes to indoor coils.
3. An air handler then moves the heat into the ductwork for circulation throughout your home.
4. As the refrigerant cools, it becomes a liquid again and flows outside to pick up more heat, and the cycle continues. 

Note: Adding to the heat pump’s efficiency, a reversing valve allows the direction of the refrigerant to be changed, turning the same machinery into an air cooler. See Heat pump (cooling) below.

 

Air conditioning systems

People living in tropical climates have always sought ways to keep cool. We’ve all seen pictures of Egyptian slaves fanning their royal masters with giant fronds. The earliest AC systems used some variation of mechanical room fans until very recently.

Electric fan. As it did for the heating industry, electricity changed everything for cooling, too. By the end of the 19th century, large, reasonably quiet electric fans made buildings from knitting mills to movie theaters more tolerable in sweltering heat. Box fans for home windows still provide low-cost cooling around the world.

Commercial air conditioning. Just over 100 years ago, engineer Willis Carrier began work on designs for removing humidity for a large publishing firm, which led to large-scale air conditioning and refrigeration systems. (The Carrier Corporation is a world leader in the industry today.) By 1930, the U.S. government and businesses throughout the country had installed Carrier’s brainchild. Initially, only the rich were able to air-condition their homes.

Central and room air conditioners. Creating refrigerated air involves some tricky concepts including gas-to-liquid phase changes, pressurization, condensation and evaporation. Until the late 1940s, AC systems were too large and noisy to fit inside our homes. Postwar advances in materials processes finally produced air-cooling units small enough to fit inside home refrigerators and, therefore, into window-sized room air conditioners. Also created was “central” air conditioning, in which a larger-capacity unit is mounted on a solid pad outdoors. Pipes lead coolant from there to a chamber located on a furnace or blower unit inside the house. A system of supply and return registers circulate the air throughout the home.

Today, about 5% of our nation’s electrical consumption goes to power our air conditioners, according to the U.S. Department of Energy.

Window air conditioner. Placing an air conditioner in a window is an easy way to deliver cool air without the need for ductwork. This type of air conditioner is typically a unitary system, meaning everything needed to draw heat from the room and discharge it outside is contained in that unit. The reach of a window air conditioner is limited, so several units may be needed to cool an entire building, which can result in a large electrical bill. They can also be noisy and present a fire risk if not well-maintained and monitored. On the plus side, the temperature in each room can be controlled separately, allowing for personal preferences.

Evaporative cooler (swamp cooler).  In hot, dry areas you can cool air using evaporation of water. This follows the same principle our bodies use when sweating to cool off. A fan or blower moves hot air across a wet membrane. By this method, swamp-cooled air gains humidity, rather than losing it—a distinct advantage in arid climes. A variant of this approach, suitable for commercial buildings, uses roof-mounted cooling towers for high-capacity cooling.

Chilled-water circulation system. In the same way that a boiler heats water to circulate into radiators, a chilled-water system can use pipes and coils to reduce air temperatures. Cold water cooling works well over longer distances and in higher capacities, making it ideal for large office buildings. An advantage of this method is that no significant humidity changes result.

Heat pump (cooling). Though a heat pump does not sound like it can cool, it can. It does not create heat; it just transfers it. It may feel as though cool air is being pumped into your home, but, in reality, heat is being absorbed from inside your home and then moved outside, leaving cool air in its wake. Note: The same process in the opposite direction, made possible through a reversing switch, can be used to heat the home through the same unit. See Heat pump (heating) above.

Ductless air conditioner. A ductless air conditioner, also known as a mini-split system, uses tubing to connect a evaporator unit mounted on an inside wall to an exterior condensing unit. This means no ductwork is needed and each room’s temperature can be adjusted independently, making it a popular choice for hotels and multifamily dwellings. The system can also be used effectively by homeowners who don’t want to deal with the expense of adding ductwork and who prefer the less obtrusive look of the interior system to that of a window unit. The main disadvantage is the cost— about 30% more to operate than a central air system and twice as much as window units of similar capacity, according to the Environmental Protection Agency. A ductless air conditioner can also be adapted to provide heat during winter.

Did you know? The advent of air conditioning changed home and building design concepts and helped lead to the population explosion in America’s Sun Belt cities.

Ventilation systems

HVAC professionals call it “air handling”—any system that moves and/or modifies building air. You probably have an "air handler" in your bathroom. It’s that fan/vent mounted on the ceiling above your tub. Any building larger than a garden shed needs good ventilation. Although many variations exist, all ventilation divides into 2 simple classes:

Passive. An open window, a grated opening between floors, or an attic louver—all of these allow crucial airflow without consuming any energy.

Active, or forced. Most of us rarely run our furnace without heating or cooling the air, but you can if you want to. For malls and office buildings, moving fresh air in and stale air out is just as important as heating and cooling. Generally, this ventilation is accomplished using the same ductwork that delivers hot or cool air.

Did you know? Furnaces rate heating capacity using BTUs (British Thermal Units). However, air-conditioning units are also rated by tonnage—which can lead to some confusing mental images! (It has nothing to do with how much the unit weighs.)