What is a thermostat and how does it work?
You might have one of those old school thermostats with the slide switch, or a newer digital display one, or even a smart thermostat that you can control with your phone. But in the end, thermostats are just the box that lets you turn the temperature up and down, right?
Well, there’s a bit more to thermostats than that. A thermostat is a temperature sensitive electronic switch that controls your air conditioner, furnace, or both. Early analog, electromechanical thermostats—the kind your parents or grandparents used—had a small bimetallic coil that expanded or contracted as the air around changed temperature. The temperature selector switch adjusted the position of the thermostat coil so that when the temperature dropped below (in cold weather) or rose above (in hot weather) the target temperature, the coil would tip a small bulb of mercury in one direction or the other, making the mercury shift and complete a circuit, starting the AC or furnace. (For a more in-depth examination, check out this tear down of an old Trane thermostat.)
While these thermostats worked well, they weren’t terribly energy efficient, and the mercury they used became a health concern. This wasn’t a big loss, as today’s computerized thermostats are much more accurate and efficient, and give you advanced settings that can help you cut your energy bill.
Common Types of Thermostats
There are many types of thermostats in use today:
Electromechanical: These are the old style analog thermostats described above. Older ones relied on mercury switches, though later models did away with this toxic metal. These are still commonly used, but they lack the cost-saving programmable features enjoyed by many homeowners. These are best suited for people with regular schedules, and are compatible with most heating and cooling systems, except for heat pumps.
Digital: Most thermostats used today are digital. These can be very bare bones, with just a black LCD and push buttons, or may include extra bells and whistles like backlighting, touchscreens, iPad-like color displays, and phone app integration. However, even very basic models are programmable, allowing you to schedule temperature “setbacks”—when the furnace or air conditioner is set back to more moderate temperature, reducing energy usage.
Hybrid: These are fancier touchscreen models that also feature manual slides and push buttons. Hybrid thermostats are a great choice when you are comfortable with technology, but some members of your family aren’t very tech savvy.
Occupancy: Occupancy controlled thermostats have the same sensors used to control lighting in business offices. When an occupancy thermostat senses that people are present, it shifts into occupancy mode and adjusts the temperature accordingly. When it doesn’t sense anyone present for a preset period of time—anywhere from 30 minutes to 12 hours—then it shifts into unoccupied mode. This allows you to have the cost-saving benefit of a programmable thermostat, without actually having to set a schedule and overriding those settings when you break your patterns. However, these units are rather inflexible, and are best suited for spaces that are unoccupied for long periods of time, such as offices and retail stores.
Light Sensing: These utilize the same logic as occupancy thermostats, but instead use a light sensor to determine whether you are home or not. When the amount of light exceeds the preset level, the system turns on the air conditioner or furnace. When lighting falls below this threshold, the thermostat shifts into unoccupied mode. As with occupancy thermostats, these are best suited for areas left unoccupied for much of the day.
The most basic digital thermostats start at around $20, and can run up to around $250 for app-controlled thermostats, such as the popular Nest line of products. Thermostats with occupancy or light sensors cost around $350.
Using a Programmable Thermostat with Automatic Temperature Adjustment
Installing a programmable digital thermostat is a great way to maximize your energy savings without sacrificing comfort. By maintaining your desired temperature for four or five hours a day instead of 24 hours—as was typical with analog thermostats—a programmable thermostat will cut your energy costs tremendously, and more than pay for itself in a very short while.
Most programmable thermostats have one or more of the following features:
- Store and repeat multiple daily and weekly setback schedules, which you can manually override temporarily without affecting the rest of the daily or weekly program.
- Store six or more temperature settings per day.
- Adjust heating and air conditioning turn-on times automatically as the outside temperature changes.
Choosing a Thermostat That’s Right for You
It’s a good idea to do a little homework before you choose a programmable thermostat. When considering which thermostat to buy, be sure to consider these questions:
- Does the thermostat draw power from the heating system’s low-voltage electrical control circuit instead of a battery? While not having to replace the battery can seem convenient, the clock in some thermostats can be disrupted by power outages and when the furnace cycles on and off. A thermostat with a battery backup is preferred by many homeowners.
