While we think of the thermostat as one of the simplest components of an HVAC system, there are actually many decisions that need to be made by both the home or building owner and the contractor.

Think about the choices available when deciding what thermostat to use:

  • Type of equipment: single-stage, multi-stage, heat pump, dual fuel, and universal.

  • Programming: nonprogrammable, 24-hour, 5-2, 5-1-1, and 7-day.

  • Power: mechanical, battery, hardwired, power-stealing, and dual-powered.

  • Changeover: manual and automatic.

  • Display: nonbacklit and back lit.

  • Button type: traditional and touch screen.

  • Service reminders: none, filter and/or air cleaner, humidifier, and UV light.

  • Communications: non-communicating and communicating.

  • Special applications: temporary, imprinted, talking, easy-to-read, and explosion-proof.

    Now let's take each of these and provide some details.

    Type of Equipment

    Obviously the thermostat must be able to control the type of equipment installed. The contractor must know if it is heat-only, cooling-only, heat/cool (could be gas, propane, or electric heat), heat pump, or dual fuel. Dual fuel means there is both a heat pump and a furnace used in the same system to optimize energy savings and comfort. Once this is known, one can begin to choose a thermostat.

    Almost all manufacturers today produce these different types of thermostats. Recently, manufacturers have started offering "universal" thermostats that handle all types of equipment. These allow the contractor to stock only one thermostat on their trucks; the only drawback is they cost more.

    When it comes to thermostats, there are many options. There are actually many decisions that need to be made by both the home or building owner and the contractor.


    Home and building owners are more concerned than ever before with energy savings. One of the easiest ways to save money is with a programmable thermostat. With a programmable thermostat, different temperatures throughout the day can be automatically maintained. So during the day, the furnace or air conditioner does not have to run as much if no one is home. The only issue with programmable models is that they actually have to be programmed and not set to permanent "hold."

    Nonprogrammable thermostats are simpler to set up and use, a benefit many individuals like. You just basically set the temperature and are done.

    If the customer opts for a programmable model, some additional decisions must be made. Are different times and temperatures required each day (known as 7-day), or can Monday through Friday have the same schedule? If Monday through Friday can be one schedule, what about the weekend? Is a different schedule required for Saturday and Sunday (known as 5-1-1), or can Saturday and Sunday be the same (known as 5-2)?

    A less common programmable thermostat is a 24-hour thermostat, mostly used in California. With a 24-hour thermostat, there is only one program, meaning that each day will be the same.


    This is probably the most misunderstood aspect of thermostats, especially for new technicians.

    The simplest type of thermostat is a mechanical thermostat. There are no electronics involved and are mainly nonprogrammable. One of the most popular thermostats in the world, the Honeywell T87F (also known as the Honeywell "Round"), is a mechanical thermostat.

    There are two main types of mechanical thermostats, mercury and magnetic reed. This refers to the type of contact closure they use. In other words, the method it uses to turn the HVAC equipment on and off.

    Because mercury is bad for the environment, most manufacturers are moving away from this type. (The mercury is only harmful if it is released from the sealed glass in which it is contained.) Many wholesalers now recycle mercury thermostats free of charge.

    Today, battery-powered thermostats are the most common. The batteries power the digital display as well the relay coils that brings on the HVAC equipment. Because they are electronic, they have more accurate temperature control and offer many more features over mechanical thermostats.

    Because battery-powered thermostats do not require power from the HVAC equipment, they make an excellent choice when replacing a mechanical thermostat. In other words, they require the same number of wires as the mechanical thermostat. Most of the time that means 4 wires, one for heat (W), one for cooling (Y), one for the fan (G), and one for power, often referred to as hot (R).

    One drawback of battery-powered thermostats is that they require the contractor or owner to replace the batteries on a regular basis. The good news is that most thermostats have a symbol that appears on the display, giving plenty of notice to change the batteries.

    Hard-wired thermostats are very similar to battery-powered, except they require power from the HVAC equipment. This is achieved by having one additional wire between the thermostat and the equipment, usually labeled "C" or "X." Hard-wired thermostats are typically used in new construction where they can easily run the extra wire required.

    New construction contractors prefer hard-wired thermostats because they do not have to worry about replacing batteries. Most commercial contractors also use hard-wired thermostats for much the same reason; they do not want to deal with replacing batteries in large commercial buildings where there may be hundreds of thermostats.

    In the last five years, one of the most significant improvements to thermostats is the lighted display. There are several different technologies used for lighting the display.


