This is likely a noob question, apologies if the answer is obvious.
I'm trying to control a furnace using a 3.3V microcontroller. The furnace operates by connecting a common 21V AC "hot" line to one of two AC "neutral" output lines: Fan and Heat. When connected they have relatively low current flow, ~100mA.
I've had some success using a TRIAC to do the switching - but I'm directly mixing the power sources by connecting the DC battery negative to the common AC "hot" line. Battery negative/hot then becomes the reference at the TRIAC's MT1, and I can switch the furnace on and off by sending +3V or 0V to the TRIAC base.
A simplified schematic is below - SW1 would be in reality a microcontroller output pin with a current limiting resistor.
Question is, is it kosher to mix power sources like this? Do I risk an exploding battery, excessive battery drain, weird circuit behavior or anything like that? If so, what would be the proper way to do this, to isolate the circuits?