What To Do If You Don't Have a C Wire For Your Thermostat

HeatingHelp.com

The common wire is the neutral wire from the 24V transformer that provides the power that WiFi thermostats require in order for their features to work. Most existing homes do not have a C wire since non-connected thermostats don’t require them. Read on.

STEVEusaPA

Simple, easy. Every since @Robert O'Brien wrote this article, I've used it probably a dozen times.

Erin Holohan Haskell

Thank you, @Robert O'Brien, for sharing this article with us.

jmulla

Thank you.
I want to replace my old, 2-wire thermostat with a Nest that requires the C-wire. I have a 1-zone Taco SR501 controller. I am able to run a new 3 conductor cable from the thermostat to the boiler room.

Question: Is there any preference whether I run the C-wire to the neutral on the thermostat, or the "COM" terminal on the Taco? They are both located in close proximity.

Steamhead

Use the C terminal of whichever transformer actually powers the thermostat.

HomerJSmith

Question: Is there any preference whether I run the C-wire to the neutral on the thermostat

I don't understand that statement. The third wire you going to add should be connected to the "C" terminal on the Nest thermostat to the 24 VAC/COM terminal which is next to the "RW/TT" terminals on the Taco. That would add 24Vac charging to the Nest with the Taco transformer providing the power. I think that the Taco transformer can handle the 2 pole relay on the Taco board and the Nest, too.

HVACNUT

@jmulla, I believe the SR501 only has a 15 or 20 va transformer, so it should be wired using a separate 40 va transformer and an isolation relay. 

Hollywood111

On the general topic on the "c" common wire feeding your new Nest and or other power consuming stats, most contractors will run in a low voltage cable, with multiple (extra) wires in case an issue arises with shorts, broken wire etc. 18/5-18/6 -18/8 etc. Depends on the flavor of the day or what they had on the truck. Most new power consumption stats require constant hard wired lo volt (24 VAC) to the t stat. Battery backup is just that, if you lose power the batt. can usually run the unit as long as sufficient battery backup is available, but only as back up for a certain period of time. Most internet connected devices need an interrupted power source to not only stay connected, but to do their job. Not like the older stats mentioned above. Common wire if needed, check your cable and or have a qualified contractor help you with this issue.

Jells

I had this problem. I stripped the relay and 24v transformer from an junked Minitherm boiler and popped them into a 4.5 x 4.5 x 1.5 steel box attached to the main control box of the burner. Works like a charm and cost me nearly nothing.

EdTheHeaterMan

HVACNUT said:

@jmulla, I believe the SR501 only has a 15 or 20 va transformer, so it should be wired using a separate 40 va transformer and an isolation relay. 

Nest does not need 40 VA for powering the Wifi features. The 15 VA transformer is more than adequate. But don't believe me, Ask one of the TACO reps that frequent this site. I seem to remember that the only time you need to add a transformer is when you use the multi zone relays and are powering up more than 2 thermostats. Then you may need to use a more powerful transformer than the one in the relay. The SR501 can only handle one thermostat because there is only one relay to operate one zone.

@HVACNUT is correct when you apply this to the multi zone relay and you are powering 3 or more thermostat from that small transformer. There have been many discussions on this topic at HH and other forums.

ChrisJ

Honeywell prestige doesn't need a C wire from the equipment as you can use a separate 24v xformer and the thermostat head only needs two wires as the equipment controller can be installed by the equipment.

ngp155

I have a Burnham gas fired steam boiler that runs with a 40V transformer. I'm trying to replace a Honeywell programmable thermostat, connected with 2 wires (W & R) & runs on 2 AA batteries, with a Nest thermostat. I have additional wires to the thermostat and a separate 24v transformer available. The transformer has 2 terminals, Load & C, but the Nest only has a C connector. Where would the Load wire get connected to close the circuit? The remaining connectors are: W (connected), R (connected), C (available), Y (open), G (open) and OB (open). Thank you.

EdTheHeaterMan

There is sometimes a problem with this. Can you post a picture of your wiring diagram or the model number of the boiler? The age and model number will make a difference.

EdTheHeaterMan

There is sometimes a problem with this. Can you post a picture of your wiring diagram or the model number of the boiler? The age and model number will make a difference.

If you have a newer boiler the wiring is fairly easy.

If I guess correctly the R from the thermostat gets connected to the R on the transformer. There are some other wired connected to the R on the transformer.

The C on the thermostat get connected to the C on the transformer

The W on the thermostat get connected to the other thermostat wire nut. But I must see the actual wires on your boiler to be sure. Unlike a 2 wire thermostat, will not work if the R and the W from the thermostat are reversed.

ngp155

Thanks for the quick response Ed. I don't remember, exactly, but the boiler is over 15 yrs. old. I tried to attach 2 pix to the initial post, but they don't seem to appear on the thread. I am attaching 2 pix of the schematic for the boiler and the ones of the transformer & Nest backplate and hope they transfer along with this post. Thanks again.

ngp155

@EdTheHeaterMan
I have changed the files to jpeg, don't know how they saved with a strange extension, and attached them again along with a word document containing the pix as well. Also, I was looking at the current transformer directly attached to the boiler and even though the diagram on the back of the cover says the unit runs on a 40 v transformer, the transformer itself has a tiny label that says it is a 24v transformer. Thanks again.

Jamie Hall

That's a 40VA transformer, putting out 24 volts. VA is a measure of the power the transformer can provide, not the voltabe.

ChrisJ

Jamie Hall said:

That's a 40VA transformer, putting out 24 volts. VA is a measure of the power the transformer can provide, not the voltabe.

Volt amps is a measure of apparent power.

Jamie Hall

fussy party. Watt's all this about power factors?

ChrisJ

Jamie Hall said:

fussy party. Watt's all this about power factors?

No no, no watts there, just volt amps

EdTheHeaterMan

You do not need the additional transformer. You can use the transformer that is inside the boiler. The transformer that is on the wiring diagram. Here is a close up of the wires you need to connect to inside the junction box on the heater.

This is essentially the diagram I posted in my first response The Ladder Diagram is easy to understand. The wiring diagram on the left illustration is easier to see where the components are actually connected.

If you look closely the power wire on the boiler transformer has a BLUE wire connected to it. The Common wire has a YELLOW wire connected to it. Make sure the Nest R is connected to the BLUE on the boiler transformer. Make sure the Nest C is connected to the YELLOW wire on the boiler transformer. the Nest W wire is connected to the BROWN wire that operates the relay.

That should be simple.

ngp155

THANK YOU ALL ! Very appreciative of all of your expertise and advice. I just finished the connections and tested it and BY GEORGE your directions were perfect. Thanks again so much

EdTheHeaterMan

ChrisJ said:

Jamie Hall said:

fussy party. Watt's all this about power factors?

No no, no watts there, just volt amps

I'm sorry, Who is on first... Watt's on second!