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Member Since: March 27, 2009

Country: United States

  • The battery is needed because older thermostats were mechanical and not wired with a common wire. The only way to get power in this configuration is to turn on heat/ac or fan. Then current flows through the hvac's relay to engage the system and I assume the nest charges the LiPo battery at the same time. When power is low I bet it runs your hvac fan for no other reason than to charge the battery. Also Honeywell has a patented method of "stealing" power but I'm guessing adding the battery was cheaper than dealing with a potential IP issue.

  • My 3.3V micro could not switch this relay as-is. I bypassed R2 (short) and got it working. The better solution might be to increase R1 to 100K (I had no 100K's on hand).
    Also, if 14 gauge solid conductor barely fits into the 2pin screw terminals (an not very securely). I threw them out and soldered directly to the board.

No public wish lists :(