All thermostats are switches. They turn the heating or cooling systems on when there is a demand for it and off when the demand is met. In most thermostats, if you take it off the wall you will see a red wire and a white wire. If you touch these two wires together the unit will come on and if you separate them the unit will go off.
Because of its simplicity, these units rarely ever fail. The concerns with thermostats are location and the anticipator. The thermostats should be located on an inside wall. It should not be near any heating or cooling element. Like being direct above a radiator or below a cooling register or opposite side of a wall oven, in direct sunlight, etc. It should be located approximately five feet from the floor.
The anticipator does what it implies. It just like warming up your car in the winter before you get on your way. As your thermometer gets near the point where there is a demand, the anticipator gets the unit started and warmed up. So that when there is a demand, the unit is ready to go. There are two types of anticipators; one that is manually set and the other electronically and you do not have to set.
On the manual anticipator, if you take the cover of the thermostat, you will see a small dial. You will need a magnifying glass to read the numbers on this dial. Then go to your heating system and find the gas line. Follow it to almost where the burners are and on the gas line there will be this gray metal box. That's the gas cock and on it numbers will be stamped, like 60hz. The number you are looking for will be like .5A or .3A.
Then go back to your thermostat and move the arrow on the dial with a pencil or pen to the number you got off your gas cock. With electric units you will find the number on the specification plate on the unit and with oil you will find it on the burner specification plate. The symptoms when the anticipator set wrong are the system goes on and off for brief periods and it comes on and does not go off.
With electronic anticipators the only problem with them is when the system is zoned and there are more than one thermostat. The problem arises when there is a demand for heat in two or more zones.
This confuses the electronic anticipator and you get the same symptoms stated above and in some cases you do not even get heat. If programming the zones is important to you, my suggestion is that you get a day night thermostat that you can manually set the anticipator or use a regular thermostat and buy one of those battery set backs that snap on to the regular thermostat.
The type of thermostat you need depends on you. If you leave the temperature the same all the time, a regular thermostat. If you like to turn down the thermostat only at night or only during the day, a day/ night thermostat. If you like to turn it down at night, have it come up for a few hours in the morning and then go down until you come home, an electronically controlled thermostat or a Programmable Slimline Thermostat It all depends on your needs.