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AIR HANDLER / BLOWER UNITS
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BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
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CIRCULATOR PUMPS & RELAYS
CONVECTOR HEATERS - HYDRONIC COILS
COOL OFF HEAT, Thermostat Switch
DEFINITION of Heating & Cooling Terms
DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC HEAT, DIAGNOSIS, REPAIR
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FLOODED HEATING EQUIPMENT REPAIR
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MANUALS & PARTS GUIDES - HVAC
NO HEAT - BOILER
NO HEAT - FURNACE
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
PULSE COMBUSTION HEATERS
Reset Switch - Heater Primary Control
Reset Switch Broken - Quick Repair
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
THERMOSTATS, WATER HEATER
THERMOSTATIC EXPANSION VALVES
Room thermostat installation & wiring guide: this article 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. 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. Other thermostat wiring schematics are provided below.
We provide thermostat wiring connections for just about every type of residential heating or cooling room thermostat as well as a description of thermostat wiring color codes & conventions. At THERMOSTAT INSTALLATION STEPS we provide a step by step photo guide to installing a room thermostat. We also provide THERMOSTAT WIRING DIAGRAMS and a separate explanation of THERMOSTAT WIRE FUNCTIONS.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Where to Locate & Mount a Room Thermostat & what spots to avoid
Hooking up a 2-Wire Thermostat: How Do I Install & Wire Up a Room Thermostat like the Honeywell Round Wall T87F Series 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.
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.
Where do the Thermostat Wires Start and Where do they End-Up?
As we explain in more detail at THERMOSTAT WIRING DIAGRAMS,
For completeness in understanding where the red and white thermostat wires originate and end up, at below left we illustrate the red and white thermostat wires originating at the low voltage transformer, and at below right we illustrate a typical thermostat wiring connection in an R8182 Aquastat primary controller of a heating boiler, using the T and T terminals.
3-Wire Thermostats: Where Do We Connect the Red, White, & Blue (if present) Wires to a Typical Three-Wire Wall Thermostat?
6-Wire Thermostats: Where Do We Connect the Red, White, & Blue, Yellow, Green, Orange Wires in a Typical Six-Wire Wall Thermostat?
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:
Again, you will have to check to see how the installer has ran the wires from the thermostat. Also see Table of Common Heating and Cooling Thermostat Wiring Connections for added details.
Typical thermostat connections when wiring 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.
We moved this topic to LINE VOLTAGE 120V Wall Thermostat Wiring - separate article in order to keep our text on line voltage thermostats together.
Tables 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.
Complete tables for Thermostat Wiring Terminal Notes for the 3M-22 Filtrete Room Thermostat. Our photo just below shows all eleven possible thermostat control wire connection points on this device. (We don't like that sharp crimp in the red wire, nor should you.) Note the white wire connected to W and the red wire connected to RH and the jumper left in place between RH and RC. Wired as shown this thermostat works only from its own internal battery power.
Lennox thermostat and equipment wiring color codes may vary significantly from common usage. Be sure to consult with Lennox and obtain the proper installation manual for your equipment.
These White Rodgers Tables Map Old Thermostat Wires to New (White Rodgers) Thermostat Terminals & Help Identify Old Thermostat Types
Frequently Asked Questions (FAQs) about how to wire all types of room thermostats for controlling heating or cooling systems
Question: possible shorted thermostat wires
Our furnace thermostat failed after an adjacent water heater installation. I noticed that wires were pinched between the water heater and the gas pipe. After I loosened the safety straps and freed the wire, the thermostat works. (I reset the furnace). Were squished wires the cause of a temporary short and will the wires keep working if they are undisturbed? - Larry 8/2/11
Reply: check for shorted wires and for thermostat transformer damage as well as for blown fuse
Yes it's surely possible that your thermostat wires were shorted.
Now a short in those wires sometimes lets you off the hook with no trouble, since the wires and thermostat are basically an "on-off" switch that calls for heat (or no heat).
Question: 3-zone radiant heat: Honeywell RHT2300B thermostat can't talk to Azel i-Link SP83 zone controller
I am installing a 3 zone in-floor radiant system in my home. Every thing is going well so far, except I am running into an issue where Honeywell (my T-Stat manufacture) is telling me my RHT2300B t-stat cannot be used with my in-floor zone controller (Azel i-Link sp-83) when I call their tech support hotline.
They cannot tell me why or what their letter designations mean.
I am an industrial controls electrician of 23 years, who I must confess, does not have much experience with HVAC.
All I need is a simple t-stat that sends voltage (24vac) when heat is called for to my controller and opens back up (back to zero volts) when the set-point is achieved. I am having a hard time believing that this t-stat is not capable of doing that. Can any one help?
Reply: get the thermostat and controller product numbers correct and it's easy to find wiring instructions
I found two important details in researching your question:
1. you gave me an in-valid Honeywell thermostat number, but I was able to guess at the correct one and easily obtain wiring instructions from Honeywell. Correctly your thermostat is the Honeywell 5-2 Day Programmable Thermostat - RTH2300B1012
2. For the Honeywell thermostat model that you mention, correctly named, Honeywell's instructions point out that
Call back and ask for senior service tech or service manager. So if, when you spoke with the Honeywell service technician, and if your old thermostat included and used any two of the wire labels I quote above, then indeed, even if s/he couldn't explain why, indeed you cannot use this thermostat model for your application.
Remember that ultimately a thermostat is simply an on-off switch, or provides several on-off functions. But with multiple zone heat, you should expect to provide a thermostat for each individual zone - else they're not heating zones at all. In a traditional hot water heating system that does not use the Azel i-Link controller the first thermostat is wired directly to an aquastat that controls the first zone; the second two thermostats are wired to individual circulator relays (if individual circulator pumps are used) or to individual zone valves (if a single circulator runs the whole system).
