The fewer comments you receive about your HVAC system, the better.
An ideal HVAC system hums along quietly in the background. If occupants are comfortable, the equipment responsible for keeping them that way is out of sight, out of mind. But if something goes awry, it’s only a matter of time before questions start rolling in. And if those complaints are primarily about noise, it’s quite possible that some part of your system is malfunctioning. To preserve good indoor environmental quality, it’s vital to act on those complaints quickly.
Where the Noise Comes From
First, narrow down the cause of the complaint. Where are the affected occupants? Certain locations, such as penthouse offices directly under rooftop HVAC equipment, may be vulnerable to certain kinds of noise.
“It could be vibration from the equipment operating that then transfers to the structure. The walls essentially act like loudspeakers,” says Jeff Fullerton, principal consultant in acoustics for Acentech. “If it’s a piece of ventilating equipment, it often has a fan with ductwork. The sound from the fan basically follows the air down the ductwork, and wherever there’s an opening to deliver the air, the sound comes with it. You also might hear equipment if the constructions that separate you don’t provide enough isolation of the mechanical sound passing through the wall, floor, or roof.”
If you can rule out these basic causes, the problematic noise may be a sign that your HVAC system is malfunctioning somehow. With a VAV system, for example, the damper controlling air supply can cause several issues, explains Ron Davis, project manager for Crockett Facilities Services at the Judiciary Building in Washington, D.C., which utilizes hundreds of VAV boxes with direct digital control.
“If too much air is going past the damper, there may be a noise problem – think of when you have your car window down,” explains Davis. “If a room has reached the right temperature and the controls are slowly closing the damper, it may close most of the way but not fully, which also creates a lot of noise. You have a lot of airflow going through ductwork, and when it’s forced through a small area like that, the velocity of the air speeds up and creates more noise than it would if the damper were three-quarters of the way open.”
Proper system design should help you avoid most of these problems, Davis adds. However, wear and tear can result in noise issues over time, particularly a repeated hammering that may indicate faulty system components. Actuator issues can sound similar to water dripping onto metal, says Vincent Morales, DC third class engineer at the Judiciary Building.
“If the computer is telling the damper to close but the gears are worn out, chipped, or broken, it will try to close but only move slightly before the gears lose their mesh and the damper bangs,” says Davis. “It might sound like a water leak in the ceiling, but in reality it’s the actuator.”
What to Do About It
A similar noise problem in multiple areas might indicate that the pressure in the ductwork is too high, Davis says. If this is the case in your building, Davis recommends lowering the speed of the main fans; this will reduce the flow throughout the system and cut back on noise.
Sounds caused by equipment vibrating against the structure may require springs under the offending equipment, Fullerton explains. Mufflers inside the ductwork help dampen fan noise, similar to how a car’s muffler works.
“The internal components help reduce the noise that passes through with the air flow. You get the air delivered but with less noise,” says Fullerton. “However, there are some important details when selecting mufflers based on the frequency of the sound causing the issue. Certain silencers work well for low or high frequencies, and they also vary depending on how much air needs to flow through them and at what speed. If they don’t match, you can end up with a device that really impedes the air flow and makes the fan work even harder to force air through the ductwork. You might even see the noise level go up.”
Ductwork may also benefit from an internal fiberglass lining (which serves as thermal insulation and absorbs sound) or a canvas collar between the equipment and the supply and return mains, which would limit vibrations that resonate through the duct, says Tony Mori, senior HVAC estimator and planner for Crockett Facilities Services.
If someone is complaining of drafts and noise at the same time, pay them a visit and see how close their desk is to the return grill. Placing supply and return diffusers too close together can create an unpleasant environment that’s both noisy and breezy. Luckily, the grills are easy to move, Davis says – just pop it out and place it in an area where it’s not directly over an occupant’s head.
Also ensure that each area has enough diffusers in the first place, especially when heating and cooling seasons are ramping up – if you don’t have enough diffusers, the existing ones have to work extra hard, increasing the noise level. Be sure to watch out for this phenomenon if you’ve recently renovated or significantly tweaked the layout of your space, Morales advises.
“Many times, tenants will reconfigure the layout, but the way they modify the floor plan doesn’t work with the way the HVAC was originally designed,” explains Morales. “If you’re changing the size of a room, you might need to change the number of VAV boxes or registers that go to that room. If you have a 20- by 20-foot space with one register, two sides are glass windows, and it’s a 90-degree day, the register is going to push out 100% velocity of air all day and will create lots of noise because it can’t keep up.”
Perhaps the most important piece of advice? Don’t neglect regular maintenance. Simply using your own observational skills can detect many problems before occupants complain.
“We walk all eight floors of our building every day and essentially use all of our senses to determine whether or not there’s an issue with our system,” says Davis. “If there’s a noise issue, we’re going to hear it.”