Need a "C" wire from boiler

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  #1  
Old 12-31-13, 01:27 PM
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Need a "C" wire from boiler

I have a Burnham oil-fired boiler, baseboard hot water, with 3 zones, single circulator pump plus indirect-fired water heater. I am replacing the original mercury thermostats with 2gig z-wave thermostats. The old thermostats only have 2 wires connected, although they have a third wire that is not used. Although the thermostats can be battery-powered, I would like to power them to get full use of the z-wave functions.

I will attach pictures of the wiring. There is an external transformer mounted on the boiler with wires running into the bundle of wires above the zone valves:


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Is it possible to tap off this transformer and tie it to the unused thermostat wires, then connect that to "C" on the new thermostats or would it be better to install a new transformer just to use for the "C" connections? There is a shutoff switch above the boiler and would be easy to install a plug powered off that switch and plug in a transformer there.


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Thanks for any help.
 

Last edited by NJT; 12-31-13 at 02:34 PM.
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Old 12-31-13, 01:45 PM
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You should be able to get the "C" connection from the transformer.

The first thing you need to do though is determine which 24VAC is "HOT" and which is "COMMON" and the way to do this is going to be by examining the wiring.

I just did a drawing in another thread for the "F" models of your valve which have the terminal strip... and the 'basics' are the same, but so there's no mistake, let me see if I can come up with a drawing for the "E" model with the yellow and red wires which you have... gimmee a little time...
 
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Old 12-31-13, 02:43 PM
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You should have a WHITE wire on each of the W terminals of the existing t'stats, and that should go to one each of the YELLOW wires from the zone valves in a wire nut as shown.

There should be a RED wire on each of the R terminals of the existing t'stats, and those should all come together to the RED wire from the TRANSFORMER.

The OTHER YELLOW from each valve should all come together and go to the WHITE wire from the transformer.

If you valves are wired as above (VERIFY EXISTING WIRING MATCHES!) then the C wires from the thermostats all come together with the OTHER YELLOW from the valves and the WHITE from the transformer.

 
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Old 12-31-13, 04:38 PM
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Look closely at your transformer...the terminals are clearly marked "R" where the red wires are connected and "C" where your white wires are connected. Take your "C" directly from there..make sure to turn off the power to the transformer before you do any wiring..even though it is only 24 volts you can still very easily short the wires and "let the factory smoke" out and then it means a new transformer.
 
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Old 12-31-13, 04:58 PM
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Hey Troop..... do you think four zone valves and a wifi t'stat will run ok on a single transformer ?
 
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Old 12-31-13, 05:37 PM
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what make valve?
......................
 
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Old 01-01-14, 04:30 PM
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Valves are Honeywell V8043E1061. I can't tell what the capacity of the transformer is, all I can see is "Class 2."

There will actually be 3 thermostats connected. They also have batteries so the way I understand it, they would use the battery power if the line power was not enough. Do you think adding 3 thermostats would overload the xformer?

The hookup makes sense and the connections agree with NJ Trooper and poorplmbr. Haven't made the mod yet.

I have one more issue. Central A/C was added to the home about 5 years ago. It is a completely separate system and they simply added a thermostat for the system so I have 2 thermostats in the living room - one for A/C and one for heat. I want to get rid of the old heat thermostat and reroute the wiring to the new thermostat and just have one for both A/C and heat in that room. I understand I will have to remove the jumper between Rh and Rc and hook them up separately. What about the "C"? Does it matter if it comes from the A/C air handler or from the boiler in this case?

Thanks a million for all your help. You guys/girls are awesome!
 
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Old 01-01-14, 04:53 PM
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Depends what size xformer you have. Power consumption for those zone valves are 7.7 VA each. So thats 23 VA right off the bat. Depending on what type of power consumption you would get with the stat will tell you what size you would need. More than likely you have a 40VA xformer. Should be enough but i usually like to put in 75s with the built in reset button. Quite costly but it can definitely save my customers money in the future so i put them in.
 
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Old 01-01-14, 06:16 PM
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I understand I will have to remove the jumper between Rh and Rc and hook them up separately. What about the "C"? Does it matter if it comes from the A/C air handler or from the boiler in this case?
I believe that the C (common) gets BOTH the commons...

but check this to verify in the thermostat wiring manual.
 
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Old 01-03-14, 02:27 PM
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Actual Hookup slightly different

I'm attaching a modification to your diagram with the wiring that I have. Seems like the installers went a bit screwy with the wire colors. Not sure why, but basically the red and white wires were reversed from your diagram. I haven't done any connections yet. Does it matter which transformer terminal the wires are hooked to? Although the transformer is labelled "C" and "R", the wires are running opposite of your diagram.

Do I need to rewire everything to correspond with your diagram or should I just go with what I've got and hook up the wires to the proper places? I looks like I need to switch the wires on my thermostats. The old thermostats had no labels so I hooked the red wire to "R" but this looks incorrect according to your diagram since the red & white wires are reversed.

