Room thermostat installation & wiring FAQs:

Questions & answers on how to connect wires to heating and cooling thermostats.

This article series explains the basics of wiring connections at the thermostat for heating, heat pump, or air conditioning systems.

We provide Honeywell, White Rodgers & other thermostat wiring diagrams and explanation showing how to wire a room thermostat, including just what connections to make and how wires and connectors are color coded to make things easy.

Guide to Room Thermostat Installation, Location, & Wiring

These questions and answers were posted originally

at THERMOSTAT WIRE CONNECTIONS - be sure to see the wiring guides given at that page.

On 2020-02-10 - by (mod) - how do I change 3 wires to 2 wires on my Kenmore AC?

John

What part are you replacing?

On 2020-02-10 by John Lloyd Chipman

I have a replacement part for a Kenmore wall Air conditioner. The original part had a three wire connection and the replacement has a two wire connection. Original part is 2H01127B.

Replacement is 2H01109L. For kenmore Model number 580.72187200. Can anyone tell me how to connect the new part?

On 2020-02-10 by John Lloyd Chipman

I have a replacement part for a Kenmore wall Air conditioner. The original part had a three wire connection and the replacement has a two wire connection. Original part is 2H01127B. Replacement is 2H01109L. For kenmore Model number 580.72187200.

On 2020-01-29 by (mod) -

Donald

Here are the two relevant thermostat installation instructions for comparison:

Honeywell TH830U - [PDF] a discontinued thermostat, part of Honeywell's VisionPROTM 8000 Thermostat line

This is or was a programmable thermostat

The Honeywell TH5320U thermostat is a member of the Honeywell FocusPRO® 5000 Series - [PDF] gives the manual for this non-programmable thermostat

On 2020-01-28 by donald mcclyment

Can I replace a th830u with a th5320u with no wire modifications

On 2018-12-14 - by (mod) - successful diagnosis of wall h eater thermostat

Anne

Kudos for excellent diagnostic steps on the thermostat wiring.

Sometimes when we disconnect and re-make electrical connections, by abrading the wire, tightening a screw or even just wiggling things around we fix what was a bad connection.

Could that be the case?

Or there could be a loose component in the "bad" thermostat. Try swapping the known-good one into the "bad" one's position.

On 2018-12-14 by anne

I have two old wall heaters. One is older than the other one. They both are connected to separate simple non-programmable mechanical two wire thermostats. The older heater's thermostat stopped activating the heater (2 months ago it was working fine).

I double checked that the heater was fine by disconnecting the thermostat and connecting the two wires directly (the heater turned on immediately). So, I then presumed it was the thermostat. However, I then tried connecting that thermostat with the other heater (to test it) and it works fine.

I then try connecting the thermostat that was being used with the other heater (where everything is working fine), and it also will not activate the older heater. I am totally confused. I have asked multiple people and nobody (including an HVAC professional) can seem to figure it out. The thermostats are working fine.

And the heater appears to be fine based on it turning on when the thermostat wires are touching directly.No issue with the pilot, or the heater staying on (just no longer able to turn it on with the thermostat).

On 2018-09-15 - by (mod) - Ruud Heat Pump UPNJ-031JAZ and UBEH 21J18SUN Thermostat wiring

please forgive me for asking what may seem obvious but if the wires were not used previously with the old thermostat and none of the equipment but the new thermostat has been changed why wouldn't you simply cap those wires off ?.

On 2018-09-09 by Anonymous

Hello. I own a RUUD heat pump/hot air system (model UPNJ-031JAZ and UBEH 21J18SUNA) since Sept, 2000. I just installed a new T7900 electronic thermostat. My hot air furnace has multiple stage heating elements.

I was able to connect 6 of the wires coming from the furnace. The two remaining ones, a black and an orange wire are not connected (they were also not being used on my old thermostat).

The only contact not currently used on the new thermostat is a contact marked W3 and is identified in the documentation as "3rd stage heat". Does it need to be connected in some way?

NOTE: On my old thermostat, all 6 wires were used and additionally, I had a jumper connecting contact W2 to a contact identified as "E". Any help you can provide is appreciated. Thank you.

