How Home Thermostats Work If you have specific heating and cooling needs in order to be comfortable then you've probably spent a little time looking at and operating your home thermostat. This handy little device controls the heating and air-conditioning systems in your house -- the two pieces of equipment that use the most energy, and the ones that have the biggest impact on your comfort and quality of life. In these days of rising energy prices, you might be interested to see how your thermostat works. Believe it or not, it's surprisingly simple and contains some pretty cool technology. In this article, we'll take apart a household thermostat and learn how it works. We'll also learn a little about digital thermostats, talking thermostats, telephone thermostats and system zoning. How Air Conditioners Work How Off-peak Cooling Systems Work Modern thermostats are almost exclusively digital, but before we get to those, let's take a trip down memory lane and look at the parts of a non-digital thermostat that you might still find in older homes and motels.

Let's start with the mercury switch -- a glass vial with a small amount of actual mercury inside. Mercury is a liquid metal -- it conducts electricity and flows like water. Inside the glass vial are three wires. One wire goes all the way across the bottom of the vial, so the mercury is always in contact with it. One wire ends on the left side of the vial, so when the vial tilts to the left, the mercury contacts it -- making contact between this wire and the one on the bottom of the vial. The third wire ends on the right side of the vial, so when the vial tilts to the right, the mercury makes contact between this wire and the bottom wire. There are two thermometers in this kind of thermostat. The one in the cover displays the temperature. The other, in the top layer of the thermostat, controls the heating and cooling systems. These thermometers are nothing more than coiled bimetallic strips. And what's that, you ask? We'll find out on the next page.A thermostat's basic function is to regulate the operation of heating and cooling equipment in your home.

You set the thermostat to the desired temperature and the thermostat turns the equipment on and off to maintain that temperature. A thermostat is essentially an automatic switch. The thermostat senses the temperature and turns the air conditioner or heater on and off as required to maintain the desired temperature. There are two basic types of thermostats: electromechanical and electronic. They both do the same thing but they do it in different ways. Additionally, of the two basic types, there are several varieties from which to choose. An electromechanical thermostat typically uses a bi-metal coil or strip which moves as it expands and contracts with temperature changes. Mounted on the bi-metal strip is a glass vial partially filled with mercury. As the vial is tilted on the back on the bi-metal strip, the mercury flows to the end of the vial where two electrical contacts are exposed. When the mercury envelopes those electrical contacts, a low voltage circuit is made, thanks to the fact that mercury is conductive.

This works just like a light switch and makes a circuit to turn on the heater or A/C. An electronic thermostat operates in much the same way as an electromechanical model except that instead of the moving bi-metal strip and mercury switch, it uses a sensor to detect temperature levels. 2 ton air conditioner dimensionsWhen the temperature differs from the preselected temperature the thermostat electronically makes the circuit to the heater or A/C.types of hvac systems for residential Advanced features include programmable thermostats where different temperatures can be set for a certain time of day or the day of the week. air handling unit valve packageA setback feature allows you to set the thermostat to a preferred temperature and then a different temperature for hours of the day when you are away and don't need to heat or cool the house to your preferred temperature.

Another feature called a heat anticipator is used for certain types of heating equipment that continue to generate heat even after they are powered off. This feature shuts off the heater in advance of reaching the preset preferred temperature. While most home thermostats use only low-voltage, the power should be shut off to the HVAC equipment at the circuit breaker when work is down on the thermostat. Accidental shorting of wires may not injure you but it may damage valuable equipment. conditioning thermostats have bimetals (older thermostat) or thermistor (newThese bimetals or thermistor sense the air current returning to the return ducts or the surrounding air. basic operation principle of air conditioning thermostat is it relies on random air current that passing thought it to determine the room temperature. room temperature to compare with the setpoint temperature. are in heat mode and we set the temperature at 70°F. Let’s said the air conditioning thermostat senses the random air current and the air current

until it pickup air current that read 70°F and then it stop running. thermostat works on this principle. It doesn’t matter if it digital, programmable, talking thermostat, telephone thermostats or zoning thermostat. will rely on air current to works. thermostat is the device that controls the air conditioner units. turn the heater or AC unit ON and OFF. Hvac thermostat is a temperatureIt controls the temperature of specific building. thermostat we’ll being discussion here is a room thermostat. conditioning thermostat controls space temperature for comfort purposes. thermostat we’ll discuss here is a regular mercury thermostat. are use a lot on older central air conditioner. thermostat doesn’t apply to Heat Pump.Heat Pump has its own special thermostat. normally three parts to home thermostat: thermostat cover doesn’t do much, except it has a thermometer that mounted on it to read the space (surrounding air) temperature.

