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Thermostat wiring details & connections for the _. This article gives a table showing the proper wire connections for the _ room thermostat used to control heating or air conditioning equipment.
Room thermostat installation & wiring guide: this article series explains the basics of wiring connections at the thermostat for heating, heat pump, or air conditioning systems. Our page top sketch, courtesy of Honeywell Controls, illustrates the wiring diagram for a traditional Honeywell T87F thermostat used for 2-wire single pole single throw control of heating only in a typical gas-fired heating system.
Table of Common Heating and Cooling Thermostat Wiring Connections for Major HVAC & Thermostat Brands
The following thermostat wiring suggestions were derived from a thermostat wiring connection code list provided courtesy of J. Scott Meenan.
Watch out: do not short any wires together. Turn off power & confirm it is off. As we have warned before, don't forget to turn off all electrical power involved with your heating system before working on thermostat wiring, and confirm that power is off where you are working by using an appropriate test instrument such as a VOM. Failure to respect this advice risks equipment damage, and in some cases electrical shock or even a building fire.
Table of Common HVAC Thermostat Wiring Connections - Generic HVAC Thermostat Control Wiring Points
Table of Standard or Most-Common Thermostat Wire Colors Matched to Thermostat Terminal Names
[see other thermostat brand specific wiring notes below]
Nominal Wire "Color"1
Energize to heat. Used on some systems including Rheem/Ruud HVAC systems. See (O) below.
Blue, brown , or black
Blue, brown , or black wire, common side of the transformer. Needed on some electronic thermostats or if the system uses indicator lamps.
Watch out: do not confuse this terminal with the (B) discussed above. Check you brand, model, and installation manual/wiring diagram.
(Also see (X) below)
Common side of the transformer (see "B")
Blue, Pink, Gray, or Tan
Blue, pink, gray, or tan wire, emergency heat relay on a heat pump. [Active all the time when selected, usually not used -??]
Furnace blower fan (used in air conditioners, heat pumps, some electric furnaces.)
On most thermostats the (G) and (Y) terminals are connected together at all times when the fan switch is in the "Auto" mode.
Energize to cool, used for reversing valve on heat pump systems.
Electrically live side ("hot" side) of the transformer wiring. Note 1.
Outdoor heat anticipator reset
(W) (W1) (W2)
"Heat", such as for a gas burner, oil burner, electric heat, or auxiliary heat on a heat pump, including the defrost output from an outdoor (condenser) that is used to activate electric heat at the compressor and to turn on the AUX heat lamp indicating that backup electric heat is in use or required.
Some heat pumps require a jumper from (W) to (Y) to operate the heat pump.;
Also see (B) above.
Compressor activity: cooling, or cooling and heating if on a heat pump
1. The [Honeywell 5-2 Day Programmable Thermostat - RTH2300B1012] thermostat cannot be used if your old thermostat had and used any two of the following wires: R, RC, RH, 4 and V.
How to Wire a Generic 2-Wire Thermostat
Basic 2-wire Room Thermostat Wiring Instructions
Wiring connections for a room thermostat such as the Honeywell 24-volt T87F, the Honeywell series 10 (out of production), or Penn "Rimset" low-voltage wall thermostat models are pretty simple as are the wiring instructions for White Rogers, Mercoid, General Controls, and similar thermostats.
Two wire thermostat wiring instructions: In a two-wire installation, the thermostat backing plate is mounted level on an interior wall in the room which we want to be the master temperature control for the area served by the heating or cooling system.
The red wire from the heater or air conditioner control is mounted to the "R" terminal on the backing plate. Typically the red wire is originating at the heating or air conditioning low voltage transformer and brings power to the thermostat. (See LOW VOLTAGE TRANSFORMER TEST)
The white wire from the heater or air conditioner control is mounted to the "W" terminal on the thermostat mounting plate. Typically the white wire is taking power from the thermostat to the operating control circuit board in the heater or air conditioner.
Our photo (above left) shows the backing plate that is mounted first when installing a round Honeywell Series 20 type room thermostat. Notice that the plate shows a "level" line.
We left off the actual wires so that you could see the "R" and "W" by the two screw terminals at the lower left 7 and 8 o'clock positions on the thermostat backing plate.
Note: The thermostat in this HVAC/R set-up is simply acting as an "on-off" switch to turn the heater or air conditioner on or off in response to room temperature.
The electrician may have run a multi-wire set of low voltage wires through the walls of the building between the low voltage transformer and the thermostat (etc), but in this simple installation the other wires at the thermostat are not being used.
At the low voltage transformer you will see two wires labeled "C" for common and "R" for red. We discuss wiring the low voltage transformer itself at LOW VOLTAGE TRANSFORMER TEST.
Watch out: do not short any wires together. Turn off power & confirm it is off. Don't forget to turn off all electrical power involved with your heating system before working on thermostat wiring, and confirm that power is off where you are working by using an appropriate test instrument such as a VOM. Failure to respect this advice risks equipment damage, and in some cases electrical shock or even a building fire.
