Mystery Thermostat Wiring

jmob77

Hello All,
I have a steam system for which I recently purchased a basic Honeywell programmable thermostat to replace the old Honeywell with the mercury switch and old analog program clock wheel. When I removed the old thermostat, the wall plate showed four wires connected: C, C, R, W. I decided not to do the job at that moment as I could not square the wiring scheme with that of the new unit, which says that you should not connect the C wire. Does anyone know what it means to have two C wires and how they should be dealt with? I did not get a good look at the color of the wires themselves, though I'm not sure how useful that information even is.

Zman

It would be helpful to know what the other end of the wires are attached to. Can you post a picture?

jmob77

I'll do so ASAP. The wires go to the transformer before anything else, correct?

Paul S_3

put a wire nut on the "C" wires....just connect the "R&W" wires to new thermostat.....the extra wires were there for the mechanical clock. Make sure new thermostat is setup for a steam system...you can find the directions to change the settings in the installation manual.

Big Ed_4

Paul is correct , C&C powers the old mechanical clock , I would disconnect the two wire at at the power source which is an step down transformer if voltage is present between the two wires .

jmob77

Thank you all. Did this on Saturday and everything is working fine.

jmob77

By the way, Big Ed: is it possible I have two step-down transformers? There is one with two coiled wires running out of it and there appears to be another connected to a conduit. Would one be for the C wires and the other for the R and W?

Paul S_3

There should be just one transformer for your steam sytem...

ChrisJ

They could've used one for the heat and one for the CC.
The other could also be for a door bell.

Impossible to say from here especially without pictures IMO.

bob_46

they made two styles of analog chronotherms one had a dedicated transformer for the clock. if I can get down to my stuff in the basement I can give you the model no. and voltage and VA.

stigmax

i think i have the same chronotherm with the clock and two mercury switches inside. also with C C R W wires

I am also trying to install a new digital honeywell thermostat that needs a common. can i somehow reuse one of those C wires to power my new thermostat?

thx

hvacfreak2

stigmax said:

i think i have the same chronotherm with the clock and two mercury switches inside. also with C C R W wires



I am also trying to install a new digital honeywell thermostat that needs a common. can i somehow reuse one of those C wires to power my new thermostat?



thx

Yes , one of those wires goes to the common side of the transformer.

stigmax

thx hvacfreak2, im guessing ill know which one is right if my unit doesn't power on? will plugging in the wrong C wire do anything?

thx

hvacfreak2

As long as there is only 1 transformer it will either power up or nothing will happen ( 1 is 24v hot and the other 24v C ).

It would be wise to go down and see where the thermostat wire conductors land , no sense in finding out the hard way that someone hid another transformer up in a joist space to power the old clock.

stigmax

tried to follow the wires and it looks like the wire color/gauge is different at the furnace. will need to investigate further. if there are two transformers, will plugging the wrong c wire be bad?

hvacfreak2

It's a crap shoot , if the transformer primaries happen to be powered from the same phase than it would be fine. If not than you will end up with a different voltage to the common of the other transformer ( up to 48 or so assuming unground secondary commons ).

stigmax

cant seem to get my smart thermostat working. got my R and W wires hooked up. i tried both the C wires from the old clock and neither of them are getting my new honeywell zwave thermostat powered up. any ideas?

hvacfreak2 said:

It's a crap shoot , if the transformer primaries happen to be powered from the same phase than it would be fine. If not than you will end up with a different voltage to the common of the other transformer ( up to 48 or so assuming unground secondary commons ).

delta T

That's a thermopile powered gas valve. The two wires to the thermostat are independant of the transformer. They are effectively just switching the millivolt current supplied by the thermopile in the pilot. I think that your transformer was just to power the clock. If the new thermostat requires a C wire, you may not be able to use it with your set up, unless you install other controls, possibly even replacing the gas valve with a 24v version. A thermostat that requires batteries may be a better fit.

What features in the Zwave are you looking for?

stigmax

thanks delta T, i do have a transformer that's to the right of that picture.

bummer, was looking to connect the thermostat to my zwave hub. i just wanted to control it via the honeywell total connect mobile app. didn't have any other reason other then one app to control everything in the house.

what other controls could i possible do to get it working otherwise ill just look for a battery option, replacing the gas valve sounds like alot of knowhow and work.

thanks

delta T

And you only have one transformer correct?

stigmax

i only see one transformer at the furnace but i have no clue what the last owner did. dont have a picture of the transformer and what it's connected to but can take that when im home tonight.

thanks again

delta T

The only way I can think to do it would be to find a relay with a coil voltage low enough to run off the thermopile, but I doubt that exists. However you look at it, you are adding controls.

Most thermostats with any kind of wireless communication require a power source other that batteries in my experience. If you replace that gas valve with a standard 24v gas valve, you will not have any problems wiring that.

Wish I had better news....

bob_46

That is not a thermopile gas valve. Look at the label , it's a plain old 24 volt valve.

delta T

Nice catch bob, got distracted by the wiring connections. Looks like a TP valve, Stigmax can you post a pic that shows the whole front of your boiler, transformer, gas valve and all? We should be able figure out why you cant wire this up. If you only have one transformer, then we are missing a puzzle piece.

delta T

And I can just make out thermocouple coming down behind the wires. *smack on head*

stigmax

Here is a pic of my boiler front and the side.

delta T

You will have to run a wire from the c terminal on the transformer up to your thermostat. My suspicion is that there is another transformer somewhere that has been removed or is hidden from view that used to run the clock. You effectively have to supply power to the thermostat from the R terminal on the transformer, to the R terminal on the thermostat, then back to the transformer on the C terminal. In conjunction with that, there should be another circuit that comes off the W terminal on the thermostat, through the burner circuit (gas valve, aquastat, limits, LWCO if you have one, pump relay) and then back to the C terminal on the transformer. The burner circuit should already be there and you should not need to touch it. I can draw it up later if you want, or someone else may beat me to it.

bob_46

Looks like the pressure-trol and low water cutoff are line voltage, so if the boiler cycled on pressure you would lose power to the stat. You can do it with what you have but you will have to rewire the controls on the boiler.

delta T

Wow I need to look at things before I post, or not post in a hurry on my lunch break while I'm thinking of all the other things...hate when I give bad advice, seems to my MO on this post.

*double smack on head*

bob is totally right OP, you can do it, but it will take some re-wiring. If you are not comfortable with doing so, you need a pro out there to help.

Let us know we may able to find someone near you if you need.

Thanks for having my back bob!

stigmax

looks like i found something else in the wall when i was trying to follow the line back to the boiler and the color wire matches whats at the Stat. not sure if this helps/hurts me or doesn't matter.

in the brooklyn, ny area, if anyone knows a pro

stigmax

the old Stat wiring

bob_46

OK so you had two transformers, one for the boiler one for the clock. The one in the pic. with the red and black wires is an at75 mh clock trans. You can reuse on of those clock wires so you don't have to pull a new wire up the wall but you will still have to rewire the boiler.

stigmax

Hi Bob. alot of work to rewire the boiler?

i did try to hook up the new thermostat with the red or black wire but the thermostat wouldn't power on, is that because my boiler also needs rewiring?

thanks

bob_46

With all due respect you are in over your head. You will be lucky if you didn't damage your new stat. I would think a good serviceman could rewire your boiler in an hour.

stigmax

thanks Bob. though i would give it a try if not too hard. will call someone to take a look.

also thanks delta T.

delta T

Check the find a contractor section here, there should be plenty of good companies in Brooklyn.