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.
So a C wire is essentially the -neg wire on the transformer. That since the transformer is an isolated circuit . (Similar a car) that the transformers -neg wire is usually attached to the metal frame the transformer is mounted on making a common connection(Usually the entire air hander), unless it built into the board which still should be connected to the frame.
All you need is to connect a wire to the frame somewhere as your common wire to your thermostat C connection for it to work.
No need to buy any other special transformers. And the ones that come with the air handlers are usually enough to run everything you need.Unless you have multiple units running at the same time(floor heater, dehumidifier,multiple heat pumps) that are far from the thermostat and you need a bigger transformer because of the length of wire causes to much of a voltage drop.
Most thermostats(Nest) can only handle about 3 Amps anyways so your limited as far as that much anyway how many transformers you can connect yo the unit (just remember if connecting to separate transformers from different locations to thermostat. They both need to have their own common wire that both connect to the same Common on the thermostat.
But if your air handler does both AC/Heater one is enough.
Even if your main heater(furnace) air handler transformer will probably be enough to turn on 2-3 AC units and their own fans from just the one maybe even more. Or even to run two wires (one from the thermostat and other from air handler ground to a separate broiler) to the other units and connect to common to its frame(-)neg and the (+)wire from thermostat to the relay switch of that unit. You could run a third wire if it needs it for a fan.
That’s about it, save your money. because there are probably extra wires not being used already going to your thermostat. Just have to pick one to Use as the common on thermostat and find a place on the other end on the air handler somewhere(hint: loosen a screw)
And you don’t have to pay an HVAC guy $200 to connect two wires , or buy multiple transformers and never have to buy batteries ever again,