Thermostats are the portion of the heating and air conditioning systems that the customer is most familiar with. This is the one point of the system that the customer or end user has total control over.
The thermostat has one purpose, which is to sense the temperature of a space and either do nothing or take a corrective action. This corrective action may be to close or open a set of contacts, or signal an alarm.
A thermostat is a switching device which can be line voltage, control voltage, or direct digital voltage. Line voltage thermostats are most often used in electric heat and refrigeration applications. Control voltage thermostats are most often used in heating and air conditioning applications for residential and small office or industrial areas. Direct digital controls (DDC) are most often used in large office, commercial, or public buildings.
Types of thermostats
Thermostats fall into two basic functions. The first is the heating thermostat and the second is the cooling thermostat. However both these thermostat types are basically switches that either open or close based on temperature changes.
The Heating Thermostat
The heating thermostat is one that closes the contact on a drop in temperature. So, if a thermostat is set at 72 degrees and the temperature drops to 70 degrees the thermostat will close the contacts. Figure 25 shows the schematic symbol for a heating thermostat. During heating season if you want to keep your house warm you turn the thermostat to heat and it becomes a heating thermostat. Other appliance such as Ovens, Stoves, Driers, and Electric Blankets also use the heating thermostat.
Figure 25 – The heating thermostat
The Cooling Thermostat
The cooling thermostat is one that closes the contact on an increase in temperature. The cooling thermostat could be set at 70 degrees and if the temperature increases to 74 degrees the thermostat will close the contacts. Figure 26 shows the schematic symbol for a cooling thermostat. During the summer if you want to keep your house cool you turn the thermostat to cool and it becomes a cooling thermostat. Other appliances such as a refrigerator, ice machine, soda machines, and freezers all use cooling thermostats.
Figure 26 – The cooling thermostat
Thermostat Sensing Elements
Thermostats must have some type of sensor in order to sense the change in temperature from one time to another. These are called sensing elements. There are currently three types of sensing elements that are used in thermostats. These are bi-metal, remote bulb, and solid state.
Bi-Metal Sensing Elements
The bi-metal sensing element contains two pieces of dissimilar metals that are welded together. Different types of metal expand and contract at different rates. The metal has a contact attached to one end of it or a mercury tube attached to it. When the metal heats or cools it will bend because of the different expansion rates. This will bring the contacts closer together or farther apart (opening or closing them). Figure 27 shows a diagram of the various types of bi-metal sensing elements.
Figure 27 – Bi-metal sensing element (NATE C134)
Remote bulb sensing element
The remote bulb sensing element is a bulb filled with a liquid, or a gas that is attached to a capillary tube. When the liquid is heated it travels down the tube and puts pressure against a diaphragm. This diaphragm closes or opens a set of contacts.
The remote bulb sensing elements are most often found in commercial refrigeration and in hydronic (hot or cold water) applications. The remote bulb allows the electronic portion of the thermostat to be protected from the environment it is controlling. Figures 28 and 29 show two different styles of remote bulb thermostats.
Figure 28 – Remote bulb thermostat
Figure 29 – Remote bulb thermostat from window air conditioner
Solid State Sensing Element
The solid state sensing element is used in most digital or programmable thermostats. These use a material that changes resistance based on temperature. These have proven to be more accurate and have a longer life span than both the bi-metal and the remote bulb because they have no moving parts and they do not have a diaphragm and are not filled with a liquid or gas.
Solid state thermostats are very easy to troubleshoot if you know the resistance vs. temperature curve from the manufacturer. Disconnect the thermostat sensing element from the circuit and then check it with your ohm-meter.
On occasion you will see the solid state sensing element hanging out of the bottom of a thermostat but most often they are a chip inside the thermostat cover that air passes across.
Thermostat Wiring
Thermostat wiring changes based on the type and if it is line voltage or not. It is critical that you follow any schematics that are included with the systems. When you are wiring a thermostat check to see if it is line voltage or control voltage. And then check if it is a cooling or heating thermostat. Try always to match the original one as closely as possible.
Glossary of Terms
Thermostat: A switch that is temperature actuated. It is the main point of control for the customer or end user.
Sensing Element: The portion of the thermostat that senses the change in temperature.
Bi-metal: Two types of dissimilar metal that are welded together.
Remote Bulb: A bulb filled with liquid or gas that is attached to a capillary tube.
Solid State: The solid state sensing element is used in digital applications where a change of resistance shows the change in temperature.