On 2018-07-27 - by (mod) -

Ana

see FURNACE FAN WONT STOP at https://inspectapedia.com/heat/Furnace_Fan_Wont_Stop.php

On 2018-07-27 1 by Ana

The thermostata is off but the fan is running,How can I turn off

On 2017-12-26 by (mod) - special wiring tip for converting from a White Rogers thermostat to a Honeywell Thermostat

Thanks Ryan, that will help other readers. I'll keep your comment in our Honeywell Thermostat Wiring Instructions found at THERMOSTAT WIRING HONEYWELL

On 2017-10-04 by Ryan

Hi, I have a setup that I think is probably common, at least in the northeast, but that I found very difficult to get straight.

I have a boiler with a hydronic zone valve that controls two zones. The thermostat I was replacing was an old White-Rogers mercury switch. It's a 3-wire configuration. The new t-stat is Honeywell RTH6350.

Both thermostats are wired into the hydronic valve. The valve connectors are labeled with numbers 1-6. The wires going to the t-stat I replaced were configured like so: the red wire is attached to 4, the white to 5, and the green to 6. My final, working configuration is:

White-5 from the hydronic valve to R on the t-stat
Red-4 to W
Green-6 to Y

I had some trouble getting through to Honeywell support, but I finally succeeded in communicating with someone via the live chat they offer at the bottom of their help and support page. That worked well. One tip before using that service: start the chat on a laptop or mobile device; I was on a desktop and had to run up and down the stairs checking on wiring.

I never would have arrived at that on my own given the information in the t-stat manual and what I've found on the internet, so bravo Honeywell support person.

On 2017-09-17 by (mod) - The Honeywell RTH3100C wall thermostat installation manual

Fred,

The Honeywell RTH3100C wall thermostat installation manual shows typical wire connections by color code or terminal ID.

Before removing wires from an existing thermostat, or if installing a new thermostat by noting the source terminals for the wires, you want to identify and label the wires and then to connect them to those terminals marked on the RTH3100C connection bus.

The typical wiring connections for that thermostat are given in the wiring manual for the RTH3100C and available at this website at:

THERMOSTAT WIRING HONEYWELL

Also it might be helpful to look at these THERMOSTAT WIRING TIPS & COLOR CODES

On 2017-09-16 by Fred Emmons How do I connect RTH3100C to a Rheem Heat pump with 6 wires

How do I connect RTH3100C to a Rheem Heat pump with 6 wires

On 2017-02-09 by Geronimo Esteban

transformer thermometer wiring conection and diagram

On 2016-12-11 by (mod) - changing out an old Honeywell mercury thermostat to a 3rd. Gen Nest thermostat

AE

Please Use the on-page search box at the top or bottom of this page to search inspectapedia.com for 2 articles

THERMOSTAT WIRING COLOR CODES

NEST THERMOSTAT INSTALLATION & WIRING

On 2016-12-11 by AE

Hello. I am changing out an old Honeywell mercury thermostat to a 3rd. Gen nest learning thermostat

The old Honeywell had 3 wires connected: R(red),W(black),Y(white). Thermostat lines connect to a controller with T5(green),T6(brown), C(empty),B(black),W(white)R (red)

No luck connecting nest. Any ideas/solutions to fix?
Thanks !

On 2016-12-06 by Anonymous

I have a hydronic heating system. One of the zones is in the off/closed position because the thermostat keeps sending an electrical signal to keep the zone valve closed even when the Mercury Bulb Switches is engaged.

The only way to open zone valve for heated water to flow through is to manually cut off the power and have it fail open. Is this a problem with the thermostat? Does it need to be replaced?

On 2016-12-03 by Anonymous

If I only have a wire round honeywell thermostat how can I change it to a 4 wire hunter programmable thermostat and do they make a 2 wire programmable thermostat

On 2016-11-28 by Anonymous

i have a thermostat that just recently started to malfunction. I suspect the wiring connecting the thermostat to the furnace control is the problem. There are two wires that appear to be disconnected but I am not sure where to connect them.

There is a double box on the furnace one over the other. the lower box has two terminals with wire connected to each pole.

The upper box doesnt have any wire connected to its two poles. There is a free wire with two stripped connectors dangling nearby.

What should i do i turned the heat to off and the furnace isnt running.

