I to have inherited this problem after purchasing a new home that has a high
efficiency heating/airconditioning unit installed.
My unit was manufactured by Tappan which I discovered had this newer ECM type
DC variable speed blower motor .
What a racket it made especially on 17 and 15m where its was most pronounced
with the RFI being generated
by the solid state controls mounted inside the motor .
By adding a braided copper shield cover over the unshileded multi wiring
harness going to the motor and to the upper section which houses the other crt
boards for this unit .
Doing this ,I have almost completely eliminated the RFI produced by the ECM
speed control circuit.
I also installed a 3 amp. 120 volt RFI Filter made by Curtis Industries to
filter any RF being fed back from the motor circuit to the rest of the control
wiring of the heating/cooling unit.
I also bonded the inspection panels of the two compartments which house these
areas as they were just painted metal plates with no real grounding because of
the painted coating these items did not really do any shielding
until I removed the around the mounting screw holes and added a braided copper
bonding strap to these panels.
Now on 17m and 15m this offending RFI has all but gone from the original S 9
hash every 30 khz on these bands.
I recently called the company who made the variable speed motor circuit asking
if they had a solution to this problem.
They replied that they had never heard of this type of complaint before.
Problem solved .
On to the next RFI issue ... 20 khz. low voltage lighting power transformers.
FYI ... they are now (2011) experimenting with 20 khz. 6 pole switching mosfet
controlled , AC motor circuits ...
RFI mailing list