I have a venter room heater in a cabin. It uses a milivo;t thermostat. I am hooking it up to a wifi thermostat using a relay and 24 volt power supply. My concern is that if the power goes off that the thermostat will not power the unit on. I currenty have a digital thermostat that controls the unit and works great. can I wire the current thermostat to the wifi in parallel so if power goes off it will still allow the heater to turn on. If I can how do I wire it?
Correct. Simply wire the smart thermostat in parallel with the passive thermostat. Set the passive thermostat at a much lower temp, low enough it only kicks when needed to prevent pipe freeze or whatever. I wouldn't even use a battery thermostat for that; too unreliable; switch to a plain old bimetal type.
Obviously if the furnace has an electric blower assist, that won't work, but most of those Empire style furnaces work on convection and the blower is optional.
You'll also want to reposition it or do something with stagecraft... To deal with the human factors. The other humans who use the facility will have a simple heuristic:
- dislike the temperature
- search for a thermostat
- stop searching at the first thermostat they find
- manipulate that thermostat until they get the temperature they want
Complaining about this heuristic will be an exericse in futility. They will feel they did the right thing and consider your design to be broken. So would I, and I get what you're trying to do.
If this is a non powered venter it would work with a battery backed up thermostat the power to run and charge the new thermostat can be installed then the normally open relay contacts in parallel with the old thermostat. With this arrangement the furnace will run when 1 or the other thermostat calls for heat. Some units only need the power generated from the thermocouple to operate and they will work without power. If not working with power out is your only concern turn the power off and adjust the existing thermostat to call for heat if it turns on you may not need to add the wifi and transformer unless you want to control the heat remotely.