Indoor Thermostat
A building thermostat is a device to control the heating and air-conditioning systems in a home, office, or other indoor environment.
The majority of modern heating/cooling/heat pump thermostats operate on 24 volts A/C.
Contents
Common HVAC Thermostat Wiring
(R) Red - Voltage
Transformer provided DC voltage
(G) Green - Blower Fan
Many thermostats (G) and (Y) terminals are connected together at all times when the fan switch is in the "Auto" mode.
(Y) Yellow - Compressor
The Air Conditioner. When voltage supplied it activates cooling. On a Heat Pump system it can also activate heat via the heat pump.
(W) White - Heat
Activate heat. Heat including gas heat, electric heat, or auxiliary heat on a heat pump.
(B) Blue - Common (exceptions exist)
What is called 'common' and is typically not used on modern thermostats.
Needed on some electronic thermostats or if the system uses indicator lamps.
On some Rheem & Ruud systems it is not 'common'. May be Blue or Orange.
Hot Wire Testing
Shorting or Hot Jumping is risky and can in face damage your system. Certain brand and models work differently from what is commonly known. Use this as a general reference but do not proceed unless you have research your system brand and model.
5 Wire Red, Blue, White, Green, Yellow
- Red + Yellow - turns on the Air Conditioner
- Red + Green - turns on the blower fan
- Red + White - turns on heat
4 Wire 24V HVAC Thermostat Wire Diagram
W : White Wire : Heat
Y : Yellow or Black Wire : Cool
G : Green : Fan
R : Red : 24v
Jump W + R will turn on heater on a gas operated HVAC system. On the gas system the fan is triggered automatically at the heater when a certain temperature is reached.
Jump G + R to force the fan to turn on
Honeywell 6 Connector
Honeywell's round mechanical mercury thermostat "The Round" has 6 connectors. Only 4 are necessary for a typical HVAC system.
B : black
G : green : Fan
Y : yellow : Cool
W : white : Heat
R : red : 24v
O : orange
Modern 8 Connector Thermostat
A modern electronic thermostat may have as many as 8 connectors. To connect this to a 4 wire 24v system use the following:
W : white : heat
Y : yellow or black wire : cool
G : green : fan
B :
O :
RC : red wire : 24v - bridge to RH
RH : red wire : 24v - bridge to RC
C :
2 Wire and 3 Wire Systems
On a 2 wire installation: If using a mercury thermostat make sure it is level. 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.
On a 3 wire (series 20) installation: The Red wire coming to the thermostat from the heater or air conditioner connected to "R". The white wire connected to the "Y" terminal. The blue wire connected to the "W" terminal.
Heatpump
The reversing valve wire for cooling is typically orange (but is occasionally blue) and labelled O or R. This connects to the O terminal. Your thermostat may also have the reversing valve wire for heating, labelled B, which should be connected to the B terminal. If you don't have this wire, cap the terminal with a wire nut. The fan control relay wire is typically green and labelled F or G, and should be connected to the G terminal.
The second stage models cooling circuit wires are typically blue and labelled Y2. The hearing wires are usually pink and labelled W1, W2, or W-U. These must be connected to their terminals.
COMMON HEATPUMP BRANDS
These systems are more common and power the reversing valve in A/C mode. (O Terminal)
Orange is the OG reverse valve
To Jumpstart the A/C connect RED the voltage to GREEN, YELLOW, and ORANGE
- Green for the fan, Yellow for the Compressor, and Orange for the OC reversing valve.
To Jumpstart the Heat connect RED the voltage to GREEN and YELLOW
- The heat pump is operating for heat mode
IF IT IS A RUUD OR RHEEM SYSTEM
These systems power the reversing valve in heat mode (E Terminal)
To Jumpstart the A/C connect RED the voltage to GREEN, and YELLOW
- Green for the fan, Yellow for the Compressor, and Orange for the OC reversing valve.
To Jumpstart the Heat connect RED the voltage to GREEN, YELLOW and ORANGE
- The heat pump is operating for heat mode
Wiring Example for Thermostat for a single stage heat pump with aux backup emergency heat
- one stage heat
- one stage cool
- backup emergency heat
C = common = black wire Rc and R = cooling power, heating power = red wire O/B = Change over valve aka reverse valve = orange wire Y = compressor stage 1 = yellow wire G = fan relay = green wire AUX-E = Backup Heat/ Emergency Heat = white wire Additional: S1 = outdoor sensor = gray S1 = outdoor sensor = pink
Two Stage Gas
W – When you see a W terminal, it just means heat. Usually, you will only see W when the control only has one stage of heat.
W1 – Means first-stage heat.
W2 – Means second-stage heat.
R (usually the red wire) is the power supply (24v). G (usually green) controls the fan.
Heat Anticipator
Mechanical thermostats like the Honeywell "The Round" have an adjustable heat anticipator while digital thermostats do not have heat anticipators. The function of the heat anticipator is to fine-tune the point at which the thermostat turns off the furnace burners. Properly adjusted a heat anticipator also prevents the room from getting warmer than the thermostat set temperature, and it prevents the heat from shutting off before the room has reached the thermostat set temperature. Adjusting the anticipator up or down to higher or lower Amps numbers will lengthen or shorten the heat-on or cooling-on cycle.
The heat anticipator essentially consists of a small disc attached to the bimetallic coil of the thermostat. The disc has a thin wire on its surface and an adjustable arm that touches the wire. The position of the arm determines the electrical resistance of the wire, which in turn affects how hot the wire gets. As the wire heats up, it warms the bimetallic coil, which in turn shuts down the gas burners early, according to the heat anticipator's setting.
When the heat anticipator in your thermostat is out of adjustment, it can cause the furnace to "short-cycle" (turn on and off frequently) or to exceed or never reach the desired thermostat heat setting. Fortunately, this problem is often easy to correct by adjusting the heat anticipator or replacing an old thermostat.
Adjustment: If the furnace is cycling on and off too frequently, move the heat anticipator adjustment lever closer to the "LONGER" setting and if the furnace is exceeding or never reaching the desired set temperature, then move the adjustment lever away from the "LONGER".
- The highest (1.2A) position gives the longest heat-on cycle. (max value dependent on model)
- The 0.1A position gives the shortest heat-on cycle in thermostat heat
- Honeywell warns to never set the T87F series heat anticipator below 0.3A.
Honeywell Thermostat Heat Anticipator Settings: Honeywell specifies the following amp settings for their CT87 thermostats, based on the heating system type:
- 1.2 - Steam
- 0.8 - Hot Water heat
- 0.8 High Efficiency Hot Air
- 0.4 Standard Hot Air
- 0.3 Electric Heat
The anticipator was replaced by a thermistor in newer designs.
Electronic thermostats do not use a physical anticipator due to the microprocessor controlled temperature and delay in shut off. In a mechanical thermostat which has no microprocessor, clock, or timer the heat anticipator's purpose is to reduce the temperature swing caused by the system, and to increase overall efficiency. It does so by creating false heat, which increases the thermostat's rate of response.
Defective Anticipator: If the heat anticipator is malfunctioning, your furnace will short cycle and the room temperature will not reach the temperature displayed on the thermostat. Typical lifespan or the anticipator and mechanical thermostat is approximately ten years.
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