I recently decided to switch my old thermostat to a Nest and figured that it should be easy considering my current thermostat just controlled a simple electric wall heater. Unfortunately, it was only after taking my old thermostat off the wall that I realized that it was in fact a line voltage thermostat (Honeywell’s T410AA), which means that it uses high voltage (240 V) and acts as an in-line switch between the electric wall heater and the power supply. The problem with this is that systems like the Nest run off of low voltage (24 V) and can’t be used to switch a high voltage system.
To fix the issue I started researching. Surely I wasn’t the only person who wanted to switch their old school line voltage thermostat with a Nest! Unfortunately, a bunch of the solutions were on Nest’s old community site which had since been taken down. However, I was able to find out about a relay/transformer that according to the reviews should be able to bridge the gap between line voltage (240 V) and the typical low voltage setup used for modern thermostats like the Nest (24 V). The schematic for it shows the method for hooking it up to known thermostat wires.
The Aube Relay RC840T-240 does exactly that. It takes 3 wires carrying the line voltage (connected to Black, Blue, and Red) and then connects them out via 3 wires (R, C, W) carrying low voltage. This works to connect the line voltage wall heater with controls from the Nest system.
After connecting everything up, I tested the Nest to see if it worked and low and behold! It worked like expected. The only thing left after wiring and testing the setup is to mount it properly. However, the relay/transformer does not fit inside the original electrical box so I will need to devise a workaround to get it to all fit within the wall up to code.
Read the Continuation here: http://blog.anthonyngu.com/2017/01/10/finishing-touches-installing-nest-w-line-voltage/