How Home Thermostats Work

By: Karim Nice, Patrick Brothers & Emilie Sennebogen  | 
A programmable thermostat is but one option for your heating and cooling needs.

Key Takeaways

  • Home thermostats regulate heating and air conditioning systems in your house, impacting energy usage and comfort. They've evolved from simple mercury switch devices to digital and programmable models, allowing for greater control over indoor climate and energy savings.
  • Early thermostats used a mercury switch and bimetallic strips to control temperature. Modern digital thermostats use thermistors for temperature measurement, offering features like programmable settings, system zoning and even remote control via smartphone apps.
  • Innovations like talking thermostats aid those with visual impairments by announcing settings and temperatures, while telephone thermostats and smart thermostats offer remote control, enhancing convenience and efficiency.

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.


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.


Thermometers and Switches

A bimetallic strip is a piece of metal made by laminating two different types of metal together. The metals that make up the strip expand and contract when they're heated or cooled. Each type of metal has its own particular rate of expansion, and the two metals that make up the strip are chosen so that the rates of expansion and contraction are different. When this coiled strip is heated, the metal on the inside of the coil expands more and the strip tends to unwind.

The center of the coil is connected to the temperature-adjustment lever, and the mercury switch is mounted to the end of the coil so that when the coil winds or unwinds, it tips the mercury switch one way or the other.


In non-digital thermostats there are two switches. These switches move small metal balls that make contact between different traces on the circuit card inside the thermostat. One of the switches controls the mode (heat or cool), while the other switch controls the circulation fan. On the next page, we'll see how these parts work together to make the thermostat work.

Inner Workings

Thermostat wiring consists of wires that connect the transformer to the system relays.

When you move the lever on the thermostat to turn up the heat, this rotates the thermometer coil and mercury switch, tipping them to the left.

As soon as the switch tips to the left, current flows through the mercury in the mercury switch. This current energizes a relay that starts the heater and circulation fan in your home. As the room gradually heats up, the thermometer coil gradually unwinds until it tips the mercury switch back to the right, breaking the circuit and turning off the heat.


When the mercury switch tips to the right, a relay starts the air conditioner. As the room cools, the thermometer coil winds up until the mercury switch tips back to the left.

Thermostats have another cool device called a heat anticipator. The heat anticipator shuts off the heater before the air inside the thermostat actually reaches the set temperature. Sometimes, parts of a house will reach the set temperature before the part of the house containing the thermostat does. In this case, the anticipator shuts the heater off a little early to give the heat time to reach the thermostat.

The loop of wire above is a kind of resistor. When the heater is running, the current that controls the heater travels from the mercury switch, through the yellow wire to the resistive loop. It travels around the loop until it gets to the wiper, and from there it travels through the hub of the anticipator ring and down to the circuit board on the bottom layer of the thermostat. The farther the wiper is positioned (moving clockwise) from the yellow wire, the more of the resistive wire the current has to pass through. Like any resistor, this one generates heat when current passes through it. The farther around the loop the wiper is placed, the more heat is generated by the resistor. This heat warms the thermometer coil, causing it to unwind and tip the mercury switch to the right so that the heater shuts off.

Next, we'll take a more detailed look at the electrical circuits in the thermostat.



This thermostat is designed for a system with five wires -- the wire terminations are marked as follows:

  • RH - This wire comes from the 24VAC transformer on the heating system.
  • RC - This wire comes from the 24VAC transformer on the air-conditioning system.
  • W - This wire comes from the relay that turns on the heating system.
  • Y - This wire comes from the relay that turns on the cooling system.
  • G - This wire comes from the relay that turns on the fan.

The two transformers provide the power the thermostat uses to switch on the various relays. The relays in turn switch on the power to the fan and the air conditioner or furnace. Let's see how this power flows through the thermostat when the air conditioner is running.


Power from the air-conditioning transformer comes into the terminal labeled RC. The ball controlled by the mode switch jumps the current onto a trace that leads to the terminal in the lower-right corner of the circuit board.