On 2016-11-22 by (mod) -

Yes. See NEST THERMOSTAT INSTALLATION & WIRING

On 2016-11-22 21:22:38.079109 by Anonymous

I have a standard round thermostat that controls my gas warehouse heater. Can I change it to a nest thermostat/ What would I have to do

On 2016-10-10 by (mod) -

It's probably quite feasible to use a single 24VAC conventional thermostat, perhaps with its own low voltage transformer for power, to turn on and off a pair of gas fired garage heaters;

but you will want to identify the heater brand and model, then obtain its installation and operation manual, where you'll see how each unit is supposed to be controlled, its wiring connections, and also to read whether or not it is designed to be operated by an external thermostat.

Or use the page bottom CONTACT link to send me photos of the heaters, their data tags and brand names and I'll help research the question.

On 2016-10-10 by Tduma67

I have 2 low voltage gas fired heaters for my garage. How do I hook them up to one thermostat and where do I find a thermostat that can operate both at same time.

On 2016-07-31 by (mod) -

Perhaps a defective or mis-set thermostat, shorted thermostat wires, or a defective control board.

On 2016-07-31 by Anonymous

My ac heat pump cools and then heats when room temp gets to setting What could cause this .?

On 2016-05-10 1 by (mod) -

Anon

If you mean small print on illustrations, click to see an enlarged version.

If you mean the print in our web pages is too small, use your browser's view feature or your computers zoom feature to show the web page at higher magnification.

On 2016-05-10 by Anonymous

SOORY
TROUBLE SEEING SMALL PRINT
THANKS FOR INFO

On 2016-05-09 by (mod) -

Anon I'm not sure why you're using a third thermostat wire when the one you removed used only two wires. The wi-fi TTs I've installed used the same connections and wires as the old manual TT I removed.

On 2016-05-09 1 by Anonymous

I have a propane furnace with hot water heat

The two wire thermostat works but if I install a new wifi thermostat using the third comon wire the system goes into idle and wont start zone valve when calling for heat

Is this sytem wired wrong ; problem exists with all thermostats I have tried

On 2016-04-30 by (mod) -

Red to red

White to white

match the 3rd color to the terminal with that name

On 2016-04-30 by Anonymous

I have 3 wires on old thermostat the ew one is 2 wire how do i hook that up

On 2016-04-04 by (mod) -

Sanjay, please help me out by posting questions just once in one place. I answered this one where you first posted it - at https://inspectapedia.com/heat/Thermostats.php

I couldn't find the company nor their manual and asked you to use the CONTACT link to send me what you could.

On 2016-04-04 by (mod) -

RE-POsting:

Sanjay said:
Can a Thermostat that is designed for Fan Coil Heating/Cooling units such as this thermostat ... be used for AHU based heating and cooling units with either Gas or heat pump heat ? I like this design but cannot determine if i can use this for our home. We have two units (one for each floor).

On one unit heating is gas and on other its heat pump. Ideally I would like to get two of these, but the wiring labels at the back seem different so not sure if this can be used. Thank you for this wonderful website.

On 2015-08-28 12:53:47.370133 by Gordon Warlow

Hello. We have recently upgraded our electric baseboard heating (24 volt 2-wire electronic thermostat - Chromalox brand) what are our options for replacing these old (about 25 years) thermostats with new digital versions (programmable or simple day/night settings)?

We just bought Stelpro thermostats with 3-connectors, so although the display lit up and seemed to work, they would not activate the baseboard heaters... Theses heaters work with the older Chromalox thermostats, but we would like to upgrade these to something more current... Sorry for the long-winded explanation. Any help would be greatly appreciated. Thank you.

Question: 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

Reply:

Patrick, typically the three wire thermostat hookups would be exactly the same for the old and new thermostats.

For details of wiring a typical and simple digital Honeywell thermostat, see Honeywell 5-2 Day Programmable Thermostat - RTH2300B1012.

For help in understanding the wiring of your old thermostat, see Three-Wire Honeywell Wall T87-F type Thermostat wiring and also see Generic HVAC Thermostat Control Wiring Points

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

Reply:

Kurt, as you see in the examples at For help in understanding the wiring of your old thermostat, see Three-Wire Honeywell Wall T87-F type Thermostat wiring

and also see Generic HVAC Thermostat Control Wiring Points.

