For most residential and commercial HVAC systems, you have likely noticed that there is both an indoor and an outdoor component.
In many cases, the outdoor part is a large box that is much more noticeable than the smaller and less conspicuous indoor part; this may lead you to believe that fresh outdoor air is “conditioned” in this outdoor box and cycled into the house, but is this really the case?
In most cases, an HVAC system does not bring fresh air into a building. Your home is heated and cooled by recycling interior air, with any exterior air that enters the building the result of processes independent of the HVAC system.
The HVAC system does not mechanically bring fresh air into a building.
However, cracks and leaks in the HVAC ducts can allow exterior air to come indoors, so it is essential to regularly service your HVAC system to ensure that all filters are working properly and that there are no inefficiencies in the transfer of air.
How Does the HVAC System Work?
In most buildings, the HVAC typically works through a system of intaking indoor air, heating or cooling it in the system, and recirculating it at the desired temperature (Source: Old Coast Heating and Air Conditioning).
Providing Cool Air
During the summer months, warm indoor air is absorbed by the HVAC system using a component called a compressor.
Once obtained, this warm air is cooled by a fluid known as refrigerant, which is encased in a closed metal loop that helps circulate the warm air from the house into the outdoors while moving the chilled air back inside.
This heat exchange is broken down into the following five steps:
- The compressor intakes warm indoor air and cools it through absorption into the refrigerant.
- The refrigerant heats up due to this absorption of warmer air.
- The refrigerant transfers this hot air outdoors, flowing to the cooler exterior air (compared to the denser, more concentrated heat of the refrigerant coils in the HVAC system).
- After this warm air is released outdoors, the cool refrigerant is circulated back to the interior of the house.
- The refrigerant passes through a narrow valve in the HVAC system into the evaporator, which is kept at a low pressure and causes the refrigerant to expand upon release. This expansion causes the refrigerant to become very cold, and fans located in the system’s ductwork release the cool air throughout the building.
Providing Warm Air
The HVAC system’s process for warming a building is generally much more straightforward than cooling.
A furnace within the system has burners that generate combustion gases. These combustion gases then pass through the heat exchanger.
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Once out of the heat exchanger, a fan blows air over these warm gases, circulating through the ductwork to the various rooms of your building.
What Does the HVAC Exterior Component Do?
If your HVAC system’s interior component does all the heavy lifting in terms of heating and cooling your building, you may be wondering what the purpose of the exterior part is.
After all, the external box is large (and often unsightly) and has a fan that is seemingly running all the time.
The exterior HVAC component—alternately referred to as the condenser unit—is used as a release mechanism for your home’s warm interior air.
The warm air intake from the compressor that is circulated through the refrigerant needs a place to go (other than back into the building), and the condenser unit provides this.
The fan circulating in this exterior component is simply releasing warm air into the great outdoors.
Important components housed in this exterior condenser unit include the condenser coils and fan, which help cool the warm air and release it to the outdoors.
The refrigerant lines, which are essential for connecting the indoor and outdoor units, and cycling refrigerant, are part of the cooling process.
How Does Fresh Air Get Inside?
If fresh air is getting inside of your building, it is likely not the result of any mechanical process of the HVAC system.
The Environmental Protection Agency lists the following as common ways that exterior air can enter a building:
- Natural ventilation: Open doors and windows are the most common way that large amounts of air circulate between the building’s interior and exterior.
- Infiltration: This allows trace amounts of air to enter the building through architectural imperfections. Cracks and crevices in the framing, walls, floors, ceiling, and trim around the doors and windows will continually allow small amounts of air to enter the building, which will add up over time.
- Mechanical means: This may result from an outdoor-vented fan that is used to remove air from a single room, such as a deodorizing fan in the bathroom or a smoke-removing fan in the kitchen.
Dedicated Outdoor Air Systems (DOAS)
While most HVAC systems will not bring fresh air into a building, the EPA notes that some HVAC systems will use outdoor air as part of the heating and cooling process.
In fact, these types of units are gaining in popularity, as there are some environmentally-friendly benefits associated with dual indoor and outdoor ventilation units.
Known as dedicated outdoor air systems (DOAS), DOAS systems will use parallel ventilation of indoor and outdoor units to circulate air into and out of your home.
This process will introduce fresh air into your home through mechanical means and can be accomplished through one of two methods:
Separate Systems with Separate Ductwork
Through this process, both the HVAC system’s indoor and outdoor components are essentially dividing the labor of heating and cooling the building.
While this type of system has a high initial cost of setup, it makes it easy to control the exterior airflow into the building and reduces the load placed on the fan coil units that traditionally undermine single-direction HVAC systems.
This setup introduces chilled outdoor air directly into the ductwork that will circulate back into the building.
It will directly mix with the interior air that is distributed by the fan coil units, helping to cool the return air that is making its way indoors.
While it may be challenging to balance the airflow that is escaping into the outdoors and traveling back inside when using this method, initial setup costs will be less than with a system of separate ductwork, and the combined exterior/interior airflow will reduce the amount of airflow volume, helping the building save on energy costs associated with fan use.
How to Improve Indoor Air Quality
While fresh air is generally thought of as a good thing, there are several benefits of having HVAC systems that do not freely cycle exterior air, as noted by Covington One Hour Air.
- First, exterior air brings with it a host of particles, from dust mites to pollen to mildew. While these particles can cause allergic reactions in many people, they can also be harmful to your house’s structure, as settlement in the walls, floors, and ceilings can cause structural damage over time.
- Furthermore, an HVAC system that brings in exterior air can cover up other low interior air quality causes. If you just assume that it is coming in through the HVAC ducts, you may overlook other causes of poor air quality, such as chemicals from cleaning supplies and pesticides and mold buildup from leaks in the plumbing.
While the benefits of fresh air for human health are plentiful, it should be obtained through other means, such as taking long walks outside or sitting on the backyard deck. If you want to introduce fresh air into your home, do it through controlled means, such as opening a window to allow the breeze to circulate.
If you feel like your HVAC system is letting unwanted exterior air into your home, there are a few steps you can take to improve interior air quality:
- Check the ductwork: Making sure that there are no holes or leaks in the ducts can help ensure that no unwanted air enters the system.
- Regularly replace the HVAC filters: Clogged and dirty filters can not only cause interior air quality issues, but it can cause the system to work harder, increasing energy costs.
- Add a damper: A damper is a device that helps control the flow of air, limiting backdraft that can add inefficiency.
- Use an air scrubber: An air scrubber is a pollution control device that removes unwanted particles and gases from an air stream. These can be installed as part of your HVAC system or be portable devices.
In general, the HVAC system does not bring fresh air into a building. A space is heated and cooled using recycled indoor air, with exterior HVAC units serving as a means of air release rather than intake.
While DOAS systems are gaining in popularity and will bring some exterior air into your home through mechanical processes, most HVAC units still function on a one-way process of recycling interior air.