OT: K3 vs an RFI noisy furnace

[Don Nelson]

Hi all,

I misspoke earlier this year when I said our new Lennox high efficiency
furnace was not generating any noticeable RFI. Quite to the contrary,
the new Lennox furnace is very rich in RFI energy. One observation is
that the RFI has a very broad spectrum and it's presence is not
noticeable by audio sound alone. However, my S meter noise floor goes up
by 3 or more S units when the furnace is heating the house. The furnace
we have is the Lennox G71MMP, said to be a 95% efficient furnace.

The “reference” antenna that I use is a Cushcraft R7 vertical mounted on
the side of the house. The magnitude of the RFI decreases as I listen on
each higher band. On 40m, the RFI from the furnace is over S9 compared
to baseline noise floor of S6 when the furnace is off. The receiver is a
K3 with a pretty good S meter calibration. The difference from
background noise is on the order of 20 dB on 40m and on 20m. The noise
decreases as I listen on higher bands. What the furnace RFI is on 80m
and 160m are unknown because I do not have a “reference” antenna for
those bands.

There are two variable speed motors in this furnace. One motor blows air
throughout the house. The second motor is the “induction” motor and
delivers outside air into the burn chamber. This induction motor and the
drive circuit are the source of the RFI. The blower motor, when running
alone does not generate noticeable RFI. I have used a clamp-on toroid
with a multi-turn secondary connected to an oscilloscope to sample RF on
various leads emanating from the control board inside the furnace. The
four leads going to the induction motor have significant noise on each
as does a ground wire going to the furnace chassis. The other leads
going elsewhere are quiet. The noise I observe on the induction motor
leads is characterized as fast rise time pulses.

I have tried to engage Lennox in discussions on this RFI problem with
limited success to date. The furnace installer has basically given up
without trying pleading ignorance which is understandable.

I am trying things to mitigate the RFI with limited success so far. I
would very much like to hear what others have done to mitigate their
furnace RFI problems.

Don, N0YE

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[K2QI]

Hi Don,

I'm no expert on RFI so what I'm about to say, you've probably thought about already.

But, have you considered shielding the induction motor? Or if that isn't possible, using something to null out the noise?

James K2QI
Sent via BlackBerry from T-Mobile

-----Original Message-----
From: Don Nelson <[hidden email]>
Sender: [hidden email]
Date: Wed, 10 Nov 2010 08:52:33
To: <[hidden email]>
Subject: [Elecraft] OT: K3 vs an RFI noisy furnace

Hi all,

I misspoke earlier this year when I said our new Lennox high efficiency
furnace was not generating any noticeable RFI. Quite to the contrary,
the new Lennox furnace is very rich in RFI energy. One observation is
that the RFI has a very broad spectrum and it's presence is not
noticeable by audio sound alone. However, my S meter noise floor goes up
by 3 or more S units when the furnace is heating the house. The furnace
we have is the Lennox G71MMP, said to be a 95% efficient furnace.

The “reference” antenna that I use is a Cushcraft R7 vertical mounted on
the side of the house. The magnitude of the RFI decreases as I listen on
each higher band. On 40m, the RFI from the furnace is over S9 compared
to baseline noise floor of S6 when the furnace is off. The receiver is a
K3 with a pretty good S meter calibration. The difference from
background noise is on the order of 20 dB on 40m and on 20m. The noise
decreases as I listen on higher bands. What the furnace RFI is on 80m
and 160m are unknown because I do not have a “reference” antenna for
those bands.

There are two variable speed motors in this furnace. One motor blows air
throughout the house. The second motor is the “induction” motor and
delivers outside air into the burn chamber. This induction motor and the
drive circuit are the source of the RFI. The blower motor, when running
alone does not generate noticeable RFI. I have used a clamp-on toroid
with a multi-turn secondary connected to an oscilloscope to sample RF on
various leads emanating from the control board inside the furnace. The
four leads going to the induction motor have significant noise on each
as does a ground wire going to the furnace chassis. The other leads
going elsewhere are quiet. The noise I observe on the induction motor
leads is characterized as fast rise time pulses.

I have tried to engage Lennox in discussions on this RFI problem with
limited success to date. The furnace installer has basically given up
without trying pleading ignorance which is understandable.

I am trying things to mitigate the RFI with limited success so far. I
would very much like to hear what others have done to mitigate their
furnace RFI problems.

Don, N0YE

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[n7ws]

Welcome to the brave new world.  We are on our way to making the hf spectrum totally unusable with the proliferation of wireless this, microprocessor that, a switched-mode power supply in every electronic doodad, a CFL in every socket and high current stepper motors everywhere else.

For example, Home Depot sells fluorescent fixtures rated "for industrial use only" because they don't meet residential RFI standards, which begs the question of what do you do if you live next door to an industrial facility.