This terminal connects to the top layer of the thermostat through a screw. It connects to the pink wire, which leads to the bottom wire in the mercury switch. If the switch is tilted to the right (as it would be if the air conditioning were on), the current travels through the mercury into the blue wire.

Through a screw, the blue wire (see above) connects to a lug in the lower-left corner of the circuit card.

From there, it goes through a trace on the circuit card to the other branch of the mode switch. The ball in the mode switch jumps the current onto a trace that connects to the terminal marked G, which energizes the fan, and the terminal marked Y, which energizes the air conditioning.

Digital thermostats use a simple device called a thermistor to measure temperature. This is a resistor which allows electrical resistance changes with temperature. The microcontroller in a digital thermostat can measure the resistance and convert that number to an actual temperature reading.

A digital thermostat can do a few things that a regular mechanical thermostat cannot. One of the most useful features of a digital thermostat is programmable settings. In the winter, you can program it to automatically turn up the heat for an hour or two in the morning while you get ready for work, turn down the heat until you get home, turn up the heat in the evening and then turn down the heat while you sleep. This is a great money-saving feature because you can simply turn down the heat when it isn't needed.


System Zoning

A programmable thermostat can help regulate the temperature during different times of the day.

A lot of times, there are rooms in your house that are always warmer or colder than others are. There can be many explanations for this. For one, heat rises, so rooms on second or third floors are often too warm. In turn, basement rooms are typically too cold. Rooms with vaulted ceilings have a difficult time retaining heat, while rooms that receive long hours of sunlight are often difficult to cool down. These are just a few reasons, but regardless of why a room's temperature is uncomfortable, there's only one surefire way to even out your house's temperature: system zoning.

System zoning is pretty simple. It involves multiple thermostats that are wired to a control panel, which operates dampers within the ductwork of your forced-air system. The thermostats constantly read the temperature of their specific zone, then open or close the dampers within the ductwork according to the thermostat's settings. Not only is system zoning helpful for houses with inconsistent room temperatures, but it's also great for heating or cooling individual bedrooms based on the desired temperature setting. If you have a usually empty guest room, just shut the door and close the damper.


If used properly, system zoning can help you save money on your energy bills. According to the U.S. Department of Energy, system zoning can save homeowners up to 30 percent on a typical heating and cooling bill. Those savings can add up to quite a sum -- the Department of Energy also estimates that heating and cooling account for 40 percent of the average household's utility costs. Because guest rooms and other seldom-used rooms don't require constant heating or cooling, system zoning allows you to save money by running temperature-controlled air to those rooms only when it is necessary.

Many homeowners are hesitant or unwilling to make the transition to programmable thermostats and system zoning because of the initial cost of installation. This is an understandable concern for anyone who's not building a new home or replacing an old HVAC system, but there are other options available. Even though installing a typical zoned system is not a do-it-yourself project, the Department of Energy's Inventions and Innovation Program funded the development of a damper system that can be retrofitted to existing ductwork. The system combines RetroZone's flex damper air control inserts with an electronic controller and air pumping system. There are no heavy motors involved, so existing ductwork does not need to be altered or supported.

The flex dampers, which come in circular and square duct models, fill with air to constrict or block the airflow within the duct. They're resistant to heat, aging, moisture, airborne chemicals and ozone, and even if they're punctured, which is unlikely, most holes will not affect the performance. Flex dampers should be installed in steel or flexible ducts. The dampers can be serviced easily by gaining access through a register. Flex dampers also work with most brands of zone-control panels.

If you're planning to install a retrofitted zone-control system, here's what you'll need to put on your shopping list:

  • thermostat for each zone
  • solenoid pump
  • solenoid panel
  • zone control panel
  • plenum tubing
  • transformer
  • fire rated tape
  • control limit switch
  • flex dampers

The number of zones your home needs will affect the way you set up the system. In a two-zone system, with the zones being fairly equal in size, each zone's ductwork must be capable of handling up to 70 percent of the total CFM (cubic feet per minute) of air produced by your HVAC system. In a three-zone system, the zones need to be as close in total area as possible. In this case, each zone's ductwork should be able to handle up to 50 percent of the total CFM. Installing a four-zone system requires a bit more work. The ducts need to be enlarged by one inch, and they require a static pressure relief damper and high- and low-limit protection. To avoid major damage, be sure not to completely cut off the airflow over the heat exchanger or coil of your HVAC system.