    There are two different types of changeover, manual and automatic. Changeover refers to the switching from heating mode to cooling mode or vise versa. Most homeowners are familiar with manual changeover.

    Several times per year they have to go to their thermostat and either hit a button or move a switch to change over their system from the heat mode to cool mode or from the cool mode to heat mode. This works great in residential and light commercial, when several months per year only heating or cooling is required. Rarely in the Northern part of the country does one need cooling in the winter or heating in the summer.

    With auto changeover, the thermostat automatically switches itself from heating to cooling, or vise versa, based on the set points. When setting up the thermostat, you have to enter a cooling set point and a heating set point. The thermostat will also prevent the user from setting the cooling set point lower than the heating set point. If it did, the thermostat would get confused and not know in which mode it should operate.

    The temperature difference between the heat mode and the cool mode is known as the deadband. Almost all auto changeover thermostats have a minimum deadband of 2 degrees. So the cooling set point must be at least 2 degrees warmer than the heating setpoint.

    Additionally, most thermostats today allow the contractor to adjust this deadband (the most common setting is 4 degrees). As an example, let's say the building owner wants a cooling setpoint of 72 degrees with a 4 degrees deadband; the maximum heating setpoint the thermostat will allow is 68 degrees. Auto changeover thermostats are typically found in commercial buildings and large residences.

    A word of caution: Most residential and light commercial cooling equipment is not designed to operate when it is 50 or so degrees outside. If there is this possibility of needing cooling when it is cold outside, be sure to consult the manufacturer or distributor to make sure the proper low-ambient controls are installed to allow the equipment to run in such conditions.


    As consumers become more educated, they are demanding more features, and in turn, more benefits, from their purchases. One of the most significant improvements to thermostats in the last five years is the ability to have a lighted display.

    Many people want to check temperatures at night, but want to avoid turning on lights and waking other people in the home. There are several different technologies used for lighting the display. Some use a simple light or led on the side of the display, while others have a true backlit display. Make sure to ask the customer what they would prefer.

    Button Type

    Up until this year, there really have not been many choices on what type of buttons a thermostat could have. But due to some incredible marketing, the touch-screen thermostat has made a big impact on the market. Many contractors are finding this feature to be an excellent way to up-sell to home and building owners.

    Service Reminders

    Now that digital-electronic thermostats account for the vast majority of all thermostats sold, many options are now available. One of the most common features is a filter and/or air cleaner monitor. It basically displays a message on the screen that it is time to change the filter or air cleaner. The interval between reminders is usually based on run time of the HVAC unit or on a set number of days. While this is easy to configure on the thermostat, it is recommended that the contractor set it up to ensure the proper interval is chosen.

    In the last few years, as indoor air quality (IAQ) has become a major concern with home and building owners, thermostat manufacturers have added similar reminders for humidifiers and ultraviolet (UV) lights. These reminders work almost identically to that of the filter monitor. The humidifier reminder indicates when to change the water pad and the UV reminder indicates when to change the bulb(s).


    Though not necessary to understand the internal mechanics, it is important to at least know what is meant by the term "communicating" thermostat. In most cases, a communicating thermostat works just like any other thermostat when it comes to controlling the HVAC equipment.

    The additional benefit with these types of thermostats is that they can be connected to some type of controller. This controller has a user interface to allow the home or building owner to control the thermostat. This is important if there are a large number of thermostats in the building or there is a need to tie into an automation system.

    With the user interface (which is usually a Windows®-based computer), the user can control such functions as set points, mode, fan, temperature and time programs, and external devices such as lights. Many communicating thermostats can now connect to the Internet and allow users to have control from anywhere in the world.

    There are many developments in the area of communicating thermostats, so it will be important for contractors to stay up-to-date on these technologies (because homeowners will be demanding such control).

    Special Applications

    From time to time an application may present itself that requires a special thermostat. While it is impossible to discuss them all, here are a few of the more common ones. Temporary thermostats, for instance, are used primarily in new construction because they are nonadjustable and extremely rugged.

    It is important to be nonadjustable because it eliminates subcontractors and others from adjusting the temperature and leaving. This is most commonly seen when a home is under construction and the paint and drywall contractors want it as hot as possible to "dry out" the home. They leave and the builder or homeowner is stuck with an extremely high utility bill.

    Now, more than ever, contractors understand the need to market themselves, and not the equipment or controls manufacturers. Many contractors have their thermostats imprinted, or private-labeled, with their company logo, name, and phone number. This ensures that the home or building owner calls them when they need service. The other benefit is that when a home is sold (which happens less than every seven years), the new homeowner already knows who to call for service or replacement work.

