InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Thermostat wiring schematics: this article provides room thermostat wiring diagrams for Flair, Honeywell, White Rodgers and other thermostat brands.
For people who prefer to see an actual wiring schematic or diagram when wiring up a room thermostat, those illustrations are provided here to help understand what wires are being connected and what each wire is doing.
We recommend following the instructions provided by the manufacturer for your specific thermostat, as wiring details may vary. Also at least some manufacturers' instructions call for installation of the thermostat by a qualified technician. And in the tables given in the article above you will find explicit wiring instructions for most common room thermostats.
Nevertheless, in response to reader interest we include here examples of actual wiring diagrams or wiring schematics for common wiring diagrams for heating and cooling thermostats including Honeywell diagrams for heating & cooling thermostats and the thermostat connections for White Rodgers & other brands of thermostats.
At left the wiring diagram illustrates the Argo AR61 triple switching aquastat equipped to connect with three thermostats. Unlike the remaining wiring diagrams in this article, the AR61 is a controller, not a thermostat. In our photo at right you can see the three thermostat terminal connection pairs.
This controller can control multiple circulator relays and is also used with tankless coil boilers and cold-start boilers - options reflected in the wiring diagrams, below left.
[Click any image or thermostat wiring schematic to see an enlarged, detailed version]
Honeywell's L8124A,C and 8151A are triple aquastats similar in function, often used to control zone valves.
Chromalox Heating Thermostat Wiring Diagrams
[Click to enlarge any image]
Above left: Chromalox Heat Flo Thermostat Kit Wiring Diagram (1 of 2) for Chromalox HeatFlo® Type KUH-TK3 and KUH-TK4 used only in KUH-02 through KUH-45 heaters.
Below: Chromalox Thermostat Kits HVH-TK6 and HVH-TK6 for field installation only in Chromalox HVH-02 through HVH-15 heaters. More wiring diagrams are given in the Chromalox HeatFlow installation manuals linked-to below.
More Help with Chromalox Thermostats - Industrial Heating Applications
Chromalox Corporation with headquarters in Pittsburgh, PA in the U.S. has offices world wide and has manufacturign facilities in the U.S. (Utah and Tennessee), Nuevo Laredo, Mexico, Soissons, France, Wujiang, China, and Edmonton Canada
Chromalox Precision Heat and Control, 1347 Heil Quaker Blvd., Lavergne TN 37086, USA, Telephone: 1-800-443-2640, Website: https://www.chromalox.com/
Chromalox Type WT Room Thermostat Installation Manual (1995), this is a high voltage (240V) thermostat whose photo is included below. Retrieved 1 Sept 2015, original source:
Chromalox Type WTL Low Voltage Room Thermostat Installation Manual (1995), retrieved 1 Sept 2015, original source:
https://www.chromalox.com/-/media/files/catalog/resources/en-us/im-pk478.pdf - A simple wiring schematic for this thermostat is shown at above-left: the two 24-volt AC thermostat wires for this control are attached to terminals 5 (red) and 4 (white).
Chromalox complete list of technical manuals including more than 20 different heating thermostat installation manuals can be found at
Chromalox thermostats, models CH, WCRT, WR, WT and WTL
Chromalox bulb & capillary thermostats, models AR (single or three phase), AR-EP, ARC, ARR, B100, BCT, E100, GNIT, and PIT
Chromalox Thermwire Heat Trace Cables and connection kits
Chromalox Industrial Convection Heaters and Unit Heater Installation Manuals (including those live-linked-to above in this article), 7150, CAF-12, CAF-20,CAF-6, CCAS-12, CCAS-8, EH, H, HCH, HVT, and V Convection Heaters, and Unit Heaters: CCH, DRA, HF, HVH, KUH, SDRA, SDRA-RG and UB, and other heater types as well.
Below: The Chromalux Type WT high voltage room thermostat.