After installing some of these in my workshop/garage (using conduit) the RFI was so bad it interfered with OTA TV (pre-digital).  I pointed out to the store manager that it said "Home" in big letters on the outside of the building and demanded that they special order ballasts rated for "home" (residential) service.  To their credit, they did, but I had to furnish the labor to change them out.  Needless to say, not to their credit, they're still selling the RFI generators to everyone else.

They also sold me a Maytag front-loader washing machine that is a horrible RFI generator. Besides the microprocessor, there is the variable-speed-reversible motor driving the drum. Same issue that Don has, high-current, fast rise time pulses transmitted on unshielded wiring without any concern whatsoever about RFI.

Wa

--- On Wed, 11/10/10, Don Nelson <[hidden email]> wrote:


Hi all,

I misspoke earlier this year when I said our new Lennox high efficiency
furnace was not generating any noticeable RFI. Quite to the contrary,
the new Lennox furnace is very rich in RFI energy. One observation is
that the RFI has a very broad spectrum and it's presence is not
noticeable by audio sound alone. However, my S meter noise floor goes up
by 3 or more S units when the furnace is heating the house. The furnace
we have is the Lennox G71MMP, said to be a 95% efficient furnace.

The “reference” antenna that I use is a Cushcraft R7 vertical mounted on
the side of the house. The magnitude of the RFI decreases as I listen on
each higher band. On 40m, the RFI from the furnace is over S9 compared
to baseline noise floor of S6 when the furnace is off. The receiver is a
K3 with a pretty good S meter calibration. The difference from
background noise is on the order of 20 dB on 40m and on 20m. The noise
decreases as I listen on higher bands. What the furnace RFI is on 80m
and 160m are unknown because I do not have a “reference” antenna for
those bands.

There are two variable speed motors in this furnace. One motor blows air
throughout the house. The second motor is the “induction” motor and
delivers outside air into the burn chamber. This induction motor and the
drive circuit are the source of the RFI. The blower motor, when running
alone does not generate noticeable RFI. I have used a clamp-on toroid
with a multi-turn secondary connected to an oscilloscope to sample RF on
various leads emanating from the control board inside the furnace. The
four leads going to the induction motor have significant noise on each
as does a ground wire going to the furnace chassis. The other leads
going elsewhere are quiet. The noise I observe on the induction motor
leads is characterized as fast rise time pulses.

I have tried to engage Lennox in discussions on this RFI problem with
limited success to date. The furnace installer has basically given up
without trying pleading ignorance which is understandable.

I am trying things to mitigate the RFI with limited success so far. I
would very much like to hear what others have done to mitigate their
furnace RFI problems.

Don, N0YE




     
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[AB3EN]

Donn (WY5I) had a similar issue with a new "high efficiency" unit and when he contacted the manufacturer and mentioned the FCC requires them to take some actions they worked through the issue with Donn and took the fix back to the plant for modification of new units. Just like the electric company and RFI from the local poles, the FCC requires action to suppress the RFI.

Good Luck
73
Dan
 

[Don Ehrlich]

I have never had a noisy furnace or an appliance with the type of motor
control you describe.  But, as an engineer who fought various EMI problems
on military and other aerospace equipment I can guess what you are facing.
It is broadband conducted noise (inherent in sharp risetime pulses of
current) and there is no kind of shielding that will cure it.  The noise is
conducted (not radiated) from the motor to your house wiring and the wiring
in your house is the 'antenna' that radiates the noise everywhere.

The only hope you have is to keep those sharp risetime pulses out of the
wiring coming from the furnace.  To do that you can connect capacitors from
each wire to ground AS CLOSE TO THE ACTUAL MOTOR LOAD as possible .. which
means taking the furnace apart to some extent, possibly voiding your
warranty, etc.  Ferrite inductors in each lead including the ground coming
from the furnace will help as well.  All of that requires some knowledge of
capacitor and inductor ratings and characteristics and in the end, if my
experience is any guide, may not work very well anyway because of other
circuits (thermostat wiring, etc) as well as the fact that the ground system
to which the capacitors are attached may not be robust enough to effectively
return the interfering currents to their source in the motor controller.
The current spikes going to a variable speed furnace motor are likely to be
many amps in amplitude (much higher than the steady AC current) and it takes
only milliamps traveling through your house wiring to cause significant EMI.
That gives you a feel for the scale of the problem.

I don't want to depress you .. but controlling EMI from a large appliance
that is designed to heat efficiently with not a thought for interference is
very problematic.  In aerospace it is not easy either but at least most
equipment is physically packaged from the beginning with
compartmentalization, robust copper or aluminum ground systems, and internal
shielding where needed in anticipation of EMI needs so that when EMI
problems do occur (and they always do) they are at least manageable.

