MLD

Interesting that your problem was with the motor itself and not the PWM
circuitry. My first thought was that the fast switching of the PWM
generates strong high frequency components. I note that the whine
(modulation on the noise) varies varies with the motor speed.

My furnace is only about 4 months old and is also under the 10 year
extended warranty (offered as a promotion), so I don't want to mess
with my own shielding. If Trane doesn't provide a fix, I will continue
experimenting with split core ferrites on various wires. I haven't
tried putting ferrittes on the wiring to the motor.

Since the motor is surrounded by the metal cabinet shielding, the noise
must first be conducted out of the enclosure on wiring, from which it
then radiates.

MLD wrote:
"modelman" wrote in message
oups.com...
Several months ago I had a high efficiency Trane XV90 natural gas
furnace and heat pump installed. Since the weather has recently been
cold enough for the gas furnace to provide heat, I have been
experiencing severe radio interference. The interference wipes out AM
broadcast reception throughout my house. I also see snow on a TV
connected to an external antenna.

The radio noise occurs as soon as the draft inducer motor starts up;
the pitch of the interference is related to the motor speed. The draft
inducer motor is a dc motor driven by a PWM controller on the main
circuit board. It is well known that PWM systems generate strong
harmonics throughout the radio spectrum, resulting in radio
interference. However, properly designed PWM systems include filtering
and shielding to suppress the radio frequency currents from the wiring.

I contacted my installer, who is totally unfamiliar with the problem.
The electrician who was sent to my house to investigate the problem
also doesn't understand what's going on. I had to explain the
difference between an RF ground and dc or house ground to him.
Although everything appears to be well grounded from a dc perspective,
I suspect that portions of the system are radiating RF energy.

Several days ago I emailed Trane about the problem, but haven't
received any response.

Although there has been past discussion in this group about radio/TV
interference emanating from Trane high efficiency gas furnaces, I
haven't seen any definitive fix. Does anyone know if Trane has a fix
for this problem. Do they have a shielded cable and or rf filter kit
to suppress this noise? Since my system is under warranty, I won't
attempt any modiifcations to the system.

Has anyone experienced this problem?

Thanks.


Here's one guy that had the exact same problem. I was a bit luckier that
you in that it didn't bother my AM radio signal but it did screw up my
kitchen TV picture--rabbit ears. It's coming from the inducer motor as you
surmised. The installer was very cooperative and tried to fix it based on
input from Trane. Changed out the cable to the motor with one that was
supposed to provide better shielding and when that didn't work changed the
control board---both without any success. Prior to and after component
replacements I tried grounding everything that I could, again with no
success. Got the local Trane Rep on the phone; he made promises but never
followed through with a solution. Had a new Trane for about a year that got
into a problem resulting in it being replaced under warranty rather than
repaired--it was the replacement furnace that had the RF noise problem.
Found out that Trane went to a "new design" inducer motor. I tried to get
the Trane Rep to replace the motor with the same design that was on my first
furnace. No luck there, was just ignored and yessed to death. If Trane acts
dumb about the RF noise it's all show-because mine happened about a year
ago. My guess is that you're stuck with the problem. I finally made mine
going away by putting in a cable line to the TV.
My suggestion in your tussle with Trane or the installer is to go find an
inducer motor from an earlier vintage furnace. Even if it has to be
cannibalized off an older (2yrs or so) furnace.
MLD