Many thermostat wiring setups are straightforward or "cookbook" but why not find the brand name on your thermostat and then you can obtain the installation instructions right from the manufacturer?

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.[5]

Reader comments & kudos: 

(July 22, 2012) Your site is the BEST!!!! said: After searching hi and low, your site was the only one that provided correct information on rewiring an old thermostat to a digital thermostat. I only had 3 wires coming from the wall and the instructions included 4 wires.

My AC compressor was not turning on. I even called the support number from the product guide and even they couldn't help me! Thank you very much for your site. Keep up the good work! Thank you.

(Feb 14, 2015) Jackie said:
Thank you so much for your site!

My old thermostat had no wire lettering and that was making it frustrating trying to install my programmable thermostat. Your site had my showed my old thermostat wiring and gave me the information I needed so quickly!

Question:

(Nov 14, 2014) Dietmar said:

I have a 6 wire feed, but connected to 5 terminals - here is the layout:

Red RH
WHITE - W
BLUE - RC
WHITE + YELLOW - Y
GREEN - G

I would like to install a thermostat that would allow me to program not only the temperature variations, but the fan. Currently the fan is running 24/7/365 - it should be turned to auto mode at night.

I am also considering a Smart Wifi thermostat (Honeywell or Next). Would very much appreciate your advice

Thanks. Dietmar

(Jan 5, 2015) Anonymous said:
I want to check my Honeywell Round Wall T87F Series 2-wire thermostat. I have exposed the R and W wires. Can I just wire them together after turning off power and then turn the power back on and see if the furnace starts to run?

Reply:

Yes Anon, that's exactly right. The thermostat is basically an "on-off" switch that connects or disconnects those wires to call for heat or stop calling for heat in response to room temperature.

Remember to check for a broken or open TT wire enroute between the thermostat and the control at the wires' other end.

Question: the W terminal

(Mar 11, 2015) Nobu said:
I would like to know the reason why "W" is terminal for heating.
Is there any specification about wiring?

Reply:

Nobu the "W" is probably referring to the usual wire colour used for this feature. W - white.

...

Continue reading at THERMOSTAT WIRE CONNECTIONS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Thermostat Wiring & Installation Articles

• THERMOSTAT INSTALLATION STEPS
• THERMOSTAT WIRE CONNECTIONS
  • COMMON WIRE at THERMOSTATS
  • CONVERT LINE to LOW VOLTAGE THERMOSTAT
  • THERMOSTAT WIRE CONNECTIONS - 2-WIRE like the T87F
  • THERMOSTAT WIRE CONNECTIONS - 3-WIRE Red, White, Blue Wires
  • THERMOSTAT WIRE CONNECTIONS - 4-WIRE Red, Yellow, Green, White
  • THERMOSTAT WIRE CONNECTIONS - 5-WIRE Blue/Black, Red, White, Yellow, Green
  • THERMOSTAT WIRE CONNECTIONS - 6-WIRE Red, White, Blue, Yellow, Green, Orange Wires
  • THERMOSTAT WIRE CONNECTIONS - 8-WIRE Black, Red, White, Yellow, Green, Orange, Brown, Blue
  • THERMOSTAT WIRE SORTING to ID R W B
  • THERMOSTAT WIRING AZEL i-LINK
  • THERMOSTAT WIRING 3M-22 FILTRETE
  • THERMOSTAT WIRING AMERICAN STANDARD
  • THERMOSTAT WIRING ARGO
  • THERMOSTAT WIRING CHROMOLOX
  • THERMOSTAT WIRING FLAIR INSTRUCTIONS
  • THERMOSTAT WIRING GENERIC Control Points
  • THERMOSTAT WIRING GOODMAN
  • THERMOSTAT WIRING HONEYWELL
  • THERMOSTAT WIRING LINE VOLTAGE 120-208-240VAC
  • THERMOSTAT WIRING NEST
  • THERMOSTAT WIRING TRANE HVAC
  • THERMOSTAT WIRING WHITE RODGERS
• THERMOSTAT WIRE TERMINAL ID CODES / FUNCTIONS
• THERMOSTAT WIRING OPENING SEAL
• THERMOSTAT WIRING COLOR CODES

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS

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