How to Wire Up a 3-Wire Thermostat
Three-wire thermostat wiring instructions, also called "series 20 installations" we have three wires rather than two to connect.
The Red wire coming to the thermostat from the heater or air conditioner is connected to "R".
The white wire is connected to the "Y" terminal, and
The blue wire is connected to the "W" terminal on the backing plate.
Then the thermostat body is screwed in place. The screws that secure a round Honeywell traditional wall thermostat to its backing plate will also connect it properly to the wiring.
A plug connector may be used:
On other, in fact most contemporary room or wall thermostats it may be necessary to plug in a connector between the thermostat and its mounting plate.
How to Hook Up a 6-Wire Thermostat
Color Codes and Hookups for Thermostat Wires, typical for heat pump installations and thermostats
At left you can see the 11 possible thermostat wire connectors on the 3M-22 FIltrete thermostat. In this installation only two wires, R and W, have been connected.
Typical Thermostat Wire Connections for Heat Pumps
Typical connections when wiring a heat pump are as follows:
the red is power,
the yellow is for cooling,
the white is for heat,
the green is for the fan,
the orange is for the reversing valve, and
the blue is usually always common.
Again, you will have to check to see how the installer has ran the wires from the thermostat.
How to Wire a thermostat to control only a heating system or only an air conditioning system
If your thermostat is controlling only a heating system or only an air conditioning system, you will only have a red wire, and a white wire coming form the inside unit, to the outside unit.
On an air conditioner these two wires will go on the contactor to send 24 volts to the coil to pull the contactor in to start the air conditioner.
On a heating system these two wires will go to two thermostat connections on the primary controller such as an aquastat or air handler control that turns on the heating unit. Depending on the type of heating system, the thermostat, acting simply as an "on-off" switch will
- Hot water heat In the U.S. the TT wires will turn on a hot water circulator (or open a zone valve) on a hydronic or hot water heating system; falling boiler temperature will cause the aquastat to turn on the oil or gas burner (this is the U.S. typical installation).
- Hot water heat In Canada the TT wires will activate an aquastat control that will turn on and off the boiler based on its temperature; circulators are already running.
- Hot air heat: the TT wires will turn on the furnace heating equipment (oil or gas or electric, say); when the air temperature in the furnace heating plenum is hot enough the fan limit switch will turn on the blower fan.
These are the traditional wiring schematics, again, check the thermostat to see if this is the case with yours.
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Replacing a round Honeywell T87 with a Digital Honeywell Thermostat
Hello - I am replacing a round Honeywell thermostat with a non-programmable digital Honeywell unit. I have a hot water system with multiple zones and with no air conditioning. There are three wires at the thermostat. On the old round unit the red wire went to the R terminal, the white wire went to the w terminal and a green wire went to B terminal. I hooked up the new stat the same way but it would not work. If I selected "fan on" instead of "fan auto" the zone would heat up but would not shut off (hot water kept flowing even if I selected a temp below room temp). Appreciate your help. - Patrick - 1/20/12
Patrick, typically the three wire thermostat hookups would be exactly the same for the old and new thermostats.
Question: Replacing a mercury thermostat with a digital unit
I am replacing an old mercury thermostat with a digital. my system is a heat pump. my neighbor gave me the digital, no instructions, no box. i'm trying to decide if this is even a heat pump compatible thermostat. on the therm it has c, g, rc, rh (which are linked by a black wire.) w, y, b, and o. coming out of my wall is, white connected on old therm w2, yellow connected to y, green conn g, black conn e, red to r, blue to b and bro to x. any help would be great. - Kurt 6/4/12
Also, Honeywell's thermostat replacement advice (and most likely that of all thermostat manufacturers) warns:
Watch out: MERCURY NOTICE: Do not put your old
thermostat in the trash if it contains mercury in a
sealed tube. Contact your local waste
management authority for instructions regarding
recycling and proper disposal.
(Jan 3, 2015) james Hunt said:
Trying to replace a Righttemp 6010 with a HoneywellRTH2410 and I don't think they are compatible. The main problem seems to be what to do with the W2 (red) wire. The AC unit is not in operation and we do not need cooling. I was thinking if I connected that wire to the R terminal of the new unit (jumpered with Rc) that it would work as the remaining three wires seems to match up okay. I guess this would mean my attempting to use the new unit on a multi stage system for which it is not intended. It worked fine before, only that we did have the Emergency Heating Mode function.
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Questions & answers or comments about heating, air conditioning, and heat pump thermostat installation and wiring
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 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 Phone: +1.978.692.3375 Toll Free (U.S.): 866-IP-LIVING (866.475.4846) Fax: +1.978.692.3378 - Sales: email@example.com Marketing: firstname.lastname@example.org Customer support: email@example.com 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
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 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.)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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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.
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