Thermostat Buying Guide
Today's thermostats are more than just the “brain” of your heating and cooling system. Sure, they're responsible for keeping your home or office comfortable year-round, but depending on how "smart" they are, thermostats can significantly reduce usage and energy costs.
With so many options available, choosing the right thermostat may seem intimidating, but we've outlined everything you need to know to make sure you pick the right model for your home.
Navigation
Interface, Sensors, & Indoor Air Quality
How Thermostats Work
After you set your desired temperature on the thermostat, it will constantly compare that setpoint temperature with the actual indoor temperature. If the actual indoor temperature falls below the setpoint, the thermostat will signal for heat to turn on. If the actual indoor temperature rises above the setpoint, the thermostat will signal for cooling. Once the setpoint temperature is reached, the thermostat signals the heating or cooling system to turn off.
Thermostat Wiring
Thermostats come in low-voltage, line-voltage, and millivolt configurations.
- Low-voltage and millivolt thermostats use thin 18-gauge wire that connects to wiring terminals on the thermostat.
- Line voltage thermostats use much thicker wiring and are usually connected to the thermostat with wire nuts.
Regardless of the type of system, it is critical to shut off the power supply before working with any wires. Additionally, any work on line voltage systems should only be performed by licensed professionals. Low-voltage control wires are often color-coded and terminal naming is pretty standard from one manufacturer to another.
Wiring Tip: Label your wires when replacing your thermostat in case standard colored wires were not used.
Thermostat Wiring Terminals
The following are the various wiring terminals you may encounter on a thermostat.
Common Terminal (C): The common wire is analogous to the neutral wire in a standard electrical system. It completes the circuit and allows the thermostat to be powered from the 24V transformer in the system. Thermostats without a C terminal must rely on another source of power, such as a battery or power stealing from the control circuit.- Power Terminal (R, Rh, Rc): The R wire is the “hot” wire of the control circuit. It is the source of power to operate the thermostat and signal the various components of the system to operate. Some thermostats will have a single terminal “R," while other thermostats will have separate terminals for the cooling system (Rc) and the heating system (Rh).
When a thermostat with two separate terminals is used on a system with a single transformer for both heating and cooling, a jumper is needed to connect the two terminals. - Heating Terminal (W, W1, W2, W3, etc.): The W terminal is used to send a heating demand to the equipment in the system. For systems with multiple stages of heating, the W1 terminal calls the first stage of heat. The remaining heat terminals call the remaining stages in order.
- Compressor Terminal (Y, Y1, Y2, etc.): The Y terminal is used to signal the air conditioner or heat pump to operate. Similar to the heating terminals, if multiple Y terminals are present, they are called sequentially.
- Heat Pump Reversing Valve Terminal (O/B): Heat pumps use a reversing valve to switch between heating and cooling operating modes. Some manufacturers energize the reversing valve during heating operation and use the B terminal. Other manufacturers energize the reversing valve in cooling operation and utilize the O terminal. Still other thermostats have a single terminal labeled O/B that can be configured for the necessary operation.
- Fan Terminal (G): The G terminal is used to control blower operation.
- Auxiliary and Emergency Heat Terminals (Aux, E): Heat pumps often require backup sources of heat when temperatures drop below a certain level. The auxiliary and emergency heat terminals are used to activate a backup heat source in these applications.
Will it Work for Your Equipment?
At the most basic level, you will need to make sure that your thermostat is compatible with the equipment installed in your system based on type, voltage, and the number of stages. Once you’ve identified compatible thermostats, you can narrow your options based on programmability, user interface, price, and other available features.
Forced Air Thermostats
Forced air is the most common system type in the U.S., and major thermostat manufacturers such as Honeywell, White-Rodgers, and ecobee design their thermostats with these types of systems in mind. If your forced air system includes a heat pump, be sure to select a thermostat containing an O/B terminal to control the reversing valve.
Boiler Thermostats
Hot water and steam systems often have different control needs than forced air systems. One difference is in the nomenclature on the terminals. Boilers will typically list T1 and T2 terminals, which will connect to the Rh and W1 terminals on the thermostat. Today, most thermostats, including smart ones like ecobee and Nest, work with boilers. If you need your boiler to include in-floor controls for radiant heating, however, you might need a more specialized thermostat, like a Tekmar.
Voltage Considerations
Nearly all boilers, furnaces, air conditioners, and heat pumps use low-voltage (24V) control signals, and the majority of thermostats are designed to work at this voltage. Older systems that have pilot lights may sometimes use millivolt controls. If your system uses millivolt control signals, you must purchase a thermostat that is rated for millivolt operation.
