Thermostats are the control units for every heating and/or air-conditioning system. Their function is to operate the climate control in your house, remotely from the system. How many units installed in your home, depends on how many zones that are in your heating and/or air conditioning system. The location of the thermostats are predominately located in a central area like a hallway or the kitchen, especially if there is only one zone.
This is the most basic type of control that has been installed in homes. They are round or square and operate with a mercury filled tube located in the back of the dial face plate. When you take off the face plate surrounding the degrees plate you will see the mercury filled tube. Newer models no longer have the mercury tube, they have an electronic switch for an environmentally-friendly thermostat.
The manual controls that have been installed usually are in older homes or as a basic beginning thermostat in newly constructed homes. Now they are being replaced by more efficient and cost effective digital, programmable styles of thermostats.
Please be aware that if you do replace your older manual unit, with the mercury inside, that it needs to be disposed of properly. Most trash/transfer stations have a hazadous material section for disposing of mercury.
Most recently the typical thermostat installed in newer homes, replacing the older mercury filled round or square manual model is the digital thermostat. They can be either programmable or non-programmable. Of course, the more programming features there are on a particular thermostat the harder it is to operate and understand. The digital thermostat uses batteries for the display along with being wired into your homes heating and air-conditioning system and need to be changed regularly to operate.