Why Are HVAC Vents Under Windows?

The location of your HVAC vents depends on several factors, such as whether or not you have a basement or crawlspace. How likely you are to either heat or cool your home and how energy efficient the construction of your home may also determine vent placement. So, what’s the reason for placing HVAC vents under windows?

HVAC air supply vents are under windows to counter cool air coming from the window. Cool air sinks because less active molecules are densely packed. The lighter, less dense, active molecules in warm air spread out and rise. As the warm and cold air mix, room temperatures become more comfortable.

Read on to learn the reasons for the various placements of HVAC air supply and air return vents. I’ll also talk about the advantages and disadvantages of each placement so you can better ensure the efficiency of your home’s system. 

HVAC Vent Placement 

Traditionally, HVAC vents are on exterior walls and under windows to mix the HVAC system’s warm air and cool air from windows. Exterior wall and ceiling placements are used in homes without basements or crawl spaces. Energy-efficient homes save on installation costs with HVAC vents on interior walls.

The warm air from the air supply vent eliminates cold drafts from the window and makes the entire room warmer. It doesn’t matter whether the vent under the window is in the floor, the baseboard, or the wall; it produces the same effect.

Air supply and return vents can be in the floor, the baseboards, high or low on interior or exterior walls, or in the ceiling. The location depends upon where and when your home was built, what type of furnace an older home originally had, and whether or not your home has a crawl space or basement.

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HVAC Vents Located in the Floor, Baseboard, or Low on Exterior Walls

HVAC vents located in the floor, baseboard, or low on exterior walls send lighter warm air up into the room to mix with and warm sinking cooler air. If you live in a climate where you need your home heated and you use your furnace during the cooler months of fall, winter, and spring, having your air supply vents located in the floor or low on the wall is an ideal arrangement. 

Placing air supply vents in the floor, baseboard, or low on exterior walls has a historical precedent. The early gravity-flow furnaces relied on the heated air to rise. Gravity-flow furnaces in the basement had asbestos-wrapped ducts to guide the air to upper floors. When the furnace or heat source was located on the home’s main floor, a grating-covered hole would be cut in the floor of the second story directly above the heat source to allow the warm air to rise.

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HVAC Vents Located High on Exterior Walls or the Ceiling

Air supply vents located in the ceiling or high on exterior walls work best for homes in warmer climates that need cooling during warm spring, summer, and fall months. In warm climates, the sun shining through windows creates warm spots in exterior walls. Because of this, HVAC air supply vents are often located directly above windows rather than below.

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With the air supply vents located higher in the room, your central air conditioning system blows denser cool air into the room. The cool air sinks to mix with and cool the lighter warm air as it rises. As the warm and cool air mixes, the overall temperature in the room cools. Before window air conditioners and central air, homes had rooms with high ceilings that allowed the warmest, lightest air to rise to the ceiling and left air that was cooler nearer the floor. 

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HVAC Vents Located on Interior Walls

In homes that have been built more recently, you’ll find the HVAC air supply vents located on the interior rather than exterior walls. Newly built or recently remodeled homes are more likely to have energy-efficient, double-pane windows. Double-pane windows reduce the warm or cold spots created by older, single-pane windows. Newly built or remodeled homes also will have more energy-efficient insulation in the walls. 

Consequently, air-supply vents can be installed in interior walls. This installation requires shorter lengths of ductwork which reduces the cost of installation. The fan that blows the air through the system doesn’t need to work as hard to send it through the shorter lengths of ductwork. Consequently, this type of installation also improves the efficiency of both heating and cooling systems and reduces the need for repairs. 

Best Locations for HVAC Return Air Vents

The vents for your HVAC system should be at least 10 feet (3 meters) away from thermostats. Some homes have only one return air vent located in a central hallway or in the interior wall of an open design area. If you have a smaller home and leave the doors open to other rooms to allow air to flow, a single, centrally located air return duct can be adequate.

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The Best Location for Air Return Vents To Heat Your Home

In older homes built in cooler climates with air supply vents on exterior walls, the best location for return air vents is in an interior wall opposite the air supply vent. To encourage the mixing of the warm and cool air, return air vents should be in the floor or low on the wall to pull the warm air down from the ceiling. 

The Best Location for Air Return Vents To Cool Your Home

Again, in older homes where air supply vents are on exterior walls, return air vents should be on interior walls opposite the air supply vent. When you live in a warm climate and want to keep your home cool, the return air vents should be located high in the wall or in the ceiling to draw the warm air that rises to the ceiling out of the room. 

The Best Location for Air Return Vents in Newer, Energy-Efficient Homes

As already mentioned, newer energy-efficient houses have features that reduce the undesired exchange of warm and cold air between the homes and the outdoors. Some windows are specifically designed for cool climates. Other windows are designed for warm, sunny climates. Insulation for walls and attics also has improved.

It’s no longer necessary for air supply and air return vents to compensate for warm or cold spots. Homebuilders, remodelers, and HVAC installers reduce the ductwork to provide more efficient systems. However, air supply and return vents remain on opposite walls. They’re still placed to create the most comfortable room temperature.

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The Only Location for HVAC Return Vents in Homes Built on Slabs

If your home is built on a slab, the only place to run ductwork to air supply and return vents is through the walls and attic space. Ductwork installed in the slab gives rise to multiple problems. Both the air supply and air return vents may be in the walls. HVAC vents in these homes can also be in the ceiling. 

The supply and return vents should still be on opposite sides of the room. The vents should also still be placed to provide the most comfortable room temperature. In cool climates, return air vents should be low in the wall to draw warm air down. In warm climates, return air vents should be high on the wall or in the ceiling to draw the rising warm air out of the room.

Final Thoughts

For homes in cool climates with older windows and insulation, heating vents under windows provide warm air that counters cooler air from windows and exterior walls. In older homes in warmer climates, cooling vents above windows counter warm air from windows and walls. The goal is to keep your home comfortable in the most efficient way possible.