Help c wire

Jamie Hall

Is the cooling circuit powered by the same transformer as the heating side?

diego168

> @Jamie Hall said:
> Is the cooling circuit powered by the same transformer as the heating side?

No they are different system. I can’t see any wiring going to the ac. From the boiler room.
But the t stat works fine if I unplug the c wire.

ratio

Based on what I read upthread, you have only an R (named RC), G, and Y from the cooling side of the system. That's the power in, fan out (G), and cooling out (compressor, Y). There's no common coming from the transformer that powers the cooling side of things. The Nest knows that the C that comes from the heat side doesn't originate from the cooling system transformer (there's no electrical path), & complains. With the C wire removed, the stat operates in power-stealing mode. (See this thread for some details on power stealing.)

Best, of course, is to get a C wire from the cooling transformer; but as you noticed that might be difficult. I would leave the C wire off, & see how it operates. It should be fine, although in the dead of winter is when I would expect any issues to pop up.

SuperTech

Better yet, ditch the lousy Nest in favor of a thermostat that is less problematic.
Yes, typically the common comes from the cooling system. Its unusual for it to be connected to a heating system and a separate cooling system and have the common come from the heating system. That's probably why the stupid thing is looking for 24 volts between RC and Common.

diego168

Yes the reason I wanted the c wire is because some review show that the nest may act up without a c wire.
There are wires not being used in the ac maybe I could use one of those as a common. I don’t know what to do now.

diego168

> @SuperTech said:
> Better yet, ditch the lousy Nest in favor of a thermostat that is less problematic.
> Yes, typically the common comes from the cooling system. Its unusual for it to be connected to a heating system and a separate cooling system and have the common come from the heating system. That's probably why the stupid thing is looking for 24 volts between RC and Common.

So which thermostat would you guys recommend instead of the nest learn- that would work with how I set up the wiring.

SuperTech

I like the Honeywell T5 and T6. I think the "learning" features of the Nest is over rated. The Honeywell thermostats are available with wifi capabilities and seem to be less troublesome.
The smartest thing operating a heating or cooling system should be the owner, not the thermostat.

Jamie Hall

If you do have extra wires in the wall, the best thing to do is going to be to take your common from the Nest back to the air conditioning transformer, rather than the heating transformer (I'm assuming that there is no jumper between Rc and Rh in the thermostat?). You need one, and only one, common, and if there is no Rc to Rh jumper, it has to be to the transformer associated with Rc. Less exotic and easier to operate thermostats can have that jumper. The Nest never seems to -- and I have no idea what is going on inside the thing.

I have spent some considerable time over the past several years looking at Nest wiring diagrams. I have yet to find one which is really useful; the actual operation and charging of the thing seems to be privileged information.

Your information that the Nest sometimes (like often) can misbehave in the winter if is there is no common is correct. The reason is that when there is no common, the Nest recharges itself from Rh and W. That's fine -- for forced air heat, where the heating cycle is rarely longer than a few minutes. It doesn't work well on hot water, and works exceedingly poorly on either steam or radiant, because the little battery runs out of juice and the thermostat dies. That's in addition to the problem that if the learning feature is allowed to run the thermostat up and down, it fights with the heating system and can be remarkably inefficient, uncomfortable -- and expensive to run.

The Nest is a good choice, if outrageously overpriced, for conventional hot air systems where the homeowner doesn't know or care about how the system operates. In my humble opinion, it is a miserable choice for anything else.

diego168

Yes I didn’t buy for the learning option. I just wanted to have control of heating/cooling remotely for when I’m not in the house. And google advertises the nest as not needing a C wire. But after doing further research I found that of the problems. If I would have done c wire from start I would have probably buy something else. I bought all this thermostats about a week ago so I could return them.

Just to clarify if I was to get honey well t5-6 then I could use my current c wire ?

ratio

Probably not. The way most thermostats are, internally, is separate heating & cooling parts, with a jumper (internal as on the Nest, or external as others) to connect the 'hot' (power in) terminals together when only one transformer is used, but the cooling side usually supplies the power to operate the thermostat itself.

This is an appendix from Ye Olden Days when it was common for AC to get grafted on to an old heat-only system. The new AC system needed to control the fan directly, but the old furnace controlled the fan itself, independently of the thermostat. If it even had a fan!

