Cadet's blog provided an approach on connecting a smart thermostat to a Com-Pak wall heater by connecting it to a relay/transformer. They have recently updated and changed their post to not recommend it due to the relay not providing enough power to the smart thermostats, and they suspect it will drain the battery. Will a relay such as the Honeywell RC840T-120 24V, power the HoneyWell Wifi Thermostats, RTH6580WF, powered from a Cadet Com-Pak 1500W, 120V model CSC151? Are externals transformers/adapters best? If so, are there other specifications beyond a two wires 24 volts transformer to keep in mind?
That particular device, the RC840T-120, is a combo relay and transformer. The problem is the transformer is just big enough to operate the relay itself, and not also a smart thermostat. It is intended for a passive/dumb thermostat.
Someone else may make a combo relay-transformer which has an appropriately sized transformer. Or, you can just go with separates; it's not that big a deal. Transformers and relays can both be found in that same form factor.
The relay can be down at the service panel. So can the transformer.
The key specification is the VA specification -- the RC840T only has a 1.2VA transformer in it, which is quite small. In fact, it's too small to run a power-hungry WiFi thermostat -- it was sized to be barely large enough to run the relay itself, although old-style programmable 'stats draw very little power and probably would work as well. Standard thermostat transformers are 40VA, which is more than beefy enough to run any thermostat you can find.
So, get a separate thermostat transformer to go with your relay.