Does HVAC Bring in Fresh Air?

Due to changing building codes, fresh-air intake is now incorporated into most new HVAC systems. In the past, most systems did not bring fresh air into a building. A home was heated and cooled by recycling interior air, with any exterior air that enters the building the result of processes independent of the HVAC system.

Let’s take a quick look at how a typical HVAC system works, so you can see the purpose behind the treatment of fresh air.

How Does the HVAC System Work?

In most buildings, the HVAC typically works through a system of intaking indoor (and/or outdoor) air, heating or cooling it in the system, and recirculating it at the desired temperature. 

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: 

1. The compressor intakes warm indoor air and cools it through absorption into the refrigerant.
2. The refrigerant heats up due to this absorption of warmer air.
3. 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).
4. After this warm air is released outdoors, the cool refrigerant is circulated back to the interior of the house.
5. 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.

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. Depending on your setup, this unit will be either an air conditioner or a heat pump.

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?

It used be that, if fresh air is getting inside of your building, it was likely through unwanted air leakage – and 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 unintentionally from a back-drafting outdoor-vented fan that is used to remove air from a single room, such as an exhaust fan in the bathroom. Or else, this can be intentional, through an intake setup built into your HVAC system.

So, if you have an older home, and/or live in an area where the codes do not require fresh air intake, your HVAC system will not intentionally bring in fresh air.

Otherwise, your system will likely have provisions for intaking fresh air each time the unit runs. Though this is less efficient (conditioning outside air typically takes more energy), it is considered to be better for human health.

Dedicated Outdoor Air System (Commercial System)

While some HVAC systems will not bring fresh air into a building, other 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 a building.

This process will introduce fresh air into the building 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.

Combined System

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.

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.

Final Thoughts

Depending on the codes in your area and the age of your system, the HVAC system might bring a certain amount of fresh air into a building. A space is heated and cooled using mostly recycled indoor air, with exterior HVAC units serving as a means of air release rather than intake. 

While DOAS systems are gaining in popularity in commercial applications and will bring some exterior air into a building through mechanical processes, most HVAC units still function on a one-way process of recycling interior air.