This thermometer is made of bimetal strip not mercury. The bimetal strip is sensor that makes of two different types of metal that bonded together as one metal. other purpose of thermostat cover is to protect the thermostat assembly from being damage by children or accidentally knocking on it. conditioning thermostat assembly is the brain of the thermostat. has an adjustment lever to increase or decrease the room temperature. thermostat assembly has the basic components:In AC thermostat there this small resistor calls heat anticipator or coolingThis heat anticipator is adjustable and located right on theIts use mainly to cut the heating unit early before it reaches the setpoint temperature. adjustment lever is mounted on a pivot inside the thermostat assembly. rotates left or right to increase and decrease the room temperature. anticipator is a resistor that given off small amount of heat to the bimetal. This small amount of heat prevents the temperature from rising above the

bimetal to be warmer than the room temperature. anticipator is located inside the air conditioner thermostat and mounts on theThe bimetal coils is warmer than the room temperature due to the resistor. It causes the furnace to cut off early. As the furnace is off the blower will run to dissipate the heat from the furnace. example: We’re in heat mode. We set the thermostat to 70°F. anticipator will cut – OFF the furnace at 68°F and allow the blower to run until it removes the heat in the furnace to reach the setpoint. mercury thermostat has a bimetal strip that acts as thermometer to tell the central air units to run or not. These bimetal strips are commonly used brass and steel as the metal. These two metals are laminated or bonding together. could said it attach back to back as one metal. metal strip is use to sense the temperature of the surrounding air nears theEach metal react differently to hot and cold air and each metal expand and contract at different rate.

one metal has a high rate of expansion and contraction while the other metal has low rate of expansion and contraction. how bimetal reacted to temperature. We have the brass and steel as the bimetalBrass and steel expand and contract at different rate andIf we were to cool the bimetal with ice, it would contract and if we’ve to heat the bimetal it would expand. in distance the bimetal expand and contract depends on the length of theIf it were short it would be difficult to expand and contract. bimetal strip in a mercury thermostat is coiled into a circle. One end of the circle is attached to mercury bulb switches and the other end is in a pivot with temperature adjustment lever. was used in the earlier thermostat to make and break the electric circuit in a is a poisonous atom to people. The earlier or older air conditioner thermostat will have the mercury in it. The reason they used mercury in AC thermostat is because it conducts electricity and in liquid state.

mercury is inside small glass. This glass bulb is filled with inert gas and had a small drop of mercury in it. The mercury is freely to move from one location of the glass bulb to other end. mercury bulb has thin wires inside it. There are three wires within the mercuryThe three thermostat wires that are attach to the mercury bulb is in first thin wire goes across all the way to the other end of the mercury bulb and the mercury is always in contact with this wire. However, this wire is insulating within the mercury bulb and only the short tip is not insulated. wire tips what cause the mercury to make contract with. thin thermostat wire is to adjust the temperature of cooling mode. connective or screw to Y terminal of the air conditioner thermostat subbase and it’s also attach to one end of the cool anticipate. second thin wire is short and near the beginning. It’s in the middle of the first and the third wire in vertical position.

This second wire from mercury bulb is permanent attach to heating anticipate. third wire also goes across to the end of the mercury bulb and it’s in theThis wire is not insulating as the first (cooling wire). is explored and always remains in contact with the mercury. wire is screw to W terminal of the air conditioner thermostat subbase and it’s also attach to the other end of the cooling anticipate. The cooling anticipates is in the back of the Ac thermostat assembly. behind a thin electrical wire within mercury bulb is to make and break electrical current easily and doesn’t have weight on the bimetal coils. within a mercury bulb is another form of contractor. wires within the mercury contact each other, there is an arc occur within theThis arc is hot enough to cause oxidation within the mercuryHowever, due to mercury and the arc within close location, it won’t mercury bulb is attaching to the movable end of the bimetal coils and the other

end of the bimetal is fastened to the stationary pivot with temperature adjustment end of the mercury bulb is freely to rotate with the bimetal coils. bimetal move, it tips the bulb and the mercury with the bulb move to the other end of the bulb and provides a path for the current to flows from wire within conditioning thermostat subbase is the last component. The subbase is the first component that must mount on the wall. conditioning thermostat subbase has two selector switch levers: second selectors switch is to turn air conditioner units to COOL - OFF -This known as mode. thermostat subbase selector switching is not for Heat Pump thermostat. pump has the extra switching went comparing to regular air conditioner and conditioner thermostat subbase has field wire connector from the air conditioner, heating, fan, and power wire. wires are color coded in white, yellow, green, and red. It also has label onHere are the air conditioners wiring