Home Improvement Skills & Specialties HVAC

Understanding Common Furnace Thermostats

Thermostat on wall
Nine OK / Getty Images
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The furnace thermostat is the nerve center of your home heating system. It's the main control point that determines when and how much heat will be delivered by the furnace. At the most basic level, thermostats are simply temperature-sensitive switches but they have become sophisticated with more features. Learn more about types of thermostats and how to choose the right one for your home and needs.

Types of Thermostats

Most thermostats that control central heating equipment, including furnaces, boilers, and heat pumps, are low-voltage thermostats that typically use 24 volts of electricity. Electric baseboard heaters and in-wall heaters may use line-voltage thermostats that use 120 or 240 volts of electricity. 

Low-voltage thermostats are powered by a transformer that reduces the incoming line voltage from 120 volts down to a level between 12 and 24 volts, depending on your furnace design. Within this low-voltage thermostat category, there are four major types: 

  • Mercury contact
  • Mechanical contact
  • Digital
  • Electronic programmable
  • 01 of 04

    Mercury Contact Bimetallic Thermostat

    Mercury contact thermostat
    Home-Cost

    The mercury contact bimetallic thermostat is an older type of analog (non-digital) thermostat that was once the most common type found in homes. They are still found in many residences, although it's no longer legal to produce or sell them in the United States, and local governments are urging homeowners to safely recycle them

    For an analog thermostat to work, something has to move as the temperature increases or decreases. That's the job of the bimetallic strip, made of two pieces of metal joined together. The two metals expand at different rates at a given temperature, known as the coefficient of expansion. As one side expands more than the other side, the metal curves one way or the other.

    The thermostat contact is like a switch for your lights. As the bimetallic strip moves, it either opens or closes the contact. In the case of a mercury thermostat, the contact is a glass vial, called an ampoule, that contains mercury. The ampoule is attached to a coiled bimetallic strip, also called a bimetallic coil. As the coil moves, it tips the vial of mercury until the mercury moves and the switch contact is opened or closed.

    Finally, the heat anticipator is an electrical resistance wire mounted on a center disc connected to the bimetallic coil. It is adjustable to allow fine-tuning of when the thermostat turns the furnace blower on and off. The bimetallic thermostat has modes for heating and cooling and fan positions for on/off and automatic. It is not programmable.

  • 02 of 04

    Mechanical Contact Thermostat

    Mechanical Contact Thermostat (White-Rodgers)
    Home-Cost

    The mechanical contact thermostat uses a simple mechanical contact instead of a mercury contact switch. Like a mercury contact thermostat, a mechanical thermostat uses a bimetallic coil (to open or close the mechanical contact points) and includes a ​heat anticipator. Some older versions of mechanical contact thermostats used a straight blade bimetallic strip rather than a coiled strip. They are a bit less accurate but work under the same principle.

  • 03 of 04

    Digital Thermostat

    Lux Digital Thermostat
    Lux Products Corporation

    The ​digital thermostat uses electronic controls instead of a bimetallic coil to accurately sense temperature changes. These thermostats usually have large, easy-to-read displays and are often used to replace older mechanical thermostats. Digital thermostats have modes for heating and cooling and fan positions for on/off and automatic, but they are not programmable.

  • 04 of 04

    Electronic Programmable Thermostat

    Woman adjusting digital thermostat
    Tetra Images / Getty Images

    The electronic programmable thermostat is a digital thermostat with a "brain." It has user controls that allow you to set the days of the week, the time, and the temperature, as well as ​custom programming functions. An electronic programmable thermostat saves energy by letting you lower your temperature when it fits your lifestyle, such as at night or when no one is in the house. Some models can even alert you to change your ​furnace filter

    The latest programmable thermostat is the smart thermostat, such as the Nest. These are fully programmable and controlled manually or with a smartphone or other device. They are also capable of programming themselves by learning how you typically set the temperature throughout the week. 

Choosing a Thermostat

There are plenty of considerations when choosing a thermostat, including design, controls, ease of installation, features, and compatibility. First, choose whether you want a manual or programmable/smart thermostat. Second, make sure what you want is compatible with your HVAC system. Though many thermostats are designed for do-it-yourself installation, it pays to hire an HVAC professional to make certain the wiring works with your existing furnace and AC system.

FAQ
  • What is the average cost of a thermostat?

    The average cost of a thermostat varies dramatically, from $15 to $500 or more. The cost depends on the type and features you choose. A mechanical thermostat is the least expensive at around $15 and a smart thermostat is the most expensive coming in at $100 to $500-plus.

  • What is the best type of thermostat?

    The best type of thermostat depends on your preferences between manual and programmable or smart models. In addition, the best thermostat for your home will be compatible with your HVAC system. However, for energy efficiency, it's best to consider a programmable thermostat.

  • What are the most popular thermostat brands?

    Honeywell is a popular brand that makes non-programmable and programmable thermostats, and they make smart thermostats. Other popular brands for smart thermostats include Nest, Google Nest, Emerson, and Ecobee.

  • How much does it cost to install a new thermostat?

    The average cost of installing a new thermostat is about $182. However, you can pay anywhere between $113 and $264 depending on the scope of the installation and the type of thermostat you choose.

  • What is the difference between a thermostat and a sensor?

    There is a difference between a thermostat and a sensor though a sensor is usually built into or located on a thermostat or nearby. A sensor detects the ambient air and converts the cold or heat into an electrical signal to trigger the thermostat to work. Very old thermostats had mercury-based sensors.

  • How would you know if your thermostat is bad?

    You will know if your thermostat is bad if your system either won't turn on, continuously runs, doesn't respond to your settings, or your system is short-cycling. Note that if you have a thermostat connected to Wi-Fi, the connection could be poor.

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  1. Contact Us About Mercury: Recycling Thermostats. EPA.

  2. How Much Does Thermostat Installation or Replacement Cost? HomeAdvisor.

  3. Thermostats (Wall-Mounted Mercury Switch). Zero Waste Sonoma.