HVAC duct work radiates 60Hz
I know because I wear hearing aids. When passing by a wall with duct work and the aids in the tele-coil position I get a very loud low frequency buzz (I'm assuming it's 60 Hz maybe 120 Hz). I've noted this in other places including my workplace when around strong electrical fields. My old house has a mix of knob and tube, newer 2 wire ungrounded (relative to the k&t) and the kitchen is 2 wire w/ground. I'm in the process of replacing the older wiring and have posted a few questions here about that process and, hopefully, soon all be updated. There is a single ground from the main outside disconnect to a cold water pipe in the basement. Can't say at the moment that the power supplying the heating unit(in basement) is grounded but I doubt it. Would that ground omission cause some kind of neutral current flow in the duct work or could that be caused by some other problem? Or is it neutral current flow at all? I think the duct work should be grounded but don't want to do anything that might damage any of the system. I have worked with the duct work, as I'm zoning into an upstairs down stairs system and have never received any kind of shock. BTW I also have the magical ability to hear leaking microwave ovens:laughing:
A stray wire touching the metal frame of the furnace, old wire with insulation falling off touching the ductwork, wire run around a duct and the duct is cutting into the wire, a "leaking" electric motor in the furnace, etc. Any of these could cause the duct work to become energized.
Or a water pipe system to become energized.
Or anything metal running through a house or a building.
For that reason, it is a good idea to bond metal electrically conductive systems like plumbing, natural gas pipes, ductwork, etc. to ground.
This could be a very dangerous situation. If you are not familiar with troubleshooting these sorts of electrical problems, I suggest you call an electrician. I suppose the electrician would run a long wire to ground, then measure with a multimeter from that wire (ground) to the ductwork to see if there was a voltage on it.
You could confirm which circuit it is by turning off one breaker at a time. See which stops the noise in your hearing aid.
ductwork MUST be grounded so don't worry about harming something by grounding it. If it damages something by grounding it, there is something wrong that needs to be fixed anyway.
In fact, code requires any metallic structure that is likely to become energized to be bonded to ground. That means; water pipes, air duct, and aluminum or steel siding.
In fact, at my fathers house, he had foil backed insulation that had to be bonded as it functioned as a capacitor. Made some neat lightning shows in the basement but a bit scary as well as an arc ran across the ceiling to the grounding electrode system on the other side of the house.
The ductwork should be effectively grounded simply because it is connected to an air handler that should be grounded. If there is a canvas boot that would electrically isolate the ductwork, there should be a bonding jumper installed to allow continuity.
What I would do before bonding the ductwork would be to take a voltmeter and measure from the ductwork to a ground source. If you have grounded receptacles, this would be as easy as checking from the grill/louver to the ground on a receptacle.
Do you notice this only when the air handler is running or at all times>
as Billbob suggested, you can turn off once circuit at a time to identify the circuit possibly. That may help lead you to any problems you may have.
It is acceptable although not always enough (sufficient) to bond (connect) to ground any point on plumbing or ducting. Should bonding the duct to ground result in a short circuit, do not disconnect (unbond) the ground connection but instead find and fix the short even if the breaker has to stay off for awhile.
I am guessing that there is only a minute induced current in the ductwork from wires in the wall passing nearby. Your hearing aids may be unusallly sensitive to the 60 Hz electrical field that surrounds the power wires or that results from the induced current in the ductwork.
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