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Guide to Installing & Wiring Heating or Air Conditioning Thermostats
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• Guide to Thermostats for Heating and Air Conditioning Systems
• How Do I Install & Wire Up a Room Thermostat like the Honeywell Round Wall Thermostat?

This article explains the basics of wiring connections at the thermostat for heating or air conditioning systems. This article is part of our Guide to Finding, Using, and Adjusting Thermostats for Heating & Air Conditioning Furnaces & Boilers, Heat Pumps or Electric Furnaces or Boilers.

This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards.

Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks. We continue to add to and update this text as new details are provided.

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How Do I Install & Wire Up a Room Thermostat like the Honeywell Round Wall Thermostat?

Our photo shows the backing plate that is mounted first when installing a round Honeywell 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.

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.

Newer thermostats that rely on other sensor and switch designs may not have this sensitivity to being out of level.

Where Do We Connect the Red, White, & Blue (if present) Wires to a Wall Thermostat?

Wiring connections for a room thermostat are pretty simple. So what are the proper wiring connections for a room thermostat?

This little photo is included so that you can see that on many thermostats the wiring instructions are embossed right on the thermostat body. Look for them on your thermostat as well as reading them in the instructions that came with the thermostat. (You kept those, right?)

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.

The white wire from the heater or air conditioner control is mounted to the "W" terminal on the thermostat mounting plate.

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 thermostats it may be necessary to plug in a connector between the thermostat and its mounting plate.

Wiring of Line Voltage 120V Wall Thermostats

See this 1949 Honeywell Control Handbook for an explanation of older line voltage (120-Volt) and 24-Volt thermostats and other controls such as the T40 series 4-wire 120V T44 Thermostat.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

THERMOSTATS
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat

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