A consistent and relatively strong airflow is one of the main signs of a fully functional HVAC system. The air coming out of your vents should be roughly equal throughout your whole house. A bathroom fan, for example, should be able to pump a minimum of 50 cubic feet of air per minute.
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What are these ‘cubic feet per minute’? Can you measure them yourself? And what should you do, if the airflow in your vents has become weaker? Here is our easy-to-understand guide.
How Strong Should Air Come Out of Vents?
The answer depends on your system and the settings. In a nutshell, you would want the airflow from the vents across your house to be roughly equal. If the fan is blowing stronger in one room and is very weak in the other, then it looks like something is wrong with the unit.
The ‘V’ in the ‘HVAC’ literally stands for ventilation, so your system has to be able to not only heat or cool a room but also exchange air within a certain space. Of course, you would want your house to have the correct air movement as it drastically affects the air you breathe in.
In general, the air in one room should be changed 8 times per hour. To achieve that a small bathroom, for example, has a recommended air movement rating of at least 50 CFM (cubic feet per minute).
But how can you measure the strength of the air that is coming out of the vents? Should you simply trust your subjective feelings or is there a more scientific way to measure the airflow?
How to Measure CFM / Airflow
Did you know that there are qualified air balancers who can professionally measure, adjust, and balance airflow? These experts can use different measurement methods.
- If the grille is located flat against the wall or ceiling, then you can use a calibrating air balancing hood. This thing is placed over the grille and it manages to catch all of the fan’s airflow. Bear in mind that airflow specifications are around plus or minus 10% of design airflow.
- An airflow traverse would require a long and straight exhaust duct. This method includes drilling a few holes into the duct. After that, an anemometer is used to measure the velocity of the air. These two measurements will help you determine the speed of the air in the duct.
- The manufacturer of your fan has to have a fan performance table. In larger fans, you can measure the fan speed (RPM) using a non-contact tachometer. You would also need a static pressure kit (a manometer, a hose, and static pressure tip) to measure the static pressure.Once you know the RPM and the static pressure, you can consult the manufacturer’s fan performance table to find out the CFM.
The methods mentioned might seem a bit too challenging or time-consuming and, of course, not everyone is going to measure their airflow like that.
In a nutshell, in case you are aware of what a well-working HVAC system feels and sounds like, then you will for sure be able to spot if anything ever goes wrong.
Most Common Causes of Low Air Flow from Your Ducts
The Vent Is Closed or Blocked
Some vents come with a closable grate or damper, just so you can choose to block the airflow if you ever need to. This might be too straightforward, but one of the main reasons why there isn’t strong air coming from the vents is because the damper is closed.
The vents might also be blocked by furniture or boxes, for example. So, make sure to remove all of the obstructions.
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In case the airflow does not recover, you can continue looking for another cause.
The Damper Valve Is Closed
The dampers are located within the HVAC system and are responsible for regulating the airflow. If there is a room in your house that needs more air, the damper valve will block off some of the paths for the air to make it easier for the system to focus on one room.
Your dampers can be either manual or automated.
| What might happen | The solution | |
| Manual dampers | Can be accidentally left in the wrong position. | Locate the dampers in your ductwork. Use a little lever to change the position of the valve. |
| Automated dampers | The valves can be automatically controlled by the thermostat. In such a case, the dampers can get stuck. | The problem is, most likely, with the thermostat and not the actual dampers. Calling an HVAC technician might be the best decision. |
You Have Blocked or Dirty Ducts
Air filters do a great job at keeping the air ducts clean from the majority of particles, but, unfortunately, some small particles can still get into the system. As a result, they are going to stick to the ducts due to static pressure and moisture.
Moreover, different pests might become an issue and an additional source of dirt. So, at one point, you might face airflow-related issues because of all this dust and debris.
In such a case, you would have to hire a team of professionals to clean your ductwork.
You Have Leaky Ducts
The returns and vents have to be the only entrances and exits in your HVAC system. Ideally, your ductwork shouldn’t have any holes and gaps in it. Otherwise, the system will lose air pressure and will start to leak air. This will also lead to a much weaker airflow.
By the way, ducts leaking 20% of the air passing through the system make the unit work 50% harder.
To cope with this problem, you would have to remove the ducts that have been severely damaged (if there are any), seal all the leaks, and, possibly, install new insulation.
The Flex Duct Is Misshapen
If you have an attic, then the chances are high that your ductwork has some flex ducts. These things are amazing as they can bend. But simultaneously, the ducts’ flexibility can become a problem.
Firstly, they are more prone to damage than the harder ducts. Secondly, flex ducts can form a kink that is going to restrict the airflow in the system.
If you can’t undo the dent on your own, you would have to call a specialist to do that.
The Air Filters Are Restrictive
There are extremely fine filters that are able to filter out practically all airborne particles. However, this part of your HVAC system might get dirty and clogged in just a few months (sometimes, even a single month, if you have pets, for example).
Of course, if the filters are clogged, the system will be struggling to have a strong airflow.
You can choose to replace your filters with a less-fine option (do check the system’s recommendations though!), manually remove all the dirt, or simply change the filter as soon as it gets dirty.
Frozen Evaporator Coil
This part of the HVAC system is responsible for cooling the air. Sometimes, it might get frozen. If you have noticed pools of water close to the indoor unit, then the evaporator coil is to blame.
Of course, the thing is going to affect the strength of the airflow as well.
In case your system has any signs of ice build-up, you should call a professional.
How to Improve Airflow in Your House
To drastically improve the airflow, you should first tackle any issues mentioned above.
However, there are a few other tips that you can follow to achieve a nice and strong airflow in your house.
- Place your vents near an outside wall. If you manage to place them against an outer wall, the system will end up pulling more air in and this, in its turn, will make the airflow stronger.
- You can try placing your vents on different floors. Vent placement is a whole other matter, but having vents on various levels certainly does improve air circulation.
- You may consider investing in an Energy Recovery Ventilator. This device is able to flush out the stale air and draw fresh air into the system. As a bonus, these things also help lower humidity levels.
To Sum Up
How strong should the air that is coming out of your vents be? Even though this might seem like a simple question, there is no single answer to it. In fact, it is challenging to even find some numbers that involve the strength of the airflow and that can be used as benchmarks.
What we do know for sure is that as long as you feel comfortable while your HVAC system is working, there is nothing to worry about.
However, if you have noticed that the air strength has drastically decreased, there has to be something wrong with the unit.
Check the filters, the flex ducts, the dampers, and the vents – sometimes, you will be able to find and fix the problem yourself. In all the other cases, make sure to call a professional.