Originally Posted by Asbestos
If its a control voltage thermostat its really easy. Most of your modern furnace thermostats fit into that category. If its line voltage, its usually 220 and you should be comfortable understanding 220/240v electrical systems before you putter with it too much. Most of your thermostats that control electric heaters fit into the line voltage category.
Make sure you have and know how to use a multimeter before you even pick up a screwdriver. Shut off the breaker if it is a line voltage thermostat, and know how to use the multimeter to verify.
Oh, also, when you screw the new thermostat back into the j-box, dont pinch any of the wires between the thermostat and the box. It results in a beautiful shower of sparks that cascades down the wall. Ask me how I know...
220/240....Uh . never seen a line voltage stat that operated on 240V / 220v. Even heat pump/electric heat stats are 24 volts, not even 125 volts.
And changing a thermostat is not nearly as dangerous as your post speaks of. C'mon, its a thermostat not a friggin thermo-nuclear reactor here.
You must be sure the replacement is for heating/cooling and that it will accomodate the number of wires your old one does, take old one along.
Most important, turn off units that this stat controls and BE CERTAIN IT IS LEVEL...this is critical if it uses mercury. Just a hair off would be 5 degrees.
Also, IF it has a heat anticipator inside be sure to set the correct amps. that the old one was set at. J -box? Should screw right to drywall, paneling whatever your walls are made of.