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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
