What is HVAC, and how does it work?

Key points

• HVAC refers to systems that distribute conditioned air to regulate temperature.

• Forced-air systems can provide heating and cooling and are the most common HVAC systems.

• Ductless mini-splits are an affordable option for homes that aren’t fitted with ductwork.

HVAC stands for heating, ventilation and air conditioning, and it refers to any system that uses heated or cooled air to regulate temperature. Forced-air systems are the most common HVAC systems — they use ducts to distribute warm and cool air, provided you install a central unit. Radiant systems that provide heat cannot be adapted for cooling.

You can speak to an HVAC specialist to find out which heating and cooling system is right for your home, but understanding the different types of HVAC systems and how they work is a good place to start before getting estimates.

What exactly does an HVAC system do?

HVAC systems heat and/or cool air and then distribute it throughout your home to keep your living area at a stable, comfortable temperature. Some HVAC equipment solely provides heating or cooling, while other equipment can provide both. HVAC systems also include components that provide ventilation by introducing fresh air into your home.

How does an HVAC system work?

HVAC systems can work in one of two ways, depending on whether the point of heating or cooling is centralized.

Centralized HVAC systems condition the air at a single location, then use air handlers to push the air through hidden ductwork to your temperature zones. Noncentralized HVAC systems, such as ductless mini-splits, use small air handlers to distribute air into a single room or area, coupled with one or more outdoor units that provide the heating and cooling.

Heaters

An HVAC system’s heater provides the source of heat for conditioned air. The heater type you have depends on the system. Here are the most common heater types:

• Furnace: A furnace system — often called a forced-air system — is the most common type. It usually burns natural gas, propane or oil to heat air in a central chamber before distribution. Electric furnaces also exist but are less common. “Gas furnaces are the most common and most effective way of heating your home. Electric heating is used in an environment where not much heating will be required,” said Jamin Katzer, senior construction manager at Earth Saving Solutions, a contracting company located in Denver.
• Heat pump: A heat pump uses refrigerant in an outdoor compressor and condenser to pull heat from the outside and move it inside. “In simple terms, it uses the refrigerant to heat your home, blown over the heat exchanger to warm the air,” Katzer continued.
• Packaged system: A packaged system includes both heating and cooling equipment. It includes a furnace or a heat pump that produces hot air — the heater is housed in the same box as the cooling equipment. These are available for residential use but are more common in commercial buildings.

Air conditioners

Many HVAC systems provide cooling as well as heating for year-round comfort. Here are a few types of air conditioning systems:

• Central air: In a central AC unit, refrigerant absorbs heat in an indoor evaporator, converting the refrigerant to a gas. An outside condenser pressurizes refrigerant in the condenser coil, releasing heat and converting it back into a liquid to continue the cooling process.
• Heat pump: A heat pump works like a central AC system. The only difference is that it can reverse the process to provide indoor heat as well. Heat pumps can be a part of centralized systems as well as ductless mini-splits.
• Packaged system: A packaged system can include a central AC or a heat pump to provide cooling. Either way, the cooling equipment is housed with the heating equipment.

Ventilation

Most HVAC systems include some means of introducing fresh air into the living area. They can use either mechanical ventilation or natural ventilation. Here’s a look at the difference:

• Mechanical ventilation: HVAC systems with mechanical ventilation use manual or electric dampers to bring outside air into an otherwise closed system.
• Natural ventilation: In a system without mechanical ventilation, pressure differences can naturally move fresh air inside. The process is less controlled but doesn’t require any additional electricity.

What types of HVAC systems are there?

There are five main HVAC systems that provide both heating and cooling: split systems, heat pumps, ductless mini-splits, ducted mini-splits and packaged systems.

Split system

A split system consists of a furnace for central heating and an outdoor condenser for central air conditioning. Both the heating and the cooling components rely on blower motors to distribute the conditioned air through your home’s ductwork.

Split systems require ducts and vents to be installed throughout the living area. The ducts run behind walls, under flooring and above ceilings, so retrofitting a home without them can be time-consuming, invasive and expensive.

