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How Room Thermostats Work
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• How the Building or Room Thermostat Senses & Responds to Temperature Changes
• Honeywell traditional wall thermostat operation details
• How Do I Install & Wire Up a Room Thermostat like the Honeywell Round Wall Thermostat?

This article explains the how heating or air conditioning thermostats actually work. This document will help repair technicians and building occupants to understand wall thermostats, their use, setting, and adjustment. Here we provide A Guide to Finding, Using, and Adjusting Thermostats for Heating & Air Conditioning Furnaces & Boilers, Heat Pumps or Electric Furnaces or Boilers.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

How the Honeywell Room Thermostat Senses & Responds to Temperature Changes

This and other older room thermostats sense room temperature by the combination of a bimetallic spring and a switch that the spring moves as its position changes in response to temperature changes in the room.

The round, coiled bi-metallic spring in the thermostat shown in this photo has been used various shapes in lots of other control devices that respond to temperature changes, such as the furnace or boiler stack relay switch.

Two metal strips, each of different properties, are sandwiched together and then coiled (center-right in our photo) to form a spring.

Because the thermal coefficient of expansion of the two metals are different, and because the two metal strips are adhered together, as temperature changes the spring will bend or flex.

As the bimetallic thermostat temperature sensing spring flexes it expands or shrinks as temperature rises or falls. This causes a movement of the spring.

A sensor or switch attached to the end of the spring is mechanically moved and in turn is used to turn a device "on" or "off".

In our photo above our little dental tool points to a mercury switch which makes or breaks electrical contact to actually turn on or off the control to which our thermostat has been attached. You can see some wires leaving the glass mercury bulb and heading off to other contacts inside the thermostat.

Just below, in our left hand photo the bimetallic coil spring has tilted the mercury-bulb so that it tips to roll its blob of mercury away from two metal contact wires that are sealed, along with the mercury, inside of the glass bulb.

In the right-hand photo, the bimetallic spring has contracted (it got cold), causing the bulb to tip so that the mercury rolls down the inside of the tube and contacts the two wires inside the bulb, completing an electrical contact to switch the air conditioning (off) or the heating system (on) depending on the mode (cooling or heating) in which the thermostat is being used.

This swapping of the role of the switch in turning something OFF in response to a temperature drop (cooling mode) or ON in response to a temperature drop (heating mode) is why a dual-purpose thermostat will also have an extra switch to decide whether we're controlling heating or cooling.

To avoid confusion about what a mercury bulb switch or any other kind of switch is doing, electricians call the left photo condition "switch break" mode and the right photo condition "switch make" mode because the switch is "breaking" or "opening" a circuit when contacts are disconnected, and a switch is "making" or "closing" a circuit when its electrical contacts are connected.

Anyone who understands how a mercury bulb thermostat works to simply "make" or "break" an electrical circuit will see clearly and forever that a room thermostat is not an accelerator, it is an on-off switch that responds to temperature changes. So if you're cold, and the room temperature and room thermostat are both reading 55 degF., turning the thermostat to any temperature above that will cause the heating system to turn on. Turning the thermostat up to 95 degF will not warm the room any faster than turning the thermostat up to 65 deg F.

Our little sketch explains how the traditional mercury switch worked in the old Honeywell (R) round wall thermostats.

As room temperature changes, a bimetallic coil-spring moves to tip the glass bulb in either direction, up or down. As temperature drops the bulb tips to the left (the spring contracts) and the mercury, at point (B), connects the two contacts to turn on the heating system.

Modern thermostats no longer use mercury switches (mercury is a toxic product) but they function similarly in response to room temperature.

Mercury is a toxic substance which is no longer used in thermostat switches, but there are millions of these devices still in place in homes. You should ask your local or state department of environmental protection/conservation if your area has special requirements when one of these old mercury bulb thermostats is to be thrown away.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

THERMOSTATS
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  How the Room Thermostat Responds to Temperature
  How Adjust the Heat Anticipator
  Mini Ammeter to Check Heat Anticipators
  Other Switches on a Room Thermostat
  How To Install & Wire Thermostats

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