- Is the thermostat compatible with your current electrical wiring?
- How accurate is the thermostat?
- Are the programming instructions easy to understand and remember? Some thermostats conveniently feature brief instructions inside the cover plate or housing box. If this isn’t the case, is it complicated enough that you’ll have to consult the instruction booklet every time you want to adjust the thermostat’s settings?
Most programmable thermostats completely replace old units, and are preferred by many homeowners. However, some models are designed to be placed over or alongside existing thermostats, and are mechanically controlled to permit automatic setbacks. These units are usually battery powered, eliminating the need for electrical wiring. They are often easy to program, and because they run on batteries, their clocks do not reset during power outages.
Before you make a choice, take some time to chart your weekly habits: when you wake up and leave for work or school, when you get home, and when you go to bed. Then determine what temperatures are most comfortable during these time periods. This will help you decide which thermostat will best serve your needs.
Where to Place Your Thermostat
The location of your thermostat can affect its performance and efficiency. Read the manufacturer’s installation instructions to prevent erroneous “ghost readings” and unnecessary furnace or air conditioner cycling. Place your thermostat away from direct sunlight, drafts, doorways, skylights, and windows. Lastly, make sure that your thermostat is conveniently located for reprogramming and overriding automatic settings.
Busting Myths About Thermostat Usage
Many people believe that if you turn the heat down when you’re gone and then turn it back up when you get home, that your furnace will have to work harder to get your house back up to a comfortable temperature, resulting in little savings or even a net loss. This isn’t true. Studies have shown that the amount of energy needed to bring a home back up to a comfortable temperature is roughly equal to the energy saved while the temperature dropped after the furnace was turned off. That means that you save money for the entire period during which your home stabilizes at a lower temperature. The longer your home remains at a lower temperature, the more energy (and money) you’ll save.
An even more popular myth is that if you set your thermostat to a higher temperature, your furnace will put out more heat and warm up faster. Again, this isn’t true. A thermostat isn’t like the gas pedal in your car. It’s more like a light switch—on or off. Turning the temperature higher just raises the target temperature and makes your furnace run longer, not faster.
Setting Your Thermostat for Maximum Energy Savings in Winter
Yes, we know how awesome it is when it’s cold outside and you step through your front door and feel that blast of warm air. But keeping your home warmer than necessary costs a lot of money in the long run; the warmer it is inside your home, the faster it loses heat through the walls, as the rate of heat loss is determined by the difference between indoor and outdoor temperatures. A bigger difference means faster heat loss.
You can cut this heat loss by setting your thermostat just a bit lower, which can save you a lot of money. If you consistently set your thermostat 1 degree lower than you normally do for an 8 hour period, you can shave 1% off of your yearly heating bill. That means turning it down by 10 degrees can save you 10 PERCENT. That’s pretty significant.
An easy way to take advantage of these savings is by using your thermostat’s schedule function to automatically drop the temperature by a few degrees when you’re sleeping or at work. When you’re home, an ideal temperature is 68 degrees—enough to keep you warm without using too much energy.
A Note for Heat Pump Owners: When a heat pump is in its heating mode, setting back a conventional heat pump thermostat can cause the unit to operate inefficiently, canceling out any savings achieved by lowering the temperature setting. Maintaining a consistent moderate temperature is the most cost-effective practice. However, some companies have begun selling specially designed heat pump thermostats which make setting back the thermostat cost effective. Please note, when a heat pump is in cooling mode during the summer, it operates like an air conditioner. Therefore, manually turning up the thermostat will save you money.
Setting Your Thermostat for Maximum Energy Savings in Summer
Summer is the same as winter when it comes to heat transfer, but in reverse: the cooler you keep your home, the faster the inside of your home absorbs heat from outside. Yes, it feels amazing to keep your home so cold that you can see your breath, but you won’t like your power bill. During the summer, try and keep your thermostat set at about 78 degrees. When you’re away from home, set it higher.