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?
Electrical – Smart wall heater thermostat line voltage from high to low.
electricalheaterthermostatthermostat-c-wire
Related Solutions
Simple "line voltage" T stat wiring. One wire is L1 (120V potential) from source, one wire is L2(120v potential)source connected to heating strips, then the switch, then L1 source. When the switch closes the 240V circuit is completed. If you have 240V when checking both wires together, and only two wires on switch, there is something wrong with the new fancy switch. If you have three wires coming from the switch then you need a neutral that you probably don't have.
According to this document, the internal wiring of that relay looks like this.
It sounds like it's connected something like this.
When the thermostat calls for heat, the T terminals are connected through the thermostat. This causes the relay to energize, which closes the contact between terminals 5 and 6 (It also closes a contact between 3 and 4). When terminals 5 and 6 are connected, the heater, pump, valve, etc. turns on.
Because the transformer is built into this device, it's going to be difficult (possibly impossible) to connect a C wire. You'd have to connect the wire to the other side of the transformer, something like this.
You'll also have to check with the manufacturer, to determine if the transformer can handle the additional load of the thermostat.
You might want to contact Honeywell, to see if they offer a similar relay that exposes the C terminal. If they do, you could replace this device with the one with the proper terminals.
What you're trying to do, is something like this.
Which as you can see in the diagram, is not going to work so well.
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Best Answer
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.