Trying to hook up wifi thermostat to Rinnai propane tankless heater

http://imgur.com/a/85ILI

I am trying to replace my old Honeywell programmable thermostat with a new WiFi Smart Thermostat, but the device will not power on.

I have three wires running from my water heater to my thermostat, a white, a red, and a green. They are wired to the old thermostat like this:

White = W Red = R (Jumped with Rc) Green = G

Now, when I traced the cable to the water heater, I found that only the red and white wires were hooked up, and the green wire was cut back and not used. If you look at the image above, it is a snapshot of my unit and how the thermostat is hooked up. The Red and white wires are connected to 22 and 23. I have an outside thermostat that is hooked up to 18 and 19.

Now, I understand that I need a "C" or common wire for the WiFi Thermostat and that the "C" wire is just a 24V constant power source, so I took the wire and trimmed the white and red back to the green wire, and replace the red and white wire to their original home, but added the green wire to 26. The Thermostat did not power on. From everything I read online, most furnaces and the likes label heir thermostat wire blocks with the standard letters of W, R, G, Y, RC, C, etc. But I cannot find that on my water heater. Am I wrong in assuming that the "C" equivalent would be the 24V block for 26 and 27?

submitted by /u/jlewis57
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