What The Tech? The Tissue Test

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    By Jim Knapp

    Jim KOccasionally a new piece of variable speed equipment will be responsible for an unusual noise in a heating and cooling system. This noise is more evident in the living space and can be described as a thrumming, or as I like to call it a “banshee wail.” Oftentimes the noise will change in pitch or volume as one moves about the room. The natural reaction is to blame the new blower, squirrel cage, or blower motor for making the unusual noise. Before you start changing parts in an attempt to eliminate that noise try the tissue test. Grab any paper tissue and lay it over the return grilles, one by one, while the blower is running (and making the noise). If the noise suddenly stops when a tissue is flattened against the grille then you have found the source of the noise. One or more of the louvres is being vibrated at their natural harmonic frequency. Also known as sympathetic resonance. Try identifying which louvre is the culprit by grabbing the louvres one-by-one with a pair of needle-nose pliers. When you’ve grabbed the noise maker the sound will stop again. Some people prefer to try to resolve the harmonic noise by twisting the offending louvres a bit; in essence re-tuning them. Others like to just replace the grille hoping that a new grille will have a different resonance.


    At this point you have identified the source of the noise, but not necessarily the cause. Much like putting a blade of grass between your thumbs to make a reed whistle, the vibration associated with variable speed equipment is caused by airflow over that reed; in this case, the louvre. Variable speed blowers operate at a whole range of different speeds, it could be that one of those speeds just so happens to cause the airflow across the louvre to vibrate in sympathy (like the blade of grass). However, it is more likely that the vibration is caused by excessive air velocity. It could be as simple as adjusting the airflow at the equipment; making short work of the problem. More likely, the return air system is too restrictive. Restrictive duct will cause increased face velocity at the grilles. More air pressure and movement are more likely to set the louvres vibrating.

    Finding the cause of duct restrictions, and eliminating them, will net two benefits. It will help avoid weird noises, and it will allow the blower motor to run at a lower torque. The biggest reason for motor module failures is restrictive ductwork. The little thermistor in the module overheats when the motor has to work too hard. You can get rid of a future breakdown while you are getting rid of the weird noise.




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