How Wire a Generic Room Thermostat

Standard Thermostat Wiring Connection Tables

Table of Common HVAC Thermostat Wiring Connections - Generic HVAC Thermostat Control Wiring Points

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.

Notes to the table above

1. The [Honeywell 5-2 Day Programmable Thermostat - RTH2300B1012] thermostat cannot be used if your old thermostat had and used any two of the following wires: R, RC, RH, 4 and V.[5]

How to Wire a Generic 2-Wire 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.

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.

Below you can see that the red R thermostat wire simply connects to the R terminal and the white W wire to the W terminal.

Complete details are

at THERMOSTAT WIRING HONEYWELL

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.

How to Wire Up a 3-Wire 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.

See details at Three-Wire Honeywell Wall T87-F type Thermostat wiring

How to replace 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 thermostat 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 thermostat W2, yellow connected to Y, green connected to G, black conned to E, Red to R, Blue to B, Brown to X.

Any help would be great. - Kurt 6/4/12

Reply:

Kurt, 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]

Trying to replace a Righttemp 6010 with a HoneywellRTH2410 and I don't think they are compatible.

The main problem seems to be what to do with the W2 (red) wire. The AC unit is not in operation and we do not need cooling. I was thinking if I connected that wire to the R terminal of the new unit (jumpered with Rc) that it would work as the remaining three wires seems to match up okay.

I guess this would mean my attempting to use the new unit on a multi stage system for which it is not intended. It worked fine before, only that we did have the Emergency Heating Mode function.

How to Hook Up a 6-Wire Thermostat

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:

• the red is power,
• the yellow is for cooling,
• the white is for heat,
• the green is for the fan,
• the orange is for the reversing valve, and
• the blue is usually always common.

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.

How to Wire 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.

Wiring 120V 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.

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.