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AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
CIRCULATOR PUMPS & RELAYS
CONVECTOR HEATERS - HYDRONIC COILS
COOL OFF HEAT, Thermostat Switch
DEFINITION of HEATING & COOLING TERMS
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
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
ZONE VALVES, HEATING
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.
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.
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Where to Locate & Mount a Room Thermostat & what spots to avoid
Locate the room thermostat at about chest height on an interior wall, in a location where the thermostat won't be affected by drafts or other unusual temperature conditions.
[Click to enlarge any image]
Good thermostat locations are
As Carson Dunlop Associates' sketch (left) explains, there are a number of places where you should not locate the room thermostat.
Bad Room Thermostat Locations
Don't locate the room thermostat in these locations:
Room Installation Thermostat Installation Tips
Yes for Older Mercury Bulb Switches
That's because the coiled bimetallic spring has to move that mercury bulb to a tipped position to turn the heating or air conditioning system on or off in response to room temperature.
When we found a thermostat that did not heat a room accurately to the called-for temperature, we'd take a look to see if it was installed out of level before looking further.
Because the "set" range on these thermostats may have had a lower-end of 55 deg .F., when we wanted the thermostat to maintain a building at a temperature lower than the minimum that the thermostat dial provided, we just tilted the whole thermostat backing plate on the wall in the proper direction to shift the operating range of the switch.
No for Newer Solid State, or Digital Room Thermostats
Newer thermostats that rely on other sensor and switch designs do not have this sensitivity to being out of level and some (such as the 3M-22 thermostat) note in their installation instructions that the thermostat does not have to be level.
Our photo (above left) illustrates the red (R) and white (W) wire connections in a simple two wire heating control thermostat hookup using the Honeywell RTH2300/RTH221 series programmable wall thermostat.
Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles:
If your building uses warm air heat, see FURNACE CONTROLS & SWITCHES.
If your building uses steam heat see STEAM HEATING SYSTEMS.
A quick scan of heating and cooling thermostats in the catalogs of HVAC suppliers such as Grainger can quickly become overwhelming as there are literally hundreds of thermostat models and quite a few different types and applications. If you are replacing a heating or cooling room thermostat or "wall thermostat" a shortcut is to examine what is already installed.
At THERMOSTATS, HEATING / COOLING we include photographs of just about every type of heating and cooling thermostat. Here we describe further details that can help you choose a replacement thermostat that will accomodate the wires used to control you rair conditioning, heat pump, or heating system.
Pay particular attention to the following to help sort out what you've got or what type of thermostat you need:
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..
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,
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.
In thermostats that also control a cooling system, a second set of contacts will make or close on temperature rise. This second set of contacts 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]
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?
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.
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.
For clarity & speed we have moved wiring instructions for individual types or brand of heating or cooling thermostats to the individual articles listed below
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.
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.
This article is part of THERMOSTATS our article series Guide to Finding, Using, and Adjusting Thermostats for Heating & Air Conditioning Furnaces & Boilers, Heat Pumps or Electric Furnaces or Boilers. This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.
Continue reading at THERMOSTAT WIRE FUNCTIONS or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
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: 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.
Reader comment: [delete] (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.
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