The situation for which you need this device:Your current home thermostat is an simple old device that does only one thing - it turns the furnace on/off via a cable connection with only two conductors in it. Now you want to replace it with a modern "smart" thermostat and even for a basic setup like yours, these thermostats require a 3 conductor connection to the furnace - 2 conductors for on/off, plus a third to provide power to the smart thermostat itself. Without using this "Common Maker" device you have only two options: (a) run a new 3-conductor cable between your furnace room and your thermostat or (b) install a "wall wart" power transformer near your new thermostat and use the wall wart to power it. Either of these two options may be either impractical and/or unsightly... and that is where this "Common Maker" comes in.What the "Common Maker" does:It allows you to connect a new "smart thermostat" to your furnace room using the existing 2-conductor cable.How you install it:This requires a bit of basic electricity knowledge, including an understanding of where in your furnace room (a) the two wires that turn the furnace on/off go to and (b) where the typical 24VAC power transformer used to power the thermostat circuit to the furnace is located. Some people will need to have their furnace installer do this for them, particularly for situations where it is not obvious where the connections you require are located.Do not use this device for:(a) thermostats that control both heating and cooling, or "staging" of the heat output. These always require more than just two conductors, and FAST-STAT has other devices specifically designed for these situations.(b) thermostats that "communicate" with the furnace they are connected to. For example, the Infinity controls used with some Carrier furnaces.My home has a zoned radiant heating system with 7 separate zones each independently controlled by its own thermostat.... so I have installed 7 of the original version of FAST-STAT's "common makers" (the one currently being sold is a functionally identical "version 2"). They have performed flawlessly for 6 years until just recently, when one of them failed. That is not a bad track record, but as other reviews here have also indicated there does appear to be a small failure rate for this device. Unfortunately as I pointed out above, there are often no reasonable alternatives to using this product.