Jamie Hall

Should be no problem...

diego168

Before I give up on this nest. I went up to my air handler where the extra wires behind thermostat go to. I found the transformer and tested with my multimeter for low voltage. In This picture blue and red are 24v. So where should I wire my common? The blue wire goes through a series of connections and eventually connects to a red wire that connects to compressor.

Jamie Hall

My first guess would be blue -- which, as you will note, is solidly grounded (first picture). However, I wouldn't do anything at all without access to complete wiring diagrams. Playing with electricity without that in something which may or may not be wired to some standard or other is likely to give rise to baby dragon sparks and snapping noises and smoke.

ratio

Without committing myself to actually advising you to do something (because Br. Jamie is right, there's a lot of ways things could go wrong), using blue as the common is reasonably popular. And the compressor needs a common as well, it heading out there makes sense. Does the white wire that goes out to the compressor end up on the Y terminal in the stat? That would confirm the blue as a common.

One thing to remember is that 'electricity don't know no color' (that's the actual verbage I learned some several decades ago!), meaning that the color of the insulation is unrelated to the function of the wire. What it's connected to entirely determines what it's (supposed) to do, hence my somewhat non-committal answers.

diego168

> @Jamie Hall said:
> My first guess would be blue -- which, as you will note, is solidly grounded (first picture). However, I wouldn't do anything at all without access to complete wiring diagrams. Playing with electricity without that in something which may or may not be wired to some standard or other is likely to give rise to baby dragon sparks and snapping noises and smoke.

Lol. Yes agree. This is the diagram. I was looking at And I forgot to post it. What else should I be looking for in diagram. What i could see is that blue and red come out as 24v. Red has a fuse. Blue gets ground and goes to pcb- not sure what that is. Blues eventually connect to red wire that goes to ac compressor

diego168

> @ratio said:
> Without committing myself to actually advising you to do something (because Br. Jamie is right, there's a lot of ways things could go wrong), using blue as the common is reasonably popular. And the compressor needs a common as well, it heading out there makes sense. Does the white wire that goes out to the compressor end up on the Y terminal in the stat? That would confirm the blue as a common.
>
> One thing to remember is that 'electricity don't know no color' (that's the actual verbage I learned some several decades ago!), meaning that the color of the insulation is unrelated to the function of the wire. What it's connected to entirely determines what it's (supposed) to do, hence my somewhat non-committal answers.

Agree ratio. I’m only mentioning the color of the wires so that you guys can follow what I mean on my pictures. If I’m not absolutely sure I will not risk it. I do my research and ask until things are clear and I’m confident.
My following statement is base on this picture attached. the blue wire measure 24v by multimeter. Blue wire connects to a red wire. (Red wire is part of a bundle of 3 wires red white green. This bundle is coming from Ac compresor.
White wire connects to yellow that ends up behind thermostat.
I haven’t check where the in the ac compressor do the white and red wires connect. Does it matter ?

ratio

Ok, the red & white that go out to the compressor are to energize the compressor during a cooling call. We don't care about what they're connected to outside, we'll assume that it's right (since it worked). The blue wire at the air handler is the common that needs to go to the Nest thermostat. The other wires from the air handler are right (again, they worked before). If you have an extra conductor (blue is 1st preference, but any unused color will do), connect it to the blue at the air handler & the C terminal at the Nest. Cap off with a wire nut or electrical tape the common coming from the zone panel, we don't need it.

SuperTech

Ratio is 100 percent correct. That blue wire is definitely the common wire for the air handler, the great advice posted by ratio and Jamie should end your debacle with the nest

diego168

So should I connect it where the blue connect to the red wire see pic.
Or where it grounds

diego168

Checked continuity with the yellow wire and it was the correct wire. The. Plugged my common to the blue/red nut. It detects 24 at the coming but the rh doesn’t detect.
🤦🏻‍♂️

ratio

You're getting an error code from the Nest, same as before but the R_H?

Set your meter to AC volts (100 volt range if not autoranging) & measure from R_H to R_C. See if you get zeroish (not more than a few volts) or 50ish volts.

diego168

Ok ratio, do I do it with common connected or without the common wire Connected. Currently I unplugged the common cause it was getting hot. Wife was starting to menopause crankiness
For now I’ll check without the common.

diego168

8.8V

ratio

Huh. Wire getting hot = not so good.

