AC return air vents are an important aspect of your AC/HVAC system. They work in collaboration with other parts of the HVAC system to ensure that the house is properly cooled and heated, and that the air in the room is always clean and fresh.
The function of AC return air vents is to allow the air in your house to return to the air handler for cooling or heating and therefore maintain proper indoor temperatures. They also help to maintain the pressure of the indoor air, clean the air and exhaust stale air to the outside (ventilation).
When the thermostat in the house calls for heating or cooling, the fan in the air handler pulls air from the house through the return air vents. After the air is heated or cooled, it is supplied to the house through the supply vents.
Although most of us call them AC return air vents, they don’t work exclusively with air conditioners. They are important for both cooling and heating not forgetting ventilation.
Most people however do not know the difference between AC and HVAC (heating, ventilation and air conditioning). If you are in that category, I wrote this guide here for exactly that reason.
AC return air vents are very important in an HVAC system. When you need to cool or heat air from the house, it is drawn to the air handler through the return AC vents. They also help in filtering the air as well as in ventilation, which also balances the indoor pressure.
So, does a return vent help cool a room?
Yes! The air handler fan pulls hot indoor air to the cooling coil through the AC return air vents. Before cooling, the air is passed through a filter where debris are removed so that the air being supplied to the house is not only cool but clean.
And what happens when you block the return air?
When you block the return air, the air handler will draw 100% of the required air from outside which is inefficient and therefore expensive. Since there will be no way for the stale air in the house to be exhausted, there will be pressure imbalance inside the house while the supply air ducts may crack and leak.
Functions of AC return Air Vents
Ac return air vents have 4 main functions. They are:
This is the main reason for having the HVAC system in place in the first time. When the indoor air becomes hotter that the temperature set on the thermostat, the HVAC systems kicks in to cool the house.
In order for that to happen, the hot indoor air has to be pulled out of the room and delivered to the cooling coil. And that is where the return air air vent comes in.
It provides a passage for the hot indoor air to the air handler for cooling. That is the exact thing that also happens when the indoor air needs to be heated.
2. Balance Indoor Pressure
An HVAC systems is in essence a closed loop that is constantly moving air around. Importantly, return air vents and supply air vents work in tandem to not only supply the house with air conditioned air while removing stale air, but also balancing the indoor air pressure.
When air is cooled or heated inside the air handler, it is supplied to the house by the supply air ducts. The supply of air changes the air pressure in the house.
For that reason, the air already in the house needs somewhere to go. And that is where AC return air vents play their role. The air already in the room is sucked in the return air ducts and is either sent to the air handler or exhausted out.
3. Clean the Air
There are so many activities that go on inside the house (like folks walking in with dirty shoes) which leaves the indoor air saturated with dust, pollen, lint and other impurities. That is not an ideal situation to live with.
The good thing is that when the indoor air is pulled into the return air ducts, it is forced through a filter where all this impurities are removed. As result, the air being supplied to the house will be free of impurities.
Activities such as cooking, cleaning, showering, breathing among others will reduce the oxygen levels in the house making the indoor air stale. The humidity levels will also increase.
Return AC air ducts provides a means for the stale indoor air to be removed/exhausted so that fresh air from outside can come in the house. They also offer a great way to reduce the humidity levels of the air.
How AC Returns Air Vents Work
Let us now look at a detailed guide of how AC return air vents work.
To start with, there are many types of air conditioners. However, AC return air vents are found in central air conditioning systems. Why is that though?
Central air conditioners have one big condenser unit outside the house and another unit inside the house. Cool air or heat is supplied to every room of the house from that single indoor unit.
To do that, a network of ducts are installed throughout the house to supply each room with cooled air or heat. In every room, there are supply air ducts which brings in fresh air and heat, and return air ducts which take indoor air back to the air handler for heating or cooling.
An important part of the air handler is the fan. When the thermostat in the house calls for cooling or heating, a signal is sent to all components of the HVAC system to come on.
After the HVAC system turns on, the fan sucks the indoor air out of the the room. The indoor air enters the return air ducts via the vents in the house where it first goes through a filter to remove dust, lint, pollen and other impurities.
The air is then cooled or heated and then supplied to the house through the supply air ducts and vents. As you can see, the indoor spaces, return air ducts, air handler and supply air ducts form a loop that works seamlessly whether the house is being heated or cooled.
Return, Supply, Exhaust and Ventilation Air
An HVAC system cannot be complete without ventilation. You see, the indoor air being circulated between the return and supply air vents/ducts will eventually become stale even after heating or cooling.
In simple words, ventilation is the process of purposely replacing indoor air with outdoor air. In HVAC, ventilation helps to bring in fresh air from outside and expel stale air.
Although you can ventilate a room by simply opening windows and doors, that will reduce the efficiency of your heating and cooling. That is why ventilation in HVAC has to be systematic.
I started by mentioning that every room in your house will have both return and supply air vents. Return air vents remove air from the house while supply air vents brings in air into the house.
We can therefore say with certainty that the supply and return air ducts terminate and start at the air handler. The air handler has a fan, a heating coil and a cooling coil.
What you may not know is that the air handler has 2 sources of air:
- Outside the house (ventilation air)
- Inside the house (from return air vent)
There is a modulating damper between the air handler and the intake of the ventilation air. This damper controls when and how much of the fresh air from outside can enter the air handler.
The fresh air from outside after being cleaned is heated or cooled inside the air handler and is then supplied to the different spaces inside the house.
As you have noted, the air handler does not use air exclusively from the outside or from the inside. It uses both ventilation air and return air. There are 2 reasons for that:
- The system cannot run 100% on return air since it needs fresh air for ventilation purposes.
- Depending on time of the year, the outside air will either be too hot or too cold meaning you need to use a lot of energy to cool or heat it which is not efficient.
For the above 2 reasons, it is important for the system to mix return air and ventilation air.
Again, you will note from the above explanation that not all of the return air will go to the air handler. So, where does the rest of it go?
The return air duct branches off before it gets to the air handler. Some of the return air is removed outside the house through another vent and that is the air known as exhaust air.
What Happens When You Block the Return Air?
To understand what will happen when you block the return air, you must first understand what happens when the return air is open. And that is why I started by explaining that concept.
It is important to understand that an HVAC system works like a recirculating pump. Air is constantly flowing through a defined loop. So, what happens when the return air vent is blocked?
When that happens, the system will work hard to find a way to exhaust the air already in the room. Remember at that point the supply vent is still supplying the room with air.
Pressure will build up inside the room forcing air out through cracks in walls or the gaps in windows and doors. Needless, to say, your HVAC system will be operating inefficiently resulting in high electricity bills.
It is often thought that you can block off vents in other rooms so that you can quickly cool/heat another room. I would advise against doing that.
The reason for that is that there are no sensors to tell your HVAC systems that the vents have been blocked off. As a result, the air handler will keep sending air through those air ducts.
Pressure will build up inside the ducts and they are likely to crack and leak. Your HVAC systems will be very inefficient (and therefore expensive) if you have leaking ducts, as it will have to work harder to compensate for the lost heat or air.
The solution to this problem is to HVAC zones. In HVAC zoning, different areas of the house as cooled or heated at different temperatures hence it is possible to quickly cool or heat one room by focusing all the air or heat there.
It is also a good choice if you have hot and cold spots in your house. I have written about HVAC zoning in more details in this post.