60 Hz small transformer emi radiation 1

Background:
I install Audio Frequency Induction Loop Systems (AFILS) to assist hard of hearing people to hear. AFILS connect inductively to the hearing aid via the hearing aid telecoil. Best assistive listening system around!
Some sites where I do an install have a number of small stepdown (110V to 24V or less) transformers for lighting or heating control systems. These are sometimes a source of objectionable emi that can be heard by the hearing aid. Since hearing aids don't pickup 60 Hz signals, it's likely some harmonic of 60 Hz e.g. 120, 180, 240, 300 that's being picked up.
I just tested a home doorbell transformer with a "Gauss" meter (

http://www.ampetronic.com/products/FSM-Field-Strength-Meter-.asp?cat=16).

As expected, the emi field strength drops off very rapidly with distance. By the time I've moved about 12 inches away from the transformer, the emi can no longer be measured with the meter that I have.
I would expect the lighting & heating control transformers to behave the same way, i.e. rapid drop off with distance. And yet, I find that in some instances, the field strength is excessive for up to 25 feet away from these small transformers.
Questions:
1. What might be different in the venues where the emi radiates over large distances?
2. How can this distance be reduced to one or two feet from the source?

Thank you for your help,
Bill Droogendyk
Better Hearing Solutions

http://www.betterhearingsolutions.ca

 

[VEBill]

The required concept might be: Conducted Emissions versus Radiated Emissions. Radiated emissions leave the source through 'the air'. Conducted emissions leave via the wiring.

Once the noise is on the wiring (likely in common mode), then you can have the situation where the wiring acts like the same sort of loop as your AFLIS.

Maybe...

 

VE1BLL;

I rather think that you are on to something. In this latest situation, there are stepdown transformers that connect to control valves on a hot water heating system. I'll have to check, but I think that the wiring between the transformer and the control valve does not include a ground wire. The various heating pipes probably connect to ground somewhere / somehow, but at a different potential than the building electrical system. Might grounding the heating pipes to the electrical system ground solve the problem?

Bill

 

[VEBill]

If you can track down the current path(s), then either a solution will present itself or you'll discover it's hopeless.