Does anyone know if the CT100 can be installed using the G wire as a substitute for the C wire and by changing the wire at the heater and jumping the Y to the G? This is described as a workaround for no C wire on a Honeywell YouTube site:
but I don’t know if i will work with the CT100 and SmartThings.
I went ahead and tried it and it seems to work. I will report back after a few days on whether I have had any problems.
I am using the ct101, lowes brand, of this thermostat and just use batteries not the C wire and it seems to work just fine. Batteries are at 67% after a month so I may have to go that route eventually. I am a little nervous about messing with my head unit, was it difficult?
I’m looking at this too - luckily, when I checked, although I had no C-wire, there was a spare wire that I can re-purpose just by connecting it to C on the boiler.
Thanks for the motivation. My CT100 no longer needs batteries because I ran a new CAT6 cable between the thermostat and the furnace to replace the super old wiring that only had 2 wires connected. 
Is it a safe assumption that if I leave the batteries in there it won’t forget the time, etc. in the event of a power outage?
Wow - I wish I had the ability to do this… my house has tons of dryway in between the furnace and the thermostat so I would have to blast the whole house down… LOL… anyway, cool that you solved it. Yes, the batteries should protect your clock from a power outage.
Okay I said I would report back, so here t is. I have been using this now for over a week and it works flawlessly. I would recommend the Honeywell workaround for anyone struggling with the no-C wire issue. The only thing you give up is the ability to run a fan without the heater - but I don’t know too many people that do that as a priority.