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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.

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER CONTROLS & SWITCHES, and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators. If your building uses warm air heat, see  FURNACE CONTROLS & SWITCHES. If your building uses steam heat see STEAM HEATING SYSTEMS. Also see Heat Won't Turn Off - Stop Unwanted Heat.

Readers needing to find and fix un-wanted air leaks, heat losses, or other energy wasters should see HEAT LOSS DETECTION TOOLS. Readers should see ENERGY SAVINGS RETROFIT CASE STUDY and also see HEAT LOSS DETECTION TOOLS and INSULATION INSPECTION & IMPROVEMENT for energy saving retrofit detailed guides. Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks.

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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.

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Questions & answers about heating and air conditioning thermostat wiring hookups

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Use links just below or 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, HEATING / COOLING
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT, Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
• Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
• Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
• National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
• The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
• Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
• The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
• Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
• "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
• Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
• Installation Guide for Residential Hydronic Heating Systems
• Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
• The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
• Links to our list of additional information on heating system inspection, repair, maintenance
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