The last word about c-wire thermostats and wiring diagrams.

JohnNY

Not that I actually believe this wiring diagram will eliminate all questions about the topic but this should make it fairly simple to do for the next guy. Or girl.
(photo redacted due to a labeling error)

Jamie Hall

It really is just that simple. Thank you, @JohnNY .

EBEBRATT-Ed

I have posted this one several times in the past couple of years. The only difference is a separate transformer. Solves all the issues for about $40 extra material

PeteA

I know its not the prettiest but this is how I set up a 24v transformer to give me the C wire for the other boiler I installed last year. I'll probably end up doing the same thing on the boiler I just did because I want to install the wifi thermostat here also.

STEVEusaPA

I say @Robert O'Brien 's AC Ready or Kool Kit solution is the easiest/simplest solution and the only one I use.

STEVEusaPA

PeteA said:

I know its not the prettiest but this is how I set up a 24v transformer to give me the C wire for the other boiler I installed last year. I'll probably end up doing the same thing on the boiler I just did because I want to install the wifi thermostat here also.

Dude, that gas pipe!?

HomerJSmith

Ya, John, but you still have to use 3 wires to the thermostat, I want to get it down to 1 wire. Work on that. I've been working on that for years, but I think I'm close. After all, a wire saved is a wire earned. Benny told me that.

JohnNY

HomerJSmith said:

Ya, John, but you still have to use 3 wires to the thermostat, I want to get it down to 1 wire. Work on that. I've been working on that for years, but I think I'm close. After all, a wire saved is a wire earned. Benny told me that.

1 wire is going to be tough but I think it's worth shooting for. I'll get back to you shortly.

PeteA

@STEVEusaPA I hear ya, not my best moment but it was October 1st and had to have the heat on and ready for my tenants so my plan is to go back and extend the 3/4 along the ceiling and then drop the flex line down from above so it lines up better.

Teemok

I'm confident that water heater is powering the furnace. Spill the beans! How is it done? One wire? Bro?

EdTheHeaterMan

@JohnNY, I had to LOL your discussion post. I believe there are about 50 of those same comments out there. I doubt if yours will be the last. But you can always Dream, Can't you.
Tommy Dorsey recorded a song with a similar quote in 1938

And I remember when I was in my teens working on cars, My neighbor thought the Germans figured out the whole 1 wire thing. Every light bulb on that VW he owned, only had one wire going to it. All the way back to the late 1950s when they started building the BUG. Boy those Germans were smart!

Teemok

Understanding the C wire might be as elusive as understanding some female anatomy. Everyone has a theory and they also think yours is way off base.

JohnNY

Teemok said:

Understanding the C wire might be as elusive as understanding some female anatomy. Everyone has a theory and they also think yours is way off base.

Of course you know this but here's how it makes sense to me.

A couple decades ago, the electrician we used to hire to wire our boilers gave me a piece of electrical theory that I've never forgotten: "Everything we deal with is either a power source, a switch, or a load." He went on to explain that we start at the source and end up at the load. In our case, the load was most often a gas valve or a pump. Everything in between was a switch.
The C-wire converts the thermostat from a simple switch to a combination switch and load. It both consumes power (charging, display, integral logic, etc) and allows or interrupts power through it to the burner circuit, terminating at the gas valve. Now, the same can be said of a probe-type low water cut off, and also a motorized vent damper or zone valve. These are combination loads and switches. Simple!

A switch just needs a power wire to one side of it and the switching action opens or closes the circuit to permit or interrupt the continuity of that path to the load.
A load requires a path back to neutral or ground to allow for a constant flow of power. That's the third *C wire.
If someone here can say this clearly in fewer words, please be my guest. My students will thank you.

EricPeterson

I find the comment relating to female anatomy unnecessary and offensive.
Maybe it's just me.

Eric Peterson

Teemok

Rumor has it, one nest t-stat that is charging draws max 25 milliamps or 0.6va @ 24v x4 for some CYA is 2.4va. Will confirm via measurement at some point. Many people have a single zone with the t-stat acting as a switch only and they need 0.6-2.4va. from the existing 24v system. Different from needing a C wire for 3-12 charging thermostats or larger loads. Less words...hmmm. A picture.







@EricPeterson You are right. Unnecessary. Not the first time a joke hit that button. Years of the trades have made me insensitive I suppose. You don't want to hear the stuff that's said in passing out here. So uncouth. I'm not sure what the offensive part was. The reference to a human body part or to the general ignorance about it? Anyway, I hope the offense doesn't last.

ChrisJ

JohnNY said:

Teemok said:

Understanding the C wire might be as elusive as understanding some female anatomy. Everyone has a theory and they also think yours is way off base.

