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.
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Interface, Sensors, & Indoor Air Quality
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.
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.
The following are the various wiring terminals you may encounter on a thermostat.
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 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.
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.
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.
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).
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:
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.
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.
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.
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.
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.