Don K7FJ

---------------

Hi all,

I misspoke earlier this year when I said our new Lennox high efficiency
furnace was not generating any noticeable RFI. Quite to the contrary,
the new Lennox furnace is very rich in RFI energy. One observation is

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[Don Ehrlich]

Don,

My last post was wrong in that I actually have had a serious EMI  problem
caused by a large variable speed motor.  The motor I have is in my treadmill
and, since I am the only person who uses it, it is never in use while I am
using my K3.  It did cause  a lot of interference to AM radios in the house,
however, and I tried a quick and dirty fix which consisted of wiring a pair
of two microfarad/400 volt mylar film capacitors directly to an AC plug (one
capacitor from each line to the ground pin) and then plugging it into the
same outlet as the treadmill.  This was a convenient way of connecting
capacitors from each wire to ground but it was done at the end of the
pigtail and not at the motor controller where it would be more effective.

The capacitors reduced the noise substantially but did not eliminate it.
One advantage in my treadmill system is that it had only the one connection
to house wiring and the treadmill is a compact and self-contained system.
However, the DC motor in it is rated at 2 horsepower which is likely a lot
bigger than the motor you have in your furnace.  If you can get to the
furnace wiring where it connects to your house wiring it may not be too
difficult to connect some capacitors there.  It could buy you a few db of
relief so it is worth a shot.  If you do it then be sure to use high quality
film capacitors of a microfarad or more.  They will be of substantial size
because it is important they have very low ESR (equivalent series
resistance).

Don K7FJ
---------------------------.

> I don't want to depress you .. but controlling EMI from a large appliance
> that is designed to heat efficiently with not a thought for interference
> is
> very problematic.  In aerospace it is not easy either but at least most
> equipment is physically packaged from the beginning with
> compartmentalization, robust copper or aluminum ground systems, and
> internal
> shielding where needed in anticipation of EMI needs so that when EMI
> problems do occur (and they always do) they are at least manageable.
>
> Don K7FJ
>

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[Jim Brown-10]

On 11/10/2010 10:20 AM, Don Ehrlich wrote:
> I tried a quick and dirty fix which consisted of wiring a pair
> of two microfarad/400 volt mylar film capacitors directly to an AC plug (one
> capacitor from each line to the ground pin) and then plugging it into the
> same outlet as the treadmill.

This is unsafe and illegal, because it creates large leakage currents on
the green wire.  The maximum permitted leakage current on green is 5 mA.

One proper way to filter the AC line is with a filter that includes
series inductance (usually a common mode choke) and much smaller shunt
capacitors.  Common mode current on the AC line can also be suppressed
using multi-turn ferrite chokes wound on the mains power cable.

See http://audiosystemsgroup.com/RFI-Ham.pdf

73, Jim K9YC


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[Jim Brown-10]

On 11/10/2010 7:52 AM, Don Nelson wrote:
> I am trying things to mitigate the RFI with limited success so far. I
> would very much like to hear what others have done to mitigate their
> furnace RFI problems.

There is a very good RFI email reflector at contesting.com   Go there to
sign up.

For RFI advice, see http://audiosystemsgroup.com/RFI-Ham.pdf

73, Jim Brown K9YC
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[John Ragle]

Jim...

     Thanks very much for sending along the link to your article. It is
a very nice not-too-technical summary, particularly as it addresses the
"RFI" problem that some of us experience with nearby audio equipment, as
well as the function of the "green wire." Although not a part of the
usual Elecraft submissions, it should be "required reading," and I was
very pleased to find it on the reflector.

John Ragle -- W1ZI

=====

On 11/12/2010 12:52 AM, Jim Brown wrote: ...See
http://audiosystemsgroup.com/RFI-Ham.pdf...

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[KC2WFF]

I would like to thank you all for this learned discussion. I'm a relatively new ham (licensed in 2009, but not on the air until 2/2012 for a number of reasons). I have a "Brand X" rig (Kenwood TS590S) and a rooftop vertical (Chameleon V1). I also have (unfortunately) a Lennox G71mpp furnace with the variable speed blower. The broadband noise from this furnace is out of site - S9 on 20m when it's running versus S2 when it's not running. Using a horizontal 20m dipole helps a little bit (about 2 S units).

Using a handheld AM portable radio (tuned to the top of the AM band) as a noise detector, I can confirm that it appears to be the "induction" motor and/or other electronics causing the bulk of the noise (vice the blower motor itself). The noise is every where in the house wiring.

I tried a homebrew filter cap (.47 microfarads) across the AC line (from black hot to white neutral) at the furnace itself and didn't see much improvement. Perhaps it's time to try a "line filter" as was suggested in the excellent paper on RFI.

Bob
KC2WFF