Of course you know this but here's how it makes sense to me.

A couple decades ago, the electrician we used to hire to wire our boilers gave me a piece of electrical theory that I've never forgotten: "Everything we deal with is either a power source, a switch, or a load." He went on to explain that we start at the source and end up at the load. In our case, the load was most often a gas valve or a pump. Everything in between was a switch.
The C-wire converts the thermostat from a simple switch to a combination switch and load. It both consumes power (charging, display, integral logic, etc) and allows or interrupts power through it to the burner circuit, terminating at the gas valve. Now, the same can be said of a probe-type low water cut off, and also a motorized vent damper or zone valve. These are combination loads and switches. Simple!

A switch just needs a power wire to one side of it and the switching action opens or closes the circuit to permit or interrupt the continuity of that path to the load.
A load requires a path back to neutral or ground to allow for a constant flow of power. That's the third *C wire.
If someone here can say this clearly in fewer words, please be my guest. My students will thank you.

The only thing I'd like to add is, if we started calling it the Common wire instead of "C Wire" it might make it easier to more people to understand.

ratio

ChrisJ said:

The only thing I'd like to add is, if we started calling it the Common wire instead of "C Wire" it might make it easier to more people to understand.

Hmmm. I'm not so sure; at least, I have to distinguish between a common wire & a common terminal of a two pole switch often. I've taken to calling it a C wire & com terminal.

But source, switch, & load is pretty helpful! Thanks, @JohnNY!

mattmia2

Common is confusing too. In this case common is the unswitched side of the power source that is connected directly to the load but if you just said common to me with no knowledge of hvac I would think of it as the switched side of the power source.

ChrisJ

ratio said:

ChrisJ said:

The only thing I'd like to add is, if we started calling it the Common wire instead of "C Wire" it might make it easier to more people to understand.

Hmmm. I'm not so sure; at least, I have to distinguish between a common wire & a common terminal of a two pole switch often. I've taken to calling it a C wire & com terminal.

But source, switch, & load is pretty helpful! Thanks, @JohnNY!

Whatever works for you.
You can also call it the neutral. Regardless, that's what it is.

ChrisJ

mattmia2 said:

Common is confusing too. In this case common is the unswitched side of the power source that is connected directly to the load but if you just said common to me with no knowledge of hvac I would think of it as the switched side of the power source.

When you hear "common" in electrical, you think of a switched wire?
I think of neutral.

Teemok

Some Trane furnaces call it B. Sometimes labeled as B or C or B/C, just to make it more fun. Then there's W/B and Rc. Keeps people guessing. See? Taco has a 24v and Com terminal on some boards. Theory and knowing how to test for it is best. Not so easy to teach. I've seen the proverbial light bulb go on with a few I've worked with. They are suddenly much more valuable. Always fun when they C the light.

mattmia2

B is for blue which it usually the 5th wire in a t-stat cable. Rh and Rc are red for heating and cooling, the non-c end of the transformer.

I think of common as the supply(Vcc B+ + etc.), the ungrounded(whether that is a chassis ground or an earth ground) end of a power supply. I think of the end that everything sinks to as ground.

ChrisJ

mattmia2 said:

B is for blue which it usually the 5th wire in a t-stat cable. Rh and Rc are red for heating and cooling, the non-c end of the transformer.

I think of common as the supply(Vcc B+ + etc.), the ungrounded(whether that is a chassis ground or an earth ground) end of a power supply. I think of the end that everything sinks to as ground.

Common in this case, (C Wire) is literally the ground, neutral etc.
The R wire is the unswitched "hot", so to speak.

All thermostats for decades have had an unswitched "R" wire. They lacked the other side of the circuit, the common.


My "C" wire literally goes to the grounded jacket of the boiler which is where the other side of the transformer goes to as well.

mattmia2

ChrisJ said:

When you hear "common" in electrical, you think of a switched wire?

I think of the wire before the switch that everything connects to.

EBEBRATT-Ed

Common is confusing.

How about a 3 way switch? Same as single pole double throw. One wire on the switch is always referred to as "the common" even though it is a hot wire.

mattmia2

EBEBRATT-Ed said:

How about a 3 way switch? Same as single pole double throw. One wire on the switch is always referred to as "the common" even though it is a hot wire.

It took me a long time to find a diagram of a pair of 3 way switches where they showed the actual switches instead of a black box that 3 wires confusingly attached to.

EdTheHeaterMan

ChrisJ said:

The only thing I'd like to add is, if we started calling it the Common wire instead of "C Wire" it might make it easier to more people to understand.

I'm not sure that would work Chris. I myself happen to have placed a very uncommon type of wire on the common terminal of a rare control that in not commonly used in an unusual place when we start talking about this on an regular basis, when in unfamiliar territory.