Uh, the heat was coming on? It shouldn't do that unless the thermostat is calling. Did the Nest say "heat on", or did IIRC it turns reddish when calling for heat?

diego168

Sorry, when I said hot, I meant the temperature in the house. since i was working on the wiring the ac was off for a while and it’s summer and super humid here in ct and wife was complaining. Forget what I said i was trying to be funny: I type from my iPhone and it autocorrects and I don’t check what I write I apologize.

I took the nest of the base and measure the volts between Rh and rc and it was 8.8volts.

ratio

Ah, AC was off & SWMBO noticed. I get it.

Ok, so right now, if you hook up the blue (common) & the R_C from the air handler, & the R_H from the zone panel, you get an error from the Nest that no power is detected on the R_H. And when the common from the zone panel was tied in, you got an error about no power on the R_C. That about where we are?

I think the first thing I'd do is, with the common from the air handler connected, do a hard factory reset on the Nest & go thought it's setup & configuration again & see if it picks things up right.

diego168

Ok I’ll try that.

When I first connected the common wire coming from heating the error message was no voltage detected from c wire. But if I was to unplug The Rc the it would detect the common.

With the common of the cooling side it wouldn’t detect rc.

With the common wire coming of the cooling side do I still need an Rc?

SuperTech

Yes. RC is the 24 volt from the cooling system transformer and RH is the same thing for your heating system. You definitely need both.

hot_rod

Alan (California Radiant) Forbes said:

Better yet, get a zone valve control. The terminals are clearly marked, you don't need to bundle wires together and there are diagnostic lights that come on when a thermostat calls for heat and another light that comes on when the end switch closes which makes for easy diagnostics when the thermostat or zone valve malfunction. Taco and Caleffi are good choices.

Bingo!

diego168

Ok so I resented the Nest and when I plugged it wouldn’t prompt the installation. So removed c wire and back to normal. Decided to look at the wiring make a diagram to see if maybe you guy can see a potential cause. My common wire convection is after the pcb I was wondering if that could make a conflict and I should connect to the 24v before it goes Into the small board(pcb).

ratio

The drawling you posted is correct. It should be working.

It might be time for a call to Nest tech support. I'm at a loss as to why it's not working.

diego168

I contacted nest they said to leave out the common that it should charge through the other wires. So I time to change it
One final thing what transformer do you all think I should switch 50 or 75va

ratio

Either one should do. I'd go with whichever configuration was more suitable (leads/terminals, size, mounting, etc).

diego168

Ok ratio thank you and everyone for their help. I learn a lot through this.
Yes I’m going to buy the honey well that matches the same one I have. Just 75av.
By upgrading to 75av does that mean I could over power the circuit. Meaning does the gauge of wires need to change like if you were go from
An 50amp vs 100amp circuit.
Or no because it’s always just giving 75av

Jamie Hall

No need to change the wire gauge -- although a 75 VA transformer can put out a surprising amount of current (around 3 amps) to a dead short, it has fuses -- and your existing wires are adequately protected by them.

EdTheHeaterMan

You have 2 transformers with 2 different common wires on the zone thermostat that does cooling also. When you use RC and RH the common wire from the cooling circuit is the one that the thermostat needs for the common.

I just got you confused with a different post ... look at it for more details, especially my solution on June 9 with the original poster's diagrams modified 2 ways

https://forum.heatinghelp.com/discussion/180114/cannot-power-up-t5-stat#latest

But REMEMBER Rule #1 for your case here is RH to R on the transformer for the zone valves

EdTheHeaterMan

Sorry if this was covered already. is was reading the last post on page 1 of this string

diego168

> @EdTheHeaterMan said:
> Sorry if this was covered already. is was reading the last post on page 1 of this string

Hi Ed. Yeah I run a common wire from my air handler as per my diagram on page 2. But it didn’t work. If I connected the common nest would not detect rh, why idk they are separate systems. So as of now I kept it without the c wire. After calling nest they said I didn’t need one. Will see in the winter. Idk I wish I could figure it For pease of mind

diego168

Just the actual diagram maybe someone can notice if there is something wrong in where I tab into 24v

diego168

So I ended up trying to wire the common again to the location on the ac air handler as before. And this time it worked and turned on but after a few seconds thermostat gave me error 103e. It says over current from yellow wire. Anybody know a possible cause ?