ASHRAE (The American Society of Heating, Refrigeration and Air-conditioning Engineers) has developed noise criteria for systems based on the space they are conditioning. These are referred to as NC levels or curves.
Sometimes it is quite obvious as to where the noise is originating, sometimes not; and sometimes there is more than one cause of the unacceptable noise level.
If the source is hard to identify at first observation, it is usually easiest to begin at the delivery end of the system and work back to the mechanical equipment.
Check the diffusers. If they are undersized or there are not enough of them, they will have an excess of air being delivered through too small an opening (or openings). That’s how we whistle: forcing a lot of air through a small opening. This excessive discharge velocity causes that “ssshhh” sound.
SOLUTIONS: Open the diffuser opening more (if it has been partially shut), reduce the volume at the volume control damper for that space if there is too much air being delivered, replace the diffuser(s) with larger one(s) or add more diffusers.
Check the ductwork. If it is rumbling it could be caused by turbulence at direction changes (elbows) or reducers. Adjustments in supply air volume or the installation of turning vanes may be the solution. If the duct is resonating (vibrating) a damping material should be considered. If you strike a bell it rings. If you hold the bell and strike it you stop its resonation (it goes “clunk”). Damping materials make ducts and other vibrating materials “go clunk.” The duct may be vibrating due to turbulence, excessive volume or the transmission of vibration from things connected to the duct – frequently the mechanical equipment itself. Duct silencers and duct liners are also options in preventing noise from traveling through the sound pipe that is your duct system.
Check the mechanical equipment (home base). Worn parts (bearings, belts), lose covers, housings and fan belt shrouds, are typical noise generators. The original vibration isolation mountings may be worn or frozen. Is there a sound absorbing/damping liner on the inside of the housing? These can be added. Under unusual conditions, it may be necessary to create an enclosure to contain the noise from the mechanical room or HVAC equipment.
Other Issues: Ductwork can provide a conduit for conversation which, in certain sensitive situations (secure government “need to know” environments, human resources, etc) can be a problem. Sound traps and duct liner can be useful. Additionally the addition of masking sound into the duct and/or the sensitive and adjacent space should be considered.
Open return vents can also provide a less inhibited route for conversation to pass between adjacent spaces. They also permit noise in the plenum to be transmitted to the occupied space below. Sound traps above the return grills will address that path.
These suggestions provide an overview and may enable you to identify the cause of a problem. Sometime the solutions are easily executed and sometime your HVAC engineer, contractor or others need to be brought in.
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