Now we'll look at another home-thermostat innovation -- the talking thermostat.


Talking Thermostats

Talking thermostats may seem like one of those unnecessary futuristic inventions straight out of an episode of "The Jetsons," but they're actually quite practical for senior citizens, people who are visually impaired or blind, and other people with special needs. Talking thermostats announce the time, day, temperature setting and room temperature, plus they have audio instructions for setup.

Even though talking thermostats are most helpful to people with vision impairments, they can also be useful to the general population.


It's often difficult to know when there's a problem in your heating and cooling system, and major problems can cost thousands of dollars to repair. Even minor problem can lead to far more serious and costly repairs if not diagnosed in a timely manner. Talking thermostats can end up saving you lots of time, money and stress because they alert you when you need to have your system serviced. They also let you know when you need to change the system filter. Promptly replacing the filter lowers the cost of heating and cooling your home and also helps people control allergies and asthma.

Some talking thermostats even recognize and respond to voice commands. You simply say an activation word, such as "thermostat," followed by a command like "raise" or "lower," and the rest is automated. Talking thermostats are able to do this because they use DSP, or digital signal processors, to process audio and speech. First, the DSP filters out real-world analog signals. Then, the microprocessor changes them into digital signals. After the signals have been converted, they're sent through application-specific integrated circuits, or ASICS, and the thermostat reacts in real time.

Because talking thermostats are high-end, cutting-edge accessories to heating and cooling systems, they come equipped with all of the user-friendly functions that other quality thermostats boast. A built-in time-delay function keeps your system from immediately starting or stopping if it's accidentally adjusted. Stopping and starting HVAC systems puts a lot of wear and tear on the compressor, which is the most expensive part of the system, so the delay function is quite important. Talking thermostats are also programmable, which allows you to heat or cool your home only when it's necessary.


Telephone Thermostats

You're pretty fortunate if you're able to own a vacation home, but it also means you'll be paying to heat and cool two houses. Programmable home thermostats can actually allow you to keep the heat or air turned off until the day you arrive, but it requires precise planning of your comings and goings in order to get the desired result. Telephone thermostats, on the other hand, allow you to heat or cool your home with a simple phone call.

Telephone thermostats replace your existing home thermostats. They connect to both the heating and cooling system and to your phone line. You simply have to call your property and enter a password on a touch-tone phone to access the controls. Then, you can adjust not only the temperature setting but the entire system mode as well.


Telephone thermostats can handle these functions because they use digital microprocessors as well as a touch-tone detector and telephone interface module. Essentially, you can "talk" to your home thermostat via telephone thanks to the internal telephone access module. A separate phone line isn't necessary, and the telephone thermostat can even coexist with answering machines or voice mail. However, if you have a DSL high-speed Internet connection, you will need a DSL filter on the telephone line that connects to your talking thermostat.

With the advent of the smart phone and their handy applications, or apps, you can bypass the land line altogether to control your home's temperature. Wi-fi based smart thermostats are available now and allow users to control their home's thermostat with the help of a touch-screen smart phone. There are quite a few apps already available, and like most smart phone apps, they aren't very expensive, with a range of free to a few dollars.

As technological advancements make their way to simple devices like thermostats, consumers benefit greatly from the combination of features.

For more information on home thermostats, check out the links on the following page.


Frequently Asked Questions

How does system zoning work with modern thermostats?
System zoning involves multiple thermostats controlling different zones in a house, allowing for customized temperature settings in each area.
Can smart thermostats significantly reduce energy costs?
Yes, smart thermostats can reduce energy costs by learning your schedule and preferences, adjusting temperatures automatically to optimize energy use while maintaining comfort.

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