How do RH/RC/R and Y/W relate for HVAC?

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I'll use this gif from my other answer, to try and describe how the system works. I'll focus only on the heating side, just to keep things simple.

When the thermostat is not calling for heat, the circuit is open and electricity cannot flow. Once the temperature in the room drops below the set point, the switch in the thermostat closes. When the switch is closed, electricity can flow through the control circuit.

Electricity flows from one side of the transformer, out to the thermostat on the R wire. It flows through the switch in the thermostat, and back out along the W wire. It then flows through the coil of the heating relay, and back to the other side of the transformer (C). When electricity flows through the relay coil, it moves the switch in the relay from the normally closed (NC) contact, to the normally open (NO) contact. When this happens, the furnace is signaled to turn on.

As you can see in the above animation, when the thermostat contact is...

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A thermostat basically just energizes the individual circuits (G (fan), W (heat), Y(cool) ) which in turn (via the furnace controller) powers a relay which provides line voltage to the actual units (like the fan or A/C. In the case of heat, it instructs your furnace to open the valve supplying gas).

R (or Rh and Rc) provides the 24VAC power. To turn on the fan, you'd connect R and G. To turn on the fan and AC, you'd connect R and Y (or R, G, and Y on systems where the thermostat controls the fan). To turn on the fan and heat (as you asked), you'd connect R and W together (or R, G, and W if you have say an electric furnace where the thermostat controls the fan).

If you make a mistake you could potentially fry your furnace controller.

As for finding out what is running - yes there should be 24VAC on the circuit if they are turned on. However note that this is just instructing the logic board in your furnace to turn on the other components; your furnaces...

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Since you haven't supplied much detail, I'll have to use nonspecific examples based on typical installation methods.

If you have a setup where you have a furnace, and an outdoor condensing unit. The wiring will be similar to this (Note: This is a basic overview, not actual wiring).

Basically, the R wire provides power to the thermostat. W, Y and G are energized depending on the state of the thermostat. For example. If the thermostat is set to the COOL mode, and the temperature in the room is above the set point. The Y and G terminals of the thermostat will be energized. This tells the furnace to turn on the blower at high speed, and the condensing unit to turn on. Alternatively, if the thermostat is set to the HEAT mode, and the temperature in the room is below the set point. The W terminal will be energized, which tells the furnace to turn on.

If you have a heat pump, the wiring will be similar to this.

Here the R wire again provides...

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R, Rh, and Rc are all the same, but not. In general terms, the R terminal is where you connect the signal voltage source. In low voltage controlled systems, there will be a step down transformer that provides the power to the control circuitry. One of the legs from the secondary of the transformer will be connected to the R terminal, which applies a voltage to the terminal.

When the thermostat wants to be warmed up or cooled down, it closes the appropriate switch, completing a circuit. This allows current to flow from "into" the R terminal and "out of" the appropriate signal terminal (W, Y, etc.) of the thermostat.

In some systems, both the heating and cooling systems use their own signal voltage. In these cases, the signal wire from the heating system will be connected to R or Rh, while the signal wire from the cooling system will be connected to Rc.

If only a single signal wire is used, the R terminal will be electrically connected (jumpered) to Rh and/or...

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I see this is a pretty common question, but the answer seems like it might be dependent on your thermostat and air handler, so I'm posting with my specific questions.

I'm upgrading from a White Rodgers thermostat to Honeywell RTH9580WF. The Honeywell requires a C wire. My old White Rodgers thermostat does not have a C connector. It has a wire from the air handler switchboard to the thermostats RC and is has a jumper to RH.

(air handler) - (Thermostat) W - W Y - Y G - G R - RC jumper to RH

I can see on the air handler that there is a COM 24v which I believe is C (COM = Common = C, right?)

There is a second 18/4 wire hooked into the air handler (C & Y). I'm not positive where that line runs, but it looks to follow the coolant line, so maybe to the condenser outside?

We only use this system for AC. I don't know if that impacts my situation at all, but I see lots of discussion on how things relate to heat, so I wanted to point it out.

I was reading...

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I am trying to replace my existing thermostat.

My old thermostat has:

Rh, W (coming from the heating unit).

