[RFI] Lennox furnace: when it heats it roars (RFI)

[Don Nelson]

Hi all,

We have a new Lennox furnace which 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 (5 dB per S unit). 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 working 
with the ARRL RFI guru.

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