Flair Thermostat Wiring Diagrams
2-Wire Flair APOV2 Thermostat wiring diagram
At left the thermostat wiring diagram illustrates use of a Flair APOV2 wall thermostat in a typical 2-wire application controlling a heating appliance.
In this application the thermostat is acting as an spst (single pole single throw) switch to turn heat on or off, often by operating a zone valve.
When used to operate a zone valve the thermostat wires are connected to the zone valve terminals, not to a primary control on the heating boiler.
[Click any image or thermostat wiring schematic to see an enlarged, detailed version]
3-Wire Flair APOV2 Thermostat wiring diagram
At left the thermostat wiring diagram illustrates typical use of a Flair APOV2 wall thermostat in a 3-wire application controlling a heating appliance.
In this application the thermostat is acting as a single pole double throw (spdt) switch to control heating & cooling or in some zone valve applications.
Also see our table of Flair thermostat wiring connections found above at
2-Wire Gas Heat Honeywell T87F Thermostat wiring diagram
At left the thermostat wiring diagram illustrates use of a Honeywell T87F thermostat in a 2-wire application controlling a gas fired heating appliance.
In the Honeywell T87F thermostat series the single pole double throw switch makes (closes) one set of contacts when the temperature falls - to turn on the heating appliance.
A second set of contacts will make or close on temperature rise. This second set of contacts is typically used to operate a cooling or air conditioning system but may also be used to operate other controls or valves in some heating systems.
[Click any image or thermostat wiring schematic to see an enlarged, detailed version]
At left the thermostat wiring diagram illustrates use of a Honeywell T87F thermostat in a 3-wire application as a spdt (single pole double throw) switch such as used to control low voltage motors, electric radiator valves, zone valves.
The thermostat makes a contact on both a rise and on a fall of room temperature.
At left the thermostat wiring diagram illustrates the typical wiring connections when installing a Honeywell RTH2300 (or similar) programmable room thermostat. Be sure to identify and label the existing thermostat wires before disconnecting them from the terminals on the old thermostat you are replacing.
Watch out: existing thermostat wires that will not be used in the new thermostat installation, such as wires attached to C or C1 terminals on the old thermostat should have their ends taped so as not to accidentally touch and short other thermostat wires or connectors.
Need More help? Call Honeywell at 1-800-468-1502 for assistance with your Honeywell thermostat.
Honeywell Series 10 Thermostats & Controls
Honeywell Series 10 controllers such as the Honeywell R19A use a simple relay and built-in transformer and one or more line voltage contactors.
These are spst switch type controls (such as heat-only).
The No. 1 is a low voltage holding contact while the No. 2 is a line voltage contactor. The "coil" shown may be a magnetic coil that pulls in the armature of the No. 1 relay or it may be a relay type coil used in a gas control valve. The holding contact closes when the coil is energized.
In this sketch from a 1949 Honey Heating Control Handbook the thermostat has been satisfied (both contacts are open) and the system is not calling for heat. On a drop in temperature the flexible blade makes the white contact, and subsequently connects the stiff blade. On a rise in temperature the contacts break (or open) in the opposite order. [Line voltage circuits are not shown].
Symptoms of Broken or Shorted Thermostat Wires in a Series 10 SPST type circuit
Understanding the wiring details of series 10 controls and circuits can help us diagnose heating and cooling system operating snafus. Here are more examples from the Handbook.