Rc, Y, G (coming from the AC unit).

On the Venstar T5800, I can map as follows:

Rh+Rc -> R

W -> W

Y -> Y

G -> G

Now I need to add a common wire. HVAC tech tried to put a jumper between R and C. Venstar started up, the screen went on, but then the moment the AC turned on,the unit shut itself off. Is this a problem with the thermostat or with the way the HVAC guy set it up?

I bought an ELK 24V AC dedicated supply/transformer. Now I have two wires coming off it. It is running to my thermostat. How do I connect them? Does one wire from the supply/transformer go in to C and the other in to R, in addition to the Rc and Rh wires already in the R?

i.e. the following:

ACSupply1 -> C

Rh+Rc+ACSupply2 -> R

W -> W

Y -> Y

G -> G

Or would this not be recommended?

Any...

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When working with a thermostat the cover can be snapped off to expose the wiring.


Before doing any work on the thermostat and wiring take a picture off the wires and their connections, or write them down. This way you can always refer back to the original setup.

Even though there is a industry standard for color coding wires doesn’t mean the installers followed it.

How Does a Thermostat Work?
A thermostat is a switch that can automatically turn on a furnace when a set temperature is reached. If a thermostat is thought to be a problem then it can be bypassed with a jumper wire.

Thermostats use a 24 volt transformer to control a furnace. The transformer steps down 120 volts to the 24 volts the thermostat needs, and sends out the 24 volts on two wires. The two 24 volt wires go to the R terminal and C terminal inside the thermostat.

Furnace 24 Volt Transformer


R is the hot side while C is...

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W Y RH G

STEP 1 - Connect the W wire to the W terminal on the

thermostat. This connects to the heater control line.

STEP 2 - Connect the Y wire to the Y terminal on the

3M-22. This connects to the Cooler compressor.

STEP 3 - Connect the RH or R wire to the RH terminal on

the thermostat. This connects the Heater/Cooler Power.

STEP 4 - Connect the G wire to the G terminal on the

Thermostat. This connects to the Fan.

STEP 5 - Set Config jumpers per this diagram. If you

have Electric heat set 5, if you have Gas or Oil set 7.

Your HVAC system is now connected

to the 3M-22.

Please Go To Page 9

W Y RH RC G

STEP 1 - Connect the W wire to the W terminal on the

thermostat. This connects to the heater control line.

STEP 2 - Connect the Y wire to the Y terminal on the

3M-22. This connects to the Cooler compressor.

STEP 3 - Connect the RH wire to the RH terminal...

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This thermostat is compatible with most HVAC systems, including the following:

24VAC systems Note: requires both the 24R and 24C (common) wires

Standard gas/oil/electric heating systems

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o

Heat Pump systems:

o

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o

Do NOT use for line voltage controls (120/240VAC)

Stop! Before removing your existing thermostat be sure to label the wires with the wiring

labels included. Label the wires as they were connected to the old thermostat terminals

and record them below.

We strongly recommend you take a picture of your existing thermostat wiring

connections. This will enable you to reconnect your old thermostat if you encounter a

problem installing the CH-THSTAT-W and will assist in trouble shooting the installation, if

needed.

CAUTION: The typical wire colors noted below may be different from your installation.

Standard HVAC System...

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I replaced the standard thermostat with a programmable one, and I cannot get the air conditioning to work. I tried a few configurations.

Our system is a Trane TWX036C100A0 (outside) with an XL1200 heat pump, Aprilaire Model 110 compressor with model# TWV042B140A1 (inside). The wire colors are Black (assumed 2nd stage heat), Orange (assumed Cool Transformer), Green (assumed Blower Fan), Yellow (assumed Cool/Compressor), Tan (assumed Outdoor Anticipator Reset), Red (assumed Input Power), White (assumed Heat), Blue (assumed Common), Purple and Grey. The original thermostat had X2, O, G, Y, T, R, W, U, F, B.

Background: We tried installing a new Honeywell programmable thermostat, but the wire colors didn't match up with the available ports, and none of the configurations in the instructions matched our existing wiring system. We called Honeywell and they were bewildered. The guy put us on hold for a while, then came back with a configuration. It seemed to work for...

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