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Replacement Thermostat Guide

2/10/22

When you need a new thermostat, it can be tempting to keep things simple and get the model most like the one you have. But if you understand some basic principles, you can choose an upgraded replacement thermostat that will bring new capabilities to your existing equipment.

Advanced thermostats can work with many existing HVAC systems to allow geofencing, remote operation and other conveniences. Many newer models also offer easy-to-read digital displays, easy-to-use interfaces, contemporary design and other advantages.

Line-Voltage Thermostats vs. Low-Voltage Thermostats

Before you can find a replacement thermostat, you'll need to know whether you need a line-voltage or a low-voltage unit.

A line-voltage thermostat delivers the electricity that powers the HVAC equipment in a room, turning it on and off directly to regulate the room's temperature. Line-voltage thermostats are most commonly paired with baseboard heaters and fan coil units.

A low-voltage thermostat sends signals that tell the HVAC system, or components of the system, when to turn on and off, without providing the heating and cooling power. Low-voltage thermostats are usually used to control central HVAC systems.

One simple way to figure out which kind of thermostat you need is to look at the wires connected to your old unit:

  • Line-voltage thermostats use heavier, thicker and stiffer insulated wires
  • Low-voltage thermostats connect to wires that are thinner and more flexible

Line-Voltage Thermostat Guide

When you're looking at line-voltage replacement thermostats, you can narrow down your choices based on the type of equipment the thermostat controls. You can find line-voltage thermostats that work with electric heating systems, electric cooling systems, fan coil units and attic fans.

You can also look at the action of the thermostat's switch. A line-voltage thermostat controlling a heater needs to “open on rise,” which means it will cut power to the heater when the room’s temperature rises above the set point. Air conditioners require a “close on rise” switch that will turn the cool air on when the room gets hot. And thermostats running both have a hybrid “open/close on rise” switch.

In addition to the switch action, there's also the switch type to consider. The switch type depends on the number of poles (how many circuits the switch controls) and the number of throws (how many possible output connections can be made):

  • If your old line voltage thermostat has two wires coming from its back, you probably have a single-pole, single-throw (SPST) switch. This is the simplest type of switch.
  • If the old thermostat has four wires and only controls a heater or air conditioner, it's probably a double-pole single-throw (DPST) switch.
  • If the thermostat controls both heating and cooling, it's probably a single-pole double-throw (SPDT) switch, which switches power between heating and air conditioning depending on the temperature. This configuration would have three wires coming from the back of the thermostat.

Other Kinds of Line-Voltage Thermostat

Plug-in line-voltage thermostats control portable heating and cooling equipment by connecting to a wall outlet. The advantage here is simplicity and flexibility, since neither the equipment nor the thermostats require permanent installation.

Low-Voltage Thermostat Guide

If you have a low-voltage thermostat, picking a replacement that will be compatible with your central HVAC system is largely a question of matching the old thermostat’s terminals.

Some HVAC systems are very basic, with a simple on-off function. Others have components like multiple-stage heating and cooling units, blower fans, heat pump reverse valves and emergency heating elements. More complex systems will require thermostats with more terminals to send separate signals to control those separate elements.

To make sure your new thermostat will work with your HVAC system, check out the letters marking the terminals where wires connect to your old thermostat. The replacement thermostat will need a terminal for every wire-connected terminal on your old thermostat. It’s OK if the new thermostat has extra terminals with no corresponding wires to connect to them, and if there are terminals on your old thermostat that don't have wires connected to them, you don't need these terminals on your new thermostat. 

Thermostat Features

Once you've determined which thermostats will be compatible with your HVAC system, there are many other options to consider.

Temperature Differential

Thermostats don’t immediately send hot or cold air the instant the temperature gets too warm or cold. If they did, the HVAC system would be less efficient because it would constantly cycle on and off as the room temperature oscillated by a fraction of a degree.

Instead, every thermostat has a “temperature differential.” This determines how much the temperature is allowed to drop below or rise above the set point before the heating or cooling kicks in. For example, if a thermostat with a three-degree differential is set to 70, the heat won’t turn on until the room cools to 67.

Wider temperature differentials can be more efficient, but they can let the room slip out of your comfort zone. Many thermostats come with a programmable differential, allowing you to choose the balance between comfort and efficiency. 

Changeover Type

A central HVAC system thermostat with manual changeover can be running either in heating mode or in cooling mode. A thermostat with automatic changeover is able to switch between heating and cooling modes as the temperature rises or falls.

Automatic changeover is convenient because it maintains your set temperature regardless of whether heating or cooling is needed, but it can be less efficient than manual changeover. For example, in a climate with hot days and cold nights, an automatic changeover system might provide heating at night and cooling during the day, when a more efficient temperature-control strategy would be to allow the space to cool off as much as possible at night.

Fan Speed and Settings

Some central HVAC thermostats only run the fan when the system is on, while others will allow you to circulate air without powering up the heater or air conditioner. And some thermostats can control the fan’s speed, while others simply turn it on or off.

Running the fan without engaging the heating or cooling system can improve comfort levels while conserving energy. It also passes air through the system's air filters without changing the temperature in the space, which can improve indoor air quality when there's wildfire smoke or other particulate pollution outdoors.

Programmability

Programmable thermostats can automatically adjust the room’s temperature depending on the time of day and the day of the week to cut heating and cooling costs. These thermostats can be set to lower the room’s temperature at night or on the weekends to save energy, and automatically warm the room up before occupants usually arrive. Many programmable thermostats also feature a “vacation” or “away” mode that will hold the room at an economical temperature while occupants are out of town. There are programmable line-voltage thermostats as well as programmable low-voltage thermostats.

Connectivity

An increasing number of low-voltage thermostats are Wi-Fi enabled, allowing you to control your central HVAC system from a mobile device. Combined with GPS technology, many of these thermostats offer geofencing that can automatically turn on the heat when your phone senses you’re on the way home.

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The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.