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?