We’re exposed to a variety of sounds each day. Some are enticing, like music to our ears. Others are like fingernails down the chalkboard. While at the AHR Expo in Orlando a few months ago, I ran into Art Miller from RSES (Refrigeration Service Engineers Society) and had a great chat about how sound is a forgotten aspect of comfort.

As we talked, we noted that one definition of comfort is when you don’t know the HVAC system is running. It maintains comfort, and you don’t hear any HVAC system noise. Unfortunately, many people know the second their system comes on, and it’s a major pain in their lives. If you’ve ever tried sleeping in a hotel room with a PTAC (packaged terminal air conditioner) that sounds like it will vibrate out of the wall, you know what I mean.

Chances are high that you have customers with sounds coming from their HVAC systems that drive them crazy. They often want it fixed but don’t say anything or have gotten used to it. Sound isn’t an issue we normally discuss in the HVAC industry, but we absolutely should. With that in mind, let’s review some basic principles and how you can use them to diagnose and solve sound-related HVAC issues.



Sound is measured in decibels (dB) and used to quantify sound pressure levels. You may see it abbreviated as (dBA), a reference to A-weighted sound levels. They are the closest match to noise levels our ears recognize. Small changes in decibels can result in huge variations in our hearing perception.

There are different sound tolerances that are helpful to know. These come from two of the most recognized organizations close to the HVAC industry — the National Institute for Occupational Safety and Health (NIOSH) and the Environmental Protection Agency (EPA).


NIOSH recommendations are:

  • Less than 85 dB — worker exposure limit for eight hours
  • Less than 100 dB — worker exposure limit for 15 minutes

EPA recommendations are:

  • 45 dB — acceptable indoor noise level limit for residential areas
  • 55 dB — limit for outdoor areas with human activity
  • 70 dB — limit of safety before hearing loss


Additional sound reference points are:

  • 20 dB — silent study room
  • 40 dB — soft whisper (5 feet away)
  • 60 dB — conversation (3 feet away)
  • 80 dB — freight train (100 feet away)
  • 90 dB — boiler room
  • 100 dB — construction site
  • 120 dB — operating heavy equipment
  • 140 dB — threshold of pain


I provided these examples for reference in case you decide to check them out. Introductory level sound meters are inexpensive, but if you have an iPhone, you can download the free NIOSH sound level meter app. Try it out and give your customers a gauge on how loud their system is. As you test, keep a look out for these annoying sounds an HVAC system makes.



One of the obvious HVAC sounds that drives people crazy is jet engine noise. Most airplane passengers put on noise-cancelling headphones to avoid this nuisance. To keep it in context, a jet engine taking off from 200 feet away is equivalent to 130 dB.

While an HVAC system might not get that high, the same nuisance is often experienced. If you find your customer turns up their television or yells during conversations, they might be dealing with “jet engine noise.” It’s often tied to fan-generated noise when the blower is on. The most common culprits are:

  • Electronically commutated motor (ECM) blowers on undersized duct systems
  • Return platforms and closet returns
  • Duct leaks close to the blower assembly.

The jet engine noise usually isn’t an issue when the duct system is sized correctly before installing ECM equipment. If it’s a system you’re servicing or maintaining, duct upgrades may be in order if the equipment is sized correctly. Otherwise, downsizing equipment can help lower decibels.

If you have a platform return, use sound-deadening materials like ductboard or internal liner inside the return cavity to absorb and deaden the noise. You can still have this problem on a properly sized duct system if the blower inlet is too close to the return grille. In some instances, you may need to add return ducting to reduce fan generated noise.

 Duct leaks close to the equipment can also cause jet engine noise as air passes through panels. You can install gaskets on any removeable service panels to minimize noise. Whatever you do, don’t mastic the blower door or coil access panel. I hear that technicians place a bounty on the heads of those who mastic these panels shut.



In the late 1980s, the band U2 released the album “Rattle and Hum.” While it was popular back then, it’s an unwelcome sound when coming from many HVAC systems. The most common sources are typically associated with air noise made from balancing dampers, grilles, and registers.

Balancing dampers that contact the inside of metal ducts are known to rattle as air passes across them. If you’re lucky, the thumb screw on the damper assembly is loose and just needs tightening. If it’s one of those days, you may need to cut into the duct system to isolate and repair the offending damper.

Poorly selected grilles and registers can cause excessive air velocities through the vanes. As air passes though these vanes, a hum is produced. If the branch duct connection at a boot or can is out of alignment, sound levels can also increase as much as 12 dB due to the increased turbulence.

Pay attention to the desired airflow from a grille or register and make sure it’s within acceptable velocity limits to keep the hum from occurring. Design supply registers for a face velocity under 500 fpm and return grilles in the 300–500 fpm range to keep the system quiet. You can also reference the manufacturers noise criteria (NC) rating of the grille or register in question.



When a duct system is subjected to certain changing pressure conditions, the noise sounds like pressing in on an oil can. Many of us have dealt with panned floor joists making a “kerplunk” sound as they are walked on. You can usually track down this problem with a good ear, but what about those times when the source isn’t as obvious?

One such condition is created when undersized ducts are installed. Excessive static pressure can cause movement in metal duct as the blower in the air handling equipment begins to move air.

One of the most common variables in the oil-canning ducts is the air filter’s condition. When the filter is new, there is more pressure in the return side causing ducts to “oil-can” each time the fan comes on. As the filter starts to load up with dirt or is replaced with a more restrictive media such as a pleated filter, the problem seems to mysteriously disappear.

Static pressure testing can help reveal the answer to this intermittent issue. At National Comfort Institute (NCI), we teach to compare measured static pressures against NCI static pressure budgets to see which side of the duct system is most restricted and needs the most attention. Keep in mind this problem can still occur with acceptable static pressures when ducts are made of extremely light-gauge metal.



Follow industry best practices and test your installations to verify your designs are successful. You can make your systems sing in a good way that’s doesn’t drive your customers nuts. Consider this forgotten aspect of comfort and use it to find new ways to better serve your customers.

I’m sure you can think of other annoying sounds and descriptions that I haven’t mentioned here. I would love to hear what HVAC noises drive your customers crazy and what you’re doing to correct them. If you aren’t already, start to ask questions about noise problems plaguing your customers — you’ll probably be the only HVAC professional who does.

Be aware of room acoustics and how they influence sound transmission. You can have a well-designed and well-installed HVAC system that still creates unwanted noise if there are too many hard surfaces in the space. Think about materials like hardwoods, glass, metal, and masonry, as these tend to reflect noise.

Many of the issues I mentioned are related to duct design and installation. Follow industry best practices and test your installations to verify your designs are successful. You can make your systems sing in a good way that’s doesn’t drive your customers nuts. Consider this forgotten aspect of comfort and use it to find new ways to better serve your customers.


If you’re an HVAC contractor or technician interested in learning more about the skills you need to optimize duct systems, contact me at davidr@ncihvac.com or call 800-633-7058. NCI’s website, www.nationalcomfortinstitute.com, is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.