Thanks.

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Old 01-03-14, 04:21 PM
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Do I need to rewire everything to correspond with your diagram or should I just go with what I've got and hook up the wires to the proper places?
There's no FUNCTIONAL reason that you can't leave the wiring as-is and just make the proper connections... as long as the wires land at the proper place everything will play fine.

Some reasons you might WANT to switch things around...

When you go to hook up the A/C to one of the new thermostats as you've indicated you wish to do, you need to be extremely careful that you REMEMBER which wire color is which. The A/C installers may have used the proper colors... and hooking them up backwards can end up with the heating transformer in SERIES with the cooling transformer... and you could end up with 48 VAC going to the controls... and you DON'T want to do that!

In the future, having the wire colors 'match' will mean that you won't need to remember what you did the last time... or the new owners won't have to know either (if you have an altruistic streak and you care!)

Also in the future... some poor homeowner or tech troubleshooting the system might pull the thermostat from the wall and say "OHHHHH, HERE's the problem! Some Bozo put the red on the w and the white on the R!" and damage an otherwise good thermostat.

Does it matter which transformer terminal the wires are hooked to?
The transformer is AC voltage and polarity is not important. Either terminal could be either 24 'hot' or 'common'... many transformers are not lableled.

Although the transformer is labelled "C" and "R", the wires are running opposite of your diagram.
They actually used the correct colors at the transformer. Don't know why they didn't follow with the thermostats... but the thing is, with simple 2 wire thermostats, it simply didn't matter... obviously, it's been working all these years!

It only starts to matter when you add a third common wire and/or use a t'stat with both heating and cooling systems, each with their own transformer.
 
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Old 01-03-14, 04:31 PM
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Just one more thought...

With regular 120VAC house wiring, we know that the 'neutral' is always white, and the 'hot' black or red.

When I'm wiring controls, I like to follow the same convention. White is neutral, or 'common' ( C ) and red is 'hot'.

Rule with 120 wiring is that one must NEVER EVER install a switch in a NEUTRAL line. We ALWAYS switch the HOT. This is for safety purposes with 120VAC wiring. Someone can get killed thinking the power is off and it's NOT - because a switch is in the neutral leg.

A thermostat is just a switch. HOT goes to switch, SWITCHED HOT goes to one side of load, NEUTRAL goes direct to load. Motor in zone valve is load.

Being low voltage wiring, the safety issue is not so much of a concern, but I must ask, "Why do different?" Why not apply the same rules to ALL wiring?

Still the bottom line... as long as you get the C wire on the correct transformer terminal or wire, it will work. Electrons don't care what color the wire is!
 

Last edited by NJT; 01-03-14 at 07:18 PM.
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Old 01-03-14, 05:14 PM
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Good explanation

NJ, Thanks for such a thorough explanation. I totally agree about having things standardized. It really bothers me that the wiring is backwards so I think I will rewire it according to your diagram. All the wires are there, just need to switch around the wire nuts. Just to simplify things, I didn't even tell you that the "W" wire they used to the thermostats is actually green, just to make things more confusing. I'm planning on using the red wire for "R", white wire for "W" and green wire for Common.

Am really glad I asked about all this and really appreciate the insights.
 
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Old 01-05-14, 02:48 PM
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So far, so good

Finished rewiring the boiler and thermostats today and all seems to be working fine. I ran the green wire straight from the transformer rather than tying into the valve connections.

Only thing left is to install the thermostat for the A/C-boiler combination. I thought I would have to run a new cable to add to the A/C thermostat, but they installed it using an 8-wire cable and are only using 4 wires. So I plan on doing the wiring to the boiler in the basement at the air handler panel. I still need to talk to the thermostat people and see if I connect both the transformers together for the "C" wire.

Thanks to everyone for all your help! Hopefully I'll be able to help someone one of these days.
 
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Old 01-05-14, 03:34 PM
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I still need to talk to the thermostat people and see if I connect both the transformers together for the "C" wire.
Pretty much have to. But good idea to ask them... get it from the horse's mouth.

Hopefully I'll be able to help someone one of these days.
Come on down! There's plenty of chores around the Ponderosa !
 
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Old 01-08-14, 08:53 AM
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I talked to 2gig (manufacturer of thermostat) today and they said to use either of the "C" wires but do not connect BOTH the air handler and boiler xformer wires to the "C." I understand that it is simply providing power for the thermostat, but would this cause problems? For example, if I use the "C" wire from the a/c air handler to power the thermostat, does it cause any problems when using the boiler in the winter since the power for the valves (Rh) is coming from the boiler xformer?
 
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Old 01-08-14, 08:57 AM
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I don't understand how it can work without both commons to and connected at the thermostat.

Maybe pose your question over in the 'Thermostatic Controls' forum? Guys there might have better info.
 
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