Symptoms of an open thermostat wiring circuit (broken wire, bad splice, loose screws)
White wire broken: thermostat cannot pull in the relay and cannot hold it open - after a momentary shorting of the W-B terminals at the relay or valve
Blue wire broken: thermostat cannot pull in the relay or valve, but once the relay is forced in (closed) the circuit can hold
Red wire broken (or a loose heater plug screw) causes short cycling on the blue contact - single point operation
Symptoms of a short circuit in the thermostat wiring
White wire shorted to the blue wire: relay or valve stays "on" unless all electrical power is shut off by the limit control switch, by disconnecting a wire at the low voltage transformer, or by turning off power to the system
White wire shorted to the red wire: thermostat can start the heater's burner but can't stop it
Red wire shorted to blue wire: short cycling (single pointing) on the white contacts alone
Mis-wiring of thermostat wires: wires connected to the wrong terminals
White & blue wires swapped incorrectly: causes short cycling on the blue contact
Blue and red wires swapped: short cycling on the white contact
Red and white wires swapped: normal operation except that the heater is effective too quickly - resulting in short on-cycles and inefficient operation
White Rodgers Thermostat Wiring Diagrams for 2, 3, 4, and 5-Wire Hookups for Heating & Cooling Applications
The White Rodgers 1F90 and similar models will replace most 2-wire heating thermostats and many 3-wire & 4-wire thermostats used for both heating and cooling. It is not used on systems without a transformer or relay, hot water heat systems that use 3-wire zone valves, two-stage heating or cooling systems, and certain other applications. [See the installation instructions for other examples.] A nice feature of this system is that the user can adjust the heating or cooling cycle-on time, and provides features similar to adjusting the heat anticipator in older Honeywell thermostats.
White Rodgers 3-Wire 1F90 Heating Thermostat wiring diagram
3-wire heating-only wiring details for a White Rodgers 1F90 Low Voltage Digital Comfort-Set thermostat
White Rodgers 4-Wire 1F90 Thermostat wiring diagram
4-wire heating & cooling applications using a White Rodgers 1F90 thermostat
White Rodgers 5-Wire 1F90 Thermostat wiring diagrams
5-Wire heating & cooling control wiring using a White Rodgers 1F90 thermostat
Want Help with Thermostat Wiring or Installation?
Thermostat Wiring help: If you need help figuring out the wiring for your thermostat see the instructions that came with your unit, or if you can't find the instructions (the dog ate the thermostat installation guide), see our detailed thermostat wiring guides at
THERMOSTAT WIRE CONNECTIONS - detailed room thermostat installation & wiring guide for each heating or cooling system type and each thermostat wiring specification, from the simple 2-wire thermostat through 3, 9 and 9-wire thermostat hookups
3M customer help line: 1-888-607-5824 for the Filtrete thermostat . Watch out: the Filtrete™ 3M-22 is a battery-operated thermostat. If the batteries fail the thermostat may fail to provide heating or cooling and the building may suffer accordingly.
Chromalox Advanced Thermal Technologies, customer service telephone: 1-800-443-2640
Emerson Technologies (White-Rodgers products): http://www.emersonclimate.com (they don't seem to want you to call)
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Ask a Question or Search InspectApedia
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
 Thanks to reader S.R. for discussing loss of heat due to a thermostat wiring mistake, October 2010
 Thank to Mr. Scott Meenen , G&S Mechanical Services , for providing some common thermostat wiring codes also found at Mr. Meenen's web page Malware Deleted 12/9/2014 . Mr. Meenan provides heating, heat pump, and air conditioning repair services in Maryland, Washington D.C., and northern Virginia. He can be contacted at 301-591-1646 or by Email to Malware Deleted 12/9/2014 - 10/2010. Quoting:
We service American Standard, Amana, Arco, Arco-Air, Bryant, Carrier, Coleman Evcon, Comfortmaker, Day/Night/Payne, Dunham-Bush, Fedders, Fredrich, Goodman, General Electric, Heil, Intertherm, ICP, Janitrol, Lennox (Armstrong, Johnson Air-Ease), Miller, Modine, Nordyne, Rheem/Ruud/Weatherking, Sears, Stewart Warner, Trane, Weather King, Williams, White-Westinghouse, Whirlpool, Weil Mclain, York, (Frasier Johnson/Borg Warner) and others.