Many electric heating systems use the thermostat as the switch to control power supply to equipment. In these applications, a line-voltage thermostat will be required. Make sure your thermostat is rated for the same type of power supply as your equipment.
Stages
Many modern heating and cooling systems use staging to increase efficiency by more closely matching the output of the heating and cooling equipment to the needs of the space. To get the most benefit from these types of systems, it is important to have a thermostat that supports at least as many stages as are present in the system.
The simplest thermostats on the market support a single stage and heating operation only. These types of thermostats will be labeled as 1H, for 1 heating stage. A single stage heating and cooling system will require a 1H/1C--1 heating stage and 1 cooling stage.
This nomenclature continues into more sophisticated thermostats (e.g. 3H/2C indicates a thermostat is capable of controlling 3 heating stages and 2 cooling stages). It is important to choose a thermostat that supports at least as many stages as are present in the system.
Some thermostats may support different numbers of stages depending on whether they are used in a heat pump system or not. This will often be listed as 3H/2C – Heat pump, 2H/2C – conventional. This is because heat pump operation uses the compressor terminals (Y) and reversing valve terminals (O/B) rather than the heating terminals (W).
Types of Thermostats
There are three types of thermostats on the market today.
A non-programmable thermostat will simply maintain a set temperature until that temperature is manually changed. This means your home will often be heated or cooled even when nobody is present.
A programmable thermostat allows you to set a schedule for the temperature you would like your space to be at any given time. By limiting heating and cooling operations when the home is unoccupied, you can reduce your energy costs without sacrificing comfort. There are various levels of programmability available to meet the needs of all spaces:
- Daily: The thermostat is capable of storing and executing a single schedule that repeats every 24 hours. This type of control is best suited for spaces that have similar needs regardless of the weekday.
- 5-2: This type of thermostat supports one schedule for the 5 weekdays and a separate schedule for 2 weekend days.
- 5-1-1: The 5-1-1 thermostat is very similar to the 5-2 thermostat above in that it supports a separate schedule for weekdays and weekends. However, it will support a schedule for Saturdays and a separate schedule for Sundays for a total of three schedules.
- 7-Day: A 7-day programmable thermostat will support a separate unique schedule for each day of the week.
Smart thermostats go beyond programmability, learning and adapting to your temperature habits. They can collect data using motion sensors, occupancy sensors, and behaviors to customize the perfect temperature without you ever having to touch the dial. On top of that, they can integrate with your smart home and respond to voice commands. Two very popular types of smart thermostats are the ecobee and Google Nest.
Thermostat Interface
Once the thermostat is on the wall, the interface is how you’ll interact with it for the rest of its life. Typical thermostats have an interface with basic switches and buttons allowing you to toggle settings like mode, fan, and temperature. Others have soft key buttons that change function based on the menu you are accessing.
Some modern thermostats use touch screens, creating a familiar experience for tech-savvy users. Smartphone apps and internet-based interfaces are available for higher-end thermostats like the ecobee, Nest, and select Honeywell products. These interfaces often allow a homeowner to monitor and track energy usage in addition to controlling their system.
Sensors
Some thermostats either include or offer optional sensors to provide more information about your system, your home, or outdoor conditions. 
These sensors can include outdoor temperature and humidity sensors, remote room temperature sensors, occupancy sensors, and other information-gathering devices. This information is often used to optimize system operation in changing conditions.
Internet-enabled thermostats are sometimes able to pull weather information from online and show outdoor conditions and a weather forecast. They can alert you on your mobile device any time the system is running or if dangerous conditions exist, like if it's getting cold enough for the water pipes to burst or it's so hot your pets are suffering.
WiFi thermostats can also link up with other smart home devices like smoke detectors and home monitoring devices to alert you of a potential break-in, fire, or unsafe levels of carbon monoxide.
Indoor Air Quality Controls
Many homes have additional equipment beyond typical heating and cooling, such as humidifiers, dehumidifiers, whole-house fans, and other air purification products.
Premium thermostats offer dedicated functionality to control these types of devices. There is a chance that you will sacrifice some functionality by using a single multi-purpose control rather than dedicated controls for each device. However, there are also efficiency and comfort benefits to be gained from coordinating the operation of these various components.
Other Features
There are a variety of other features designed to make operating and maintaining your heating system at peak efficiency as easy as possible. Some common features include service reminders, email alerts, and “learning” functionality.
Service reminders are a way for the thermostat to prompt you when it’s time to inspect or replace key components, like air filters. Email alerts can notify you in the event equipment malfunctions or needs attention - they can alert your contractor as well so that problems are resolved as quickly as possible.
Some thermostats can learn your schedule and temperature preferences and use this information to create a schedule that optimizes comfort and efficiency.