Where zone valves are used, an end switch closes to turn on the circulator (a logical inclusive OR function with the other zone valves) when the zone valve opens to allow hot water to flow. That same principle is used when wiring the Azel i-Link Zone Switching Relay. And the company makes clear that their controller is compatible with standard thermostats:
First let's make sure you've identified your Honeywell thermostat model correctly.
Honeywell RHT2300B - the number you gave, doesn't look right, but you probably meant RTH-2300-B. Calling Honeywell with the right product number (Tel: 800-468-1502) might produce better results. Honeywell has done a great job making installation and operations manuals available for their equipment, but you've got to search with the right product number.
Searching the Honeywell site for the corrected thermostat number delivers a single product that's probably yours, the 5-2 Day Programmable Thermostat - RTH2300B1012. In fact this thermostat is widely sold including at Home Depot stores, and I've installed and used this very model myself to control an add-on hot water heating zone and Taco circulator in an older home. According to Honeywell, this thermostat is compatible with:
For your model you want Honeywell document 69-2326ES - found at http://customer.honeywell.com/Honeywell/getliterature.axd?LiteratureID=69-2326ES.pdf - owners operation manual or
document 69-2327ES - for installation materials - found at
Here are details from that manual's instructions for wiring the Honeywell RTH-2300-B 5-2 Day Programmable Thermostat beginning with the inspection of the existing wires:
How to Correctly Identify the existing thermostat wires
3. Identify and label each of the existing thermostat wires by using not the wire colors (someone could have made a wire color error and violated convention) but rather, identify each existing thermostat wire by noting the letter next to the old thermostat wiring block terminals where each wire was connected to a screw. Identify and label the wires that are connected as
If any wires are not attached to your old thermostat or are attached to a terminal marked C or C1, they will not be connected to your new thermostat. Wrap the bare metal end of each of these wires with electrical tape, so it cannot touch and short other wires.
Watch out: since here we are focusing on connections, I am leaving out some important procedure and safety details like removing and taping each wire end to avoid shorting, etc. so be sure to consult the installation manual in detail.
On your new thermostat, the Honeywell the 5-2 Day Programmable Thermostat - RTH2300B1012, connect the wires you labeled as follows:
If the wire labels from your old thermostat hookup don't match the above, Honeywell offers additional advice that we adapt in table form:
Azel i-Link Multi-Zone Switching Relay Wiring Details
In a radiant floor heating system that uses a controller, the additional thermostats must be wired to instruct that controller to send heat to their zones. Azel Technologies, a Canadian heating controller manufacturer, provides installation instructions and wiring diagrams for their controllers, including the Azel i-Link SP83-here is a copy.
Or contact Azel Technologies Inc., P.O. Box 53138 10 Royal Orchard Blvd. Thornhill, Ontario, Canada L3T 7R9 Ph: 905-223-5567 Fax: 905-223-3778 Email: email@example.com, Website: www.azeltec.com.
Question: Adding a third heating zone with its one zone valve
I am adding a third zone (basement) to my oil fired hot water baseboard heating and have it all planned as far as the plumbing is concerned. The zone will have it's own 24V zone valve so will require a wall mounted thermostat. I'm only looking for heat so I believe a 2 wire thermostat is sufficient. My other two zones have their own valves and thermostats and the wiring at the valves is all connected with wire nuts and doesn't make any sense to me.
If so, is there anything I need to be aware of, like possibly wiring it backwards and frying something ?
If you need me to provide more detail on the types of thermostat, aquastat and zone valve I am using I can. My zone valve has two red wires and two yellow wires. The instructions seem to indicate that I only use the yellow wires and the red wires were for auxiliary, whatever that is.
Will I think this will help clear things up a little for wiring a 3rd heating zone using a zone valve approach:
Dan, thanks for the information. So I effectively have two circuits to connect. One 24v circuit runs in series as follows - transformer, zone valve, thermostat, transformer. Please can you confirm this.
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
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: reader comments on shorting out thermostat wires & fusing requirements
Greetings, not sure if anyone saw this or not, but whoever S.R. is above in the description about shorting out wires seems to maybe have missed the fact that the "E" they refer to is actually a "3" amp fuse and not a 4 amp fuse. My HVAC installer of 30 years who recently installed 2 furnaces for us and is installing my heat pump agrees that it is a 3 amp fuse. We purchased an extra one just in case. Putting a 4 amp in there may not be good for the board as it may allow too much current for a relay or something and you may fry a piece on the board which is most likely not cheap. I hope this helps - D.M. - 3/12/12
Thanks D.M., We posted your important fuse size comment right in the article at Shorting Out Thermostat Wires - Leads to Loss of Heat.
Question: Wiring a non-programmable White Rodgers Thermostat
I have a 1f86 Non programmable White Rogers i matched up the R to Red W to White and O/B to Black But I Get nothing and unfortunately I stay in a condo with base floor panels so i cant really hear if the heat comes on or not. How do i wire this thermostat correctly - Anon 4/24/12
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
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.
Question/Comment: Helpful Pointers Regarding 24V Thermostat Wiring & Heat-Anticipators
- Helpful Pointers Regarding 24V T, 10/7/2012
Helpful, thanks so much for the detailed tips on wiring 24-volt room thermostats. I've inserted your remarks into the main article body above, beginning at 9 Thermostat Wiring Tips for 24V Thermostats. You are welcome to email me with contact information if you want to be identified or receive reader referrals. Also, sorry that the comment form software chopped off the end of your text point 9 -thanks for filling it in. We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique, questions, or content suggestions for our web articles. Working together and exchanging information makes us better informed than any individual can be working alone. - Ed.
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