“If you do not have a forced air unit, the type and layout of the home can determine the cost to add ductwork to your home,” Katzer said. “Adding ducting to a home can range from $5,000 to $25,000, and going with a mini-split option may be more productive to add heating and air to your home.”

Heat pump system

This HVAC system uses a centralized heat pump to heat air as well as cool it. Heat pumps use blower motors to distribute the conditioned air through the home, so they also require ductwork installation.

Heat pumps include an indoor and outdoor unit through which they move refrigerant to control indoor temperature.

Ductless mini-split system

A ductless mini-split uses a heat pump for temperature control, but rather than using a centralized HVAC unit, it uses a mini-split head (or air handler) to move conditioned air to a single room or small zone. Ductless mini-splits can serve entire homes, but you need multiple indoor air handlers — usually wall-mounted units — to distribute the air, as well as multiple outdoor condensers to condition it.

Ductless mini-splits are the more affordable option for homes that aren’t fitted with ductwork, as each air handler only requires a small, concealed hole in the wall. Katzer recommended these in cases where it’s not cost-effective to add a ducting system. “Adding a mini-split system can help get things done when ducting is impossible or very expensive to achieve,”  he said. “Mini-splits can range from $4,500 to $25,000, depending on how many units are needed.”

Ducted mini-split system

Ducted mini-split systems are a middle ground between centralized systems and ductless mini-splits. They use heat pumps to condition air by using an outdoor condenser like a ductless system. But, they move the air to a unit inside that then uses a blower motor to distribute it to multiple rooms via ducts.

Ducted mini-splits require less ductwork than a centralized HVAC system but more than a ductless system.

Packaged system

A packaged HVAC system includes a furnace or a heat pump for heating, along with a condenser or a heat pump for cooling. All of the equipment is housed together for a simpler installation. Packaged systems can save space indoors. You can also retain your yard space if they’re mounted on the roof.

Packaged systems work just like centralized systems, so they require a full set of ducts running through the home.

What are the components of an HVAC system?

HVAC systems include the heating and cooling units mentioned above, but they can involve components such as ductwork, air handlers, vents, thermostats and evaporator coils to regulate temperature and airflow.

Air ducts

Air ducts — collectively referred to as ductwork — are rectangular conduits that run behind walls and through attic and basement spaces. They connect blower motors to supply and return vents and deliver conditioned air to the living area and back to the cooling or heating system for conditioning.

Air handlers

Air handlers are the units that push air through your HVAC system. They use blower motors to deliver heated or cooled air to rooms and to pull air from rooms to be heated or cooled at a centralized location.

Vents

HVAC systems that use ducts also make use of vents. Ducted HVAC systems feature two vent types:

• Supply vents: Supply vents push conditioned air from your ducts into the indoor space to heat and cool the area.
• Return vents: Return vents pull stale air from your rooms and route it through ductwork back to your heating or cooling unit.

Thermostats

Thermostats have three main functions:

• Monitor the living area’s temperature
• Call on the heater or AC to provide conditioned air when the temperature falls outside the desired range
• Allow you to set heating and cooling parameters for your home’s zones

Evaporator coils

Evaporator coils sit inside the air handler or the furnace. The coils contain liquid refrigerant. When air from the blower motor passes over the coils, the refrigerant absorbs heat and turns into a gas. The refrigerant is then pumped to the condenser, where it’s placed under pressure. The pressure change condenses the gas back into a liquid, releasing heat to the outside.

What’s next?

Forced-air HVAC systems provide consistent energy-efficient heating and cooling for your home, making them a better financial investment than a system that heats water. Katzer said that the upfront costs are similar, but that “the heated water system has more items to maintain, and is not as efficient.”

Regardless of which system you choose, don’t neglect maintenance. Furnaces have a life expectancy of 15–30 years, and heat pumps last an average of 15 years, according to the U.S. Department of Energy. You can extend your equipment’s life by carrying out proper HVAC maintenance. That includes “cleaning your condenser coils, changing your air filters often and having certified furnace personnel look at your system biannually,” Katzer said.

Contact a local HVAC technician to perform routine maintenance on your system twice per year, especially if you live in an extreme climate.

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