Do you hear a high-pitched, tuning-fork type of sound coming from your return air vent, the metal grille that pulls air into your heating and cooling system?
It’s a situation we hear happening to several homeowners.
This high-pitched sound is typically the sign of low return airflow.
In other words, your system’s blower is “starved for air.” It’s trying to pull in a certain amount of air, but is struggling to do so. As this happens pressure and velocity increases, therefore increasing the sound level (that’s the sound you hear).
Yes, you should be worried about this since low airflow can damage your system and increase your energy bills.
We’ll explain this problem in more detail and explain how you can solve it.
Your system can whistle for the same reasons you can
Think about this problem like breathing normally versus when you whistle.
When you breathe normally, air can easily pass through your mouth. There’s very little air pressure and low velocity. You barely hear a thing. Similarly, when there’s nothing wrong with your vents and ductwork, air easily passes through and therefore there’s very little noise.
Conversely, when you try to whistle, you pucker your lips into a small “O” shape, creating a smaller passageway. Thus, air has a hard time passing through and, therefore, the harder you blow, the higher the velocity and the sharper the whistling noise.
So here’s the main issue: What’s blocking/restricting airflow in the vents and ductwork? What’s causing the blower to work harder to pull in more air?
Well, there are problems you can solve and problems only an AC technician can solve.
6 problems you can solve
Dirty air filter
A dirty filter acts as a barrier to return air, forcing the blower to work harder and longer to circulate air (say hello to a larger electric bill!). Check the filter and change it if it looks like the one on the right in this picture:
Blocked return vent
The return vent may be blocked due to dust or other debris. Make sure to clean it off so air can easily pass through.
Blocked or shut off supply vent
Supply vents (the grates with the levers) may be shut in certain rooms of your home. You should always keep them open (yes, even in rooms you don’t use.) Also, make sure you don’t have any furniture, like a couch, covering the supply vents.
Shut or partially closed dampers
Dampers are metal valves that you can shut to prevent air from being delivered to certain parts of your home. When shut, you have the same problem as shutting supply vents. Locate damper levers near the inside unit and make sure they’re positioned in line with the ductwork.
Large debris in ducts
Small toys, screws, nails, sawdust, and a variety of knick-knacks make their way into the supply side of your ductwork. Take off the supply side vents and see if there’s anything blocking the path that you can take out.
Too many closed doors
Your system may be too air starved because you have too many closed doors inside your home.
Why does this cause problems?
Well, notice that many return grilles are located in central areas, like a hallway or stairway and many supply vents are located in smaller rooms with doors, like a bedroom. When you close those doors, the air has to sneak under the small gaps of the doors to make its way into the return grill. So the blower has to work harder to pull in more air, causing the whistling noise. So, whenever possible, keep doors in your rooms open.
Try solving the above DIY problems first. And if those don’t work, call an AC technician to investigate your system. They may find the following problems...
4 problems a technician needs to solve
Undersized return grille
If your return grille is ductwork is too small, it’s the equivalent of the puckering of your lips we mentioned earlier. A lot of air is trying to make it through a smaller area, and thus you get the high-pitched whistling noise. the same amount of air is traveling through a smaller area. The return grille should be sized to match your cooling and heating system.
Your cooling/heating system is designed to move a certain amount of air in your home. But if it’s too large it may try to move more air than your ductwork can handle. Basically, it’s like putting a size 15 foot into a size 10 shoe.
Not enough return inlets
Maybe your system and current return grilles are sized just fine. The problem may be that there are just not enough return grilles to return air to the inside unit.
Issues with blower wheel
If the sound originates at the furnace, the problem may just be with the blower wheel/fan and the sound is just carrying out into the return duct.
Need a Phoenix tech to solve your problem?
If you live in the Phoenix metro area, contact George Brazil to investigate and fix your duct system.