    There are several manufacturers now making thermostats for the visually impaired and elderly. They include talking thermostats and easy-to-read displays.

    The final type of thermostat reviewed here is explosion-proof. Certain industrial applications require these types of thermostats. It is obvious by the appearance they are not standard thermostats. They usually have heavy-gauge metal housings and are rather large. They prevent any electrical sparks or discharge from escaping from the thermostat. This in turn, prevents explosions in the space.

    For more information on thermostat recycling, visit the Thermostat Recycling Corporation website, www.nema.org/gov/ehs/trc. Tom Jackson is with Jackson Systems, LLC. He can be reached at tom@jacksonsystems.com.

    Sidebar: More New Thermostats Introduced This Year

    The improvements in thermostats keep on coming. Honeywell and White-Rodgers - just to name a few manufacturers - introduced some new thermostats this year. This is the breakdown.

    Honeywell - It introduced several additions to its thermostat family. The FocusPROâ„¢ 6000 is a new programmable thermostat featuring an easy-to-read digital, backlit display that shows both room and set temperatures. Honeywell also announced a new FocusPROâ„¢ 5000 model, as well as the PRO 3000 and PRO 4000 thermostats, which provide basic home comfort controls.

    According to the company, the FocusPRO 6000 provides precise comfort control (±1 degreesF), and when used correctly, can help homeowners save up to 33 percent on their heating and cooling bills. It features weekday and weekend programming. Homeowners can use a 5-1-1, or 5-2 depending on lifestyle. The display is available in large and standard sizes. It has a five-year warranty.

    The new FocusPRO 5000 digital nonprogrammable thermostat can be configured up to two heat/two cool conventional and heat pump. Meanwhile, the PRO 4000 (programmable) and PRO 3000 (nonprogrammable) are basic thermostats. All FocusPRO models feature a flip-out door that allows for easy battery replacement without removing or disassembling the thermostat, it said.

    "Contractors have told us that they feel comfortable offering FocusPRO because of its simplicity and its brand name," said Pat Tessler, product manager for Honeywell.

    Honeywell also introduced three new models in its family of line voltage thermostats. The LineVoltPROâ„¢ 8000 7-day programmable thermostat, LineVoltPROâ„¢ 7000 digital electric heat thermostat, and the LineVoltPROâ„¢ 7000 nonprogrammable thermostat are designed to offer high-precision electronic control for electronic baseboards, convectors, radiant heating, and fan-forced heaters, the company said.

    White-Rodgers - White-Rodgers said it sets a new industry benchmark with its 90 Series Blue touch screen thermostat. Some of the features include a 12-square-inch display, room temperature readout, and extra-large touch keys for fingers of all sizes, it said.

    The 90 Series Blue is exclusively for trade professionals and will not be sold to big-box retailers, said the company. As White-Rodgers put it, "The last thing you need is an unhappy homeowner shoving a retail ad in your face." Installer-selectable options include dedicated programmable or nonprogrammable mode, dual power (hardwired or battery), and a universal staging model.

    According to White-Rodgers, research shows that consumers prefer the color blue, which is why it selected that color for its new line.

    Sidebar: Help Customers Program Their Home for Energy Savings

    During the frigid winter months, heating bills in many homes go through the roof. This winter will be no exception, as the U.S. Department of Energy predicts increases in energy prices, especially as a result of Hurricane Katrina. Luckily, there are ways homeowners can rein in heating costs.

    By properly setting a programmable thermostat, homeowners can reap significant energy savings. However, recent Honeywell research indicates nearly 70 percent of homeowners who own programmable thermostats find them too difficult to operate, and they lose out on energy-saving benefits.

    To take full advantage of the energy-saving benefits of programmable thermostats, Honeywell suggests that homeowners turn their thermostats back 10 percent to 15 percent for eight hours, while away at work or sleeping at night. This can result in savings of up to 33 percent on annual energy bills, it said.

    In addition to using a programmable thermostat, contractors are urged to tell customers to follow these U.S. Department of Energy tips for saving energy dollars during the cold winter months:

  • Use a programmable thermostat to preset lower temperatures when you're sleeping or at work.

  • Keep your window coverings open during the day and closed at night.

  • Test your home for air leaks, particularly at doors and windows, to prevent heated air from escaping. Weatherize those that are leaky.

  • Install tight-fitting, insulating window shades on windows that feel drafty after weatherizing.

    Publication date: 09/26/2005