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Thermostat wiring reference chart - simplest case two-wire thermostat (C) Daniel Friedman How Wire a Thermostat
Thermostat Wiring Connection Tables
Hook-up Procedures for Heating, Heat Pump, or Air Conditioning Thermostats

  • THERMOSTAT WIRE CONNECTIONS - CONTENTS: Guide to Thermostats for Heating and Air Conditioning Systems. Wall mounting, leveling, and wiring requirements vary depending on the type of room thermostat and what it needs to control. How to select the proper type of replacement thermostat for air condtiioning or heating systems.
  • POST a QUESTION or READ FAQs about heating, air conditioning, and heat pump thermostat installation and wiring
  • REFERENCES
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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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Guide to Room Thermostat Installation, Location, & Wiring

Where NOT to locate the room thermostat  (C) Carson Dunlop AssociatesArticle Contents

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

Does the Room Thermostat Need to be Level on the Wall?

Yes for Older Mercury Bulb Switches

Thermostat wire connections (C) Daniel Friedman

It's important to mount mercury-bulb thermostats as level as possible since otherwise you're putting the thermostat out of accurate temperature calibration.

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

Thermostat wire connections (C) Daniel Friedman

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. Here is a copy of the Honeywell RTH2300/RTH221 Series Programmable Thermostat Owners Manual [PDF]

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles:
see BOILER CONTROLS & SWITCHES,

If your building uses warm air heat, see FURNACE CONTROLS & SWITCHES.

If your building uses steam heat see STEAM HEATING SYSTEMS.

Thermostat Replacement Types: What Kind of Thermostat Do I Have?

Traditional round Honeywell wall thermostat (C) Daniel Friedman

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.

Internal parts of a thermostat and how they work

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.

Nest thermostat

Above: A Nest Learning Thermostat. See NEST LEARNING THERMOSTATS - home, or see NEST THERMOSTAT INSTALLATION & WIRING for details about wiring Nest thermostats.

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?

Honeywell thermostat backing plate showing wiring connections

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.

Where do the Thermostat Wires Start and Where do they End-Up?

As we explain in more detail at THERMOSTAT WIRING DIAGRAMS,

Honeywell T87F Thermostat wiring diagram for 2-wire, spst control of heating only in a typical gas fired heating system - details from Honeywell Controls

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]

Low voltage transformer wires (C) Daniel Friedman

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.

Low voltage transformer wires (C) Daniel Friedman

3-Wire Thermostats: Where Do We Connect the Red, White, & Blue (if present) Wires to a Typical Three-Wire Wall Thermostat?

Honeywell thermostat backing plate showing wiring connections

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?

3M-22 Filtrete thermostat wiring terminal layout (C) Daniel Friedman

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:

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

These are the traditional wiring schematics, again, check the thermostat to see if this is the case with yours.

Instructions for Wiring of Line Voltage 120V Wall Thermostats

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.

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Continue reading at THERMOSTAT WIRE FUNCTIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see THERMOSTAT WIRING DIAGRAMS

Or see THERMOSTAT INSTALLATION STEPS for a step by step photo guide to installing a room thermostat.

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THERMOSTAT WIRE CONNECTIONS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to ARTICLE INDEX to HVAC THERMOSTATS

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