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Author Topic: RFI from furnace  (Read 13013 times)

Posts: 992

« on: January 17, 2015, 06:34:43 PM »

Recently had a new high efficiency furnace installed, an American Standard Gold Series. I had to replace a light base today so while shutting off breakers I thought I would try and find the source of the noise on the upper bands that has been plaguing me. Well it's the furnace. I flipped the breaker and the noise vanished. Only concern is the furnace was not running at the time. I flipped the breaker back on and the noise came back. I left the breaker in the ON position and went over to the furnace and flipped the power switch off. Noise gone.

I've sent an email to the manufacturer asking if they offer a fix, but since the furnace wasn't actually running at the time, what could be causing the RFI. I think this particular example rules out the motor.


Posts: 192

« Reply #1 on: January 17, 2015, 07:23:22 PM »

There are all kinds of electronic controls in modern furnaces that are "always-on".  Good luck isolating the problem.

Posts: 7998

« Reply #2 on: January 18, 2015, 07:47:42 AM »

A home furnace that is powered but not producing heat has two electronic items running. The AC-DC power supply and the digital control electronics. If the power supply is a switching power supply it will emit noise at harmonics of its switching frequency. This is roughly every 70 to 200 kHz. The controller generally emits noise at harmonics of the clock frequencies.

I assume the RFI you hear is in the HF band. While the power supply and/or controller are the noise sources the AC power wiring and the thermostat wiring are the egress points. The RF noise at these two egress points can be attenuated. You can isolate the egress point to one or the other by disconnecting the thermostat wires at the furnace. If the noise amplitude drops this is the dominant egress point. After isolating the egress point to one or the other the first fix to try is common-mode filtering on the noisy egress point.

Snap-on ferrite cores are the easiest to install. DX Engineering stocks these and the DXE-CSB-COMBO kit is a good start. A target for common-mode impedance is over 1000 ohms. With one turn of wire through the ferrite core this can take many turns. For example, the 0.750" ferrite core presents an impedance of 69 ohms at 10 MHz. At 3.5 MHz it is around 10 ohms. Fortunately the impedance is proportional to the square of the number of turns, so wrapping the wire 10 times through the ferrite core will give 100X the impedance. The ferrite core that was 10 ohms with one turn is 1000 ohms with 10 turns.

This works if there's enough extra wire in the furnace. If there isn't you can extend the wire length. For the thermostat all of the wires are wrapped together through the ferrite core. For the AC power the Hot and Neutral wires are wrapped together. Sometimes wrapping the Hot, Neutral, and Ground wire together works better. You can experiment.

Another way to tackle noise filtering of the AC power is by adding an off-the-shelf EMI filter. Two suitable filters are the Schaffner FN2010-20-07 with flying leads and the Qualtek 851-20/011 with threaded studs. Both are available at Digikey. The filter is mounted to the grounded enclosure the the Hot and Neutral wires connecting to the filter. The ground wire should connect to the filter case.
« Last Edit: January 18, 2015, 07:57:09 AM by WX7G » Logged
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