HVAC units work like lungs. They need to inhale air (supply vents) and exhale air (return vents). If your return vents don’t return enough air, you will have a series of problems. We are going to look at the symptoms of not enough return air from vents.
First of all, it is important to understand that return air is essential for HVAC. Without sufficient return airflow, you will have an inadequate (and inefficient) airflow. Benefits of healthy return air vents include:
- Maintaining air pressure.
- Maintaining adequate temperature and humidity.
- Filtering your air via air filters.
- Reducing your HVAC expenses on cooling and heating.
What happens if there is not enough return air?
Well, you will start seeing symptoms of not enough return air (we’re going to cover these symptoms one by one below).
Example of insufficient return airflow: Let’s say your home requires at least 700 CFM of return air. If you were to measure the combined return air from all of the vents and get something like 400 CFM of the return air, you are experiencing an insufficient return airflow problem.
Of course, diagnosing insufficient airflow from return vents is not all that easy. Not everybody owns an airflow meter (CFM meter). How to tell if your vents are not returning enough air without measuring the airflow?
You will need to evaluate if the return airflow is indeed lower than needed. How exactly to do that? You don’t have to measure it; you can just check for the symptoms.
To help you out diagnose this airflow problem, we are going to do through all 5 symptoms of not enough return air, starting with the most common sign of insufficient return airflow:
1. Hot And Colds Spots (#1 Indicator Of Not Enough Return Air)
The first thing most homeowners notice in the case of not enough return air are the hot and cold spots. Having some rooms that are hot and others that are cold is a tell-tale sign of not enough return air.
Example: Let’s say you have HVAC ductwork across your entire house. If you see that, for example, the bathroom and bedroom are at 80°F while the living room and kitchen are at 72°F, you probably have a case of insufficient airflow from HVAC vents.
Noticing these hot and cold spots indicated that something is wrong with either your HVAC unit (air conditioner, furnace) or you have inadequately sized return air ducts (too big or too small).
Here’s how you can check for insufficient airflow:
Rooms closest to the HVAC unit will have a bigger airflow. Rooms that are far from the HVAC unit will have a lower airflow. In essence, an HVAC unit generates high air pressure. Close to the unit, the pressure will still be high, and you will see air blowing from the return vent. Far from the unit, the pressure will fall and you will see not enough return air from those vents.
Summer situation: In the summer, you run air conditioning. If you don’t have enough return air in summer, you will notice that rooms closer to the central air are cooler. Rooms further away from the aircon are hotter.
Winter situation: In the winter, you run heating (usually a furnace). If you don’t have enough return air, in winter you will notice that rooms closer to the furnace are hotter. Rooms further away from the furnace are colder.
Noticing hot and cold spots is not the only and not the sufficient sign, based on which you can say ‘Yes, I definitely know my return air is not sufficient’.
To confirm your suspicion of inadequate return airflow, you should also check for these following symptoms:
2. Air Pressure Variation In Your Rooms
A symptom of not enough air from return vents is a difference in pressure between your rooms. Rooms closer to the HVAC unit will get more return air than rooms further from the HVAC unit.
As a result, you will have different air pressure in different rooms.
Of course, detecting if you indeed have air pressure variations is not easy. It can be done in two ways, namely:
- Measure the pressure. You will have to use a barometer (which most homeowners don’t have) in the kitchen, living rooms, bedroom, and so on.
- Feel the pressure (usually with your ears). Humans notice air pressure variations. Older folks will say their knees hurt in high air pressure rooms, for example. The most commonly used human instrument for measuring air pressure are our ears. If your ears hurt in one room and not in the next room, you are likely experiencing pressure imbalance.
If you detect different air pressures across your house, this is a sign of not enough return air.
3. Use A Wet Finger To Tell If The Return Air Is Not Sufficient
Another good human instrument for measuring airflow are our finger (especially if you use a bit of spit on them). If you were to put a wet finger close to the vent that is generating airflow, you should be able to tell if there is a lot of airflow or not enough airflow.
This is the simplest way to tell if you are getting enough return air. If you lick your finger and put it close to the return vents, you will feel the airflow.
However, it is worth noting that this technic:
- Measures air velocity.
- Don’t actually measure airflow.
We can only notice if air from return vents is moving quickly or slowly (air velocity). We can’t actually tell if there is enough airflow since airflow by itself is defined as the quantity of air that is moving (measured in CFMs).
Nonetheless, low air velocity from your return vents usually indicated low airflow. You can quickly tell if you have enough return air (or not) using this simple method.
4. Air Conditioner Blowing Warm Air Or Furnace Blowing Cool Air
As we have noted, supply and return vents work like lungs. If there is not enough air coming from the return vents (problems with exhaling), you will have problems with the air supply as well (problems with inhaling).
You will practically see this insufficient ‘breathing’ as:
- Air conditioner blowing warm air (in the summer).
- Furnace blowing cool air (in the winter).
In the summer, the return air should be cool and the supply air should be warm. However, if you don’t have sufficient airflow in your ducts, the supply air will be warmer (especially at 100°F or more outdoor temperatures) and the return air will be warm. The result – a symptom you will notice – is that the AC unit is blowing warm air (instead of cool air).
In the winter, the return air should be warm and the supply air should be cool. With insufficient airflow, however, the supply air will be less cold and the return air will be less hot (usually mild-warm).
Now, there are many culprits for air conditioners blowing warm air as we have described here and furnaces blowing cold air as described here, but it may also be an indication that you don’t have enough return air.
If you see all (or even 2 or 3) of these symptoms, you very likely have insufficient airflow from your return vents.
How to deal with not enough return air?
You will usually need an HVAC professional to check the ductwork and the HVAC unit. If you need hands-on help, you can use this form here to get 4 free quotes from HVAC experts in your area.
We hope this helps you diagnose if you indeed have less return air than needed.