It is not uncommon to be irregular when approaching this subject. And talking about the opposite of common, and the whole Irregular thing, I would do the whole exlax® joke but I don't want to offend anyone @EricPeterson. Some of us have grown out of the poop joke stage. Some not. And @pecmsg has a lot to say about this whole Nest thing. Don’t let him fool you, he is actually a secret admirer

Jamie Hall

And then you have the whole colour thing. Electricity, I have sometimes said, is colourblind. Just becasue one colour is normally used for the hot feed to a thermostat -- red -- for instance, and white for the switched lead, doesn't mean that someone didn't decide to switch them (since it doesn't matter to a battery or older conventional themostat.

Never assume that a certain wire colour has a certain meaning or function! Granted, it should, but... does it? As someone said a few years back -- "trust, but verify".

I loke the term "common return". It's clear. It's also never switched.

EBEBRATT-Ed

@mattmia usually the three-way switches sold now have 1 black (or darker) colored screw which is the common. but I always check them with an ohmmeter before hooking them up.

mattmia2

EBEBRATT-Ed said:

@mattmia usually the three-way switches sold now have 1 black (or darker) colored screw which is the common. but I always check them with an ohmmeter before hooking them up.

that it is a spdt switch with the travelers connected to the poles. most diagrams don't show that.

Alan (California Radiant) Forbes

Can you define travelers and poles for me?

mattmia2

er, single pole double throw so one pole with 2 throws. i'll draw a sketch later if i think about it.

Jamie Hall

SPST -- One wire in, one out. Handle one way on, one way off
DPST -- Two wires in, two out. Handle one way both on, Handle other way, both off
SPDT -- One wire in, two out. Handle one way input goes to one output, other way input goes to other output. May have centre off.
DPDT -- two wires in, four out. Handle one way both inputs go to one pair of outputs, other way both inputs go to other pair of outputs. May have centre off.

mattmia2

The throw is the number of positions the switch has that connect through the switch.
Th poles are the number of switches ganged together on a common lever/shaft. For example a double pole breaker switches 2 line connection simultaneously.

This is a 3 way switch:


A 4 way switch sits on the travelers and crosses or uncrosses the travelers.

If the source and load are at the same end a hot can go to the far switch instead of a neutral to the load.

STEVEusaPA

Yeah I was taught 'travelers' also.

EdTheHeaterMan

Alan (California Radiant) Forbes said:

Can you define travelers and poles for me?

A traveler is a person that is taking a trip away from home. They may take a short trip of an hour or so, but those folks are usually called commuters. Traveler is usually reserved for those that are away for weeks at a time.

As far as Poles are concerned, those folks are from a country in Eastern Europe that is just west of Ukraine and Belarus and east of Germany and just north of the Czech Republic and Slovakia. and just south of Lithuania and the Baltic Sea. Many Poles are Travelers but not all Travelers are Poles.

I hope that helps you get a better understanding of that situation.

I asked Matt from out graphics department to draw this illustration for you Thanks @mattmia2

Teemok

HAHA! Ed you did it. I wrote it out and erased it. Hats off. Travelers are another name for Gypsy's

EBEBRATT-Ed

@Alan (California Radiant) Forbes

Looking at @mattmia2 diagram usually a 3-wire cable (black, red, white and ground) are run between the three-way switches. Why they call them 3-way switches is another bad thing because there is nothing 3 way about them. They are just SPDT switches.

The two hot wires between the switches (normally black & red) only one of them is hot at any given time. Why they are called "travelers" is another mystery.

EdTheHeaterMan

EBEBRATT-Ed said:

@Alan (California Radiant) Forbes

Looking at @mattmia2 diagram usually a 3-wire cable (black, red, white and ground) are run between the three-way switches. Why they call them 3-way switches is another bad thing because there is nothing 3 way about them. They are just SPDT switches.

The two hot wires between the switches (normally black & red) only one of them is hot at any given time. Why they are called "travelers" is another mystery.

They are called what someone named them over a hundred years ago because that guy needed to explain it to someone else... Just ask @ethicalpaul about the Equalizer that does not equalize anything. There is even a video about it.

mattmia2

i believe the identified screw on the 3 way switch is called common

EdTheHeaterMan

mattmia2 said:

i believe the identified screw on the 3 way switch is called common

Is that the same common as the one on the Nest Thermostat?
Is no one going to let @JohnNY have the last word on this topic?
Source, switch, load, conductor, are we all just working on the railroad all the live long day? Take that locomotive down to the switch and load up the boxcars on track 9. and don't forget to tell the conductor where the source of that load came from.
Can't you hear the whistle blowing, Dina blow your horn!
Tooo many martini's... Time to go to bed!