Return Air and Supply Air in HVAC
In this post, we will know about the most common terminologies that are often used while speaking about airflow and ducts.
Return Air in HVAC :
Return air is used to return stale air out of your home through ducts. In this case, we are dealing with supply air which is fresh air that comes into your house from outside. If you have a well-sealed home, then you probably do not need to worry about return air. But if there are gaps in the walls, flooring, windows, etc., then you will want to get rid of some of that stale air. This will make sure your indoor air quality stays clean and healthy.
There are two ways to treat return air. One way is to use a ventilator system. Ventilators work by using fans to move air around. A fan pulls in air from the top of the unit and pushes it back out to the bottom through large vents. You can buy these units at hardware stores for $40-$50. They are cheap but they can be noisy.
Return air can be considered as the air that returns back inside your house after passing through your heating system. This is usually from your furnace intake, but could also come from the vents in your bathroom or kitchen. In larger systems, this would include the return ducts that bring warm air into the home and then go out through the various vents throughout the house.
Supply Air in HVAC :
Supply air is the fresh air coming in from outside. This should be clean and free of any odors or smells.
Supply air is what enters your home from outside the house through windows, doors, etc. There are two types of supply air that make their way into our homes - fresh (also called outdoor) air and return (or recirculated) air. Fresh air contains much higher levels of oxygen than return air, while return air has a lower amount of carbon dioxide (CO2). When we breathe out CO2, it makes its way back into the room where we are breathing through the supply air. It's this same principle that allows us to recycle indoor air by pulling air out and replacing it with fresh air.
Supply air in an HVAC system refers to the volume of air that enters the building. This can differ based on the type of heating/cooling unit installed. A heat pump uses outside air with some additional energy to transfer heat out of the house while cooling units use water or ice to absorb heat from the building. Once inside, the supply air passes through the ducts where heat or cool is removed. The supply air then mixes with return air before being released back into the structure. In larger buildings (multi-family homes, office complexes, etc.), the supply of air may need to pass through several stages of filtration to remove impurities such as dust and pollen.
If you are not able to determine what direction your supply/return airflow is going, check the exhaust fan blades. Exhaust fans are generally located at the bottom of the unit where the exhaust air is pulled out from the room that needs to be cooled.
Generally, if the exhaust fan blades spin clockwise, the air is returned to the space. This is called a “right-hand” or horizontal exhaust fan. The opposite way is known as a counter-clockwise or “left-hand” exhaust fan.
As stated before, if you cannot tell which way the fan blades are spinning, check under the hood of the unit. Most units have a small exhaust vent near the base of the unit that has a plastic cover over it. This is where the exhaust comes out. Look for a clear spot or hole in the plastic cover. This is how you should know whether it is rotating in either direction.
To find the exact location of the return air opening, look at the top of the blower motor. You might have to eliminate the filter box.. Underneath the box, there will be a round metal plate that is attached to the motor shaft. This is where the blower wheel turns. When it turns, it pushes against the top of this plate causing it to rotate. The actual location of the return air openings will be directly above these plates. Once you locate the holes, take a tape measure and mark them off to make sure they are consistent.
Return air is the air that comes from the room being cooled/heated. This air goes back into the space where it was originally drawn from. It is typically recirculated through the space via vents. 2. Supply air is the fresh air coming in from outside the building. In many cases, you will not have a choice in what type of air enters your home; however, if your system has a supply door to allow outside air in, then it can be filtered. You may find a filter at the intake of the unit, but some systems do not even have filters due to safety concerns. A filter will help remove any dirt and debris that may be circulating through your heating and cooling system.
How to measure Return and Supply Air?
Airflow is gravity-driven. With this understanding, we know that warm air rises above the cool air leaving the top floor. Cooler air rushes in at ground level and heat moves out through supply vents along with the ceiling and returns air ducts flowing down from the ceiling opening into the furnace compartment.
A simple method to measure supply and return airflows is with a smoke detector. A single smoke detector can show the difference between supply and return airflows if placed in the basement near the return register. Smoke detectors are cheap and work well.
Another method is to install a special vent cover that has an airflow measurement device inside. These covers cost about $40 - $80 each. They come in several styles and sizes. Some have been custom-designed while others use a standard size with a round hole sawed out of the center to allow the airflow indicator to protrude through the cover glass.
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