 Honeywell Controls, the company wants you to use their contact form at this web page: http://www51.honeywell.com/honeywell/contact-support/contact-us.html
Honeywell Consumer Products,
39 Old Ridgebury Road Danbury, CT 06810-5110 - (203) 830-7800
World Headquarters, Honeywell International Inc.,
101 Columbia Road,
Morristown, NJ 07962,
Phone: (973) 455-2000,
Fax: (973) 455-4807 1-800-328-5111
Honeywell product model numbers & instruction Manuals: see http://yourhome.honeywell.com/home/Applications/FindYourModelNumber.aspx
 White Rodgers Thermostats and HVAC controls,
Homeowner information: http://www.emersonclimate.com/en-US/brands/white_rodgers/Pages/wr-homeowner-info.aspx
Contractor information: http://www.emersonclimate.com/en-US/brands/white_rodgers/wr_contractor_info/Pages/white-rodgers-contractor-info.aspx
White Rodgers Product Catalog (don't misspell the company's name as White Rogers Thermostats) -
http://www.emersonclimate.com/Documents/thermostats.pdf - Thermostat Catalog
 White Rodgers 1F90 Low Voltage Digital Comfort-Set thermostat Installation Instructions, PN 37-3654, White-Rodgers Division, Emerson Electric Co., 9797 Reavis Rd., St. Louis MO 63123
 Thermostat wiring color codes & conventions,
Thanks to reader "
Helpful Pointers" Regarding 24V T, 10/7/2012
 Domestic Central Heating Wiring Systems and Controls, 2d Ed., Raymond Ward, Newnes, ISBN-10: 0750664363, ISBN-13: 978-0750664363, Quoting from Amazon.com:
This unique A-Z guide to central heating wiring systems provides a comprehensive reference manual for hundreds of items of heating and control equipment, making it an indispensable handbook for electricians and installers across the country. The book provides comprehensive coverage of wiring and technical specifications, and now includes increased coverage of combination boilers, recently developed control features and SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) boilers ratings, where known.
In addition to providing concise details of nearly 500 different boilers fuelled by electric, gas, oil and solid fuel, and over 400 programmers and time switches, this invaluable resource also features numerous easy-to-understand wiring diagrams with notes on all definitive systems. Brief component descriptions are provided, along with updated contact and website details for most major manufacturers.
 Proliphix Corporate Headquarters,
3 LAN Drive Suite #100
Westford, MA 01886
Toll Free (U.S.): 866-IP-LIVING (866.475.4846)
Fax: +1.978.692.3378 - Email: email@example.com or Customer support: firstname.lastname@example.org Website: http://www.proliphix.com/ - quoting from the company's website: All Proliphix Network Thermostats come with our free Uniphy Remote Management Service. This unique offering lets you monitor and control your HVAC systems by simply pointing your Browser to our secure Proliphix Web Site. Enjoy the convenience of programming a thermostat from any location, using a simple graphical interface. No computer equipment or software is required. And since Proliphix takes care of the network configuration for you, you’ll be up and running in no time. We’ll even proactively monitor your thermostats and send you an immediate email or SMS message when an HVAC problem is detected.
 "Heating Control Handbook for the Installer and Service Man,Oil Burner, Gas Burner and Stoker Controls", Honeywell Corporation, March 1949 [copy on file as HoneywellControlsHandbookSA1399-2-1949.pdf] . Some of the controls discussed in detail here include the
Honeywell T1 and T11A = Series 10
Honeywell T21A (T2) = Series 20
Honeywell T847A = Series 80
Honeywell RA117A (RA1) = Series 10
Honeywell LA101A = Series 10,
Honeywell LA419A (LA4) = Series 40
V155A = Series 10, V435A = Series 40, V575A = Series 50, V835A = Series 80
 Trane TCONT800 Series Touch Screen Programmable Comfort Control Ownes Guide, American Standard, Inc., Troup Highway, Tyler TX 75711, January 2005, Telephone: Customer Service: 1-877-3381, website: www.trane.com
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones
Support InspectApedia.com & See Fewer Advertisements
From Google's Contributor website: Contribute a few dollars each month. See fewer ads. The money you contribute helps fund the sites you visit.