RF destroys Nest Thermostat

Discussion in 'General Technical Questions and Answers' started by KC3KDK, Jun 20, 2016.

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  1. KC3KDK

    KC3KDK Ham Member QRZ Page

    I've really done it this time.
    I was playing around with my FT-991 with an FC-40 tuner on a 102 foot random end antenna.
    I added a 9:1 unun in an attempt to better tune 40 and 80 meters.
    The first time I hit the key, I heard my AC compressor try to turn on. Like the relay inside was opening and closing rapidly, and it almost sounded like electrical arcing. The antenna and setup is roughly 20 feet from the AC compressor.
    Also the blower kicked on. My HVAC system had been set to off from the thermostat, so it's not like the system was calling for cool.

    So after several hours of troubleshooting my HVAC system, I found that the cause was the Nest Thermostat, specifically the mounting ring that controls the currents to the different HVAC components. The nest uses a vampiric charging mechanism that "borrows" the line through the AC relay. It's enough to provide a trickle charge, but not enough to throw the relay. As a result, I think I generated enough voltage in the relay coil to cook the thermostat. I haven't had any RF in the shack issues prior to this, but I assume this is a result of that.
    With the nest not being grounded or tied into the furnace common, the current had nowhere to go I guess.

    Any way to avoid this is in the future before I cook another $250 thermostat?
  2. WR2E

    WR2E Ham Member QRZ Page

    See also the recent thread about end fed wires.

    Does your antenna have some way for the RF current to return to the transmitter? Or does it have to find any way it can to get back, I.e. through house and control wiring?
  3. KC3KDK

    KC3KDK Ham Member QRZ Page

    Well, I tied the ground in to the AC ground rod, connecting it directly to the same rod. I don't have any additional space for a dedicated RF ground, as I already have 3 ground rods in the same area

    Radio, Power Supply, and Remote Tuner are all wired to a ground bus(copper pipe) and tied into the ground rod system.
  4. WR2E

    WR2E Ham Member QRZ Page

    Sounds like the RF has multiple paths... one through the ground wiring.

    The other possibility is that you coupled RF directly into the tstat wiring. How long is that wire? Is it possible that it's near resonance?
  5. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Put toroid chokes on the input/output lines. Use 3-5 turns.

    -You could put a standard MOV across the power lines to control surge voltage
    -Put a safety rated X or XY .01 uF capacitor across the power lines to bypass RF from the thermostat.
    Last edited: Jun 20, 2016
  6. KC3KDK

    KC3KDK Ham Member QRZ Page

    Actually, I didn't have the issue until I used a toroid choke. It was adding the choke that seemed to cause the issue. 10 turns, connected directly to the single line. It seemed to tune 40 and 80 just fine. It just also cooked my thermostat.

    I didn't have that issue without the choke. Might just be a coincidence, but it was literally the first time I keyed the mic.
  7. KC3KDK

    KC3KDK Ham Member QRZ Page

    I suppose anything is possible, the tstat wiring is behind the wall.
  8. KC3KDK

    KC3KDK Ham Member QRZ Page

    Would a ferrite bead on the A/C compressor wire help?
  9. KC3KDK

    KC3KDK Ham Member QRZ Page

    Basically, I was using an end fed connected to the remote tuner without a choke for the past few days. I could tune 160m, 20m, 17, 15, 10 without any problems. 80, and 40 had too high SWR.
    I had read that using a 9:1 unun would allow you to better tune a lot of bands, so i decided to build this one:

    I got it all hooked up, selected 40m, and hit the auto-tuner. Tuned the first time without any problems at all.
    Keyed the mic, and noticed low SWR. I then noticed some weird arcing type noise, so I removed my earphones.
    Turns out it was the A/C compressor, located about 20 feet from me. It actually started to spin up and turn on as the relay inside cycled on and off.
    Now, any time I connect the Nest thermostat, the relay starts to flutter and cycle on an off.

    I don't think it was caused by the choke, so much as that's the first time I was able to hit 40m with full power. It's possible that the thermostat wire that fed from the A/C compressor into the furnace and then out to the thermostat, were just long enough to be resonant at 40m. The current that was generated was just enough to cook the sensitive relays in the thermostat, which are not grounded to common.
    *shrug*, I'm not sure what to do next. The A/C works find when connected to an older analog thermostat, but I would rather not have any other electrical issues.

    Is this just a fluke, or should I be concerned about my antenna trashing electronics?
  10. KC4YLV

    KC4YLV Ham Member QRZ Page

    The Nest has a horrible problem with RFI because it uses FETs (iirc) to close the lines instead of relays.

    My buddy fried out his on 40m with just 20 watts into an attic doublet. Ferrites on the control lines helped, but the real solution was moving to a vertical in the back yard.

    Call Nest, play dumb, you can get it replaced. DO NOT MENTION ANY "I think it's because"'s. Your Nest stopped working. Thats all you know.

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