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!

NoCwire

I have a similar situation. I just purchased the Nest 2020 Thermostat (pictured in posts above). However, my setup with the thermostat wires is quite different. I have an old Honeywell 2-wire thermostat (pictured) that runs into a box at the furnace (pictured) that only has two terminals for the thermostat wire. There doesn't appear to be a c-wire anywhere. Thoughts on how to wire my new thermostat? I'm at a loss. Thanks

Jamie Hall

Nest -- among others -- makes a wall wart device which you can use to power the Nest. That's usually the simplest solution. Sometimes it is possible to find the common connection point on the furnace control board, but not always -- and never without some real research often home made changes.

Jells

I'd have to see the back of that board, but if you can locate the output wires from the transformer in the upper right, you might be able to identify the C wire you need. Obviously you need to run 3 wire cable to the thermostat.

If not, in a similar circumstance but with an unhackable control I used a transformer and relay from a defunct boiler to get my C wire.

NoCwire

Thanks, Jamie and Jells. I'm ok with buying a "wall wart" transformer. However, is it safe to hook up the 24Vhot side of "wall wart" transformer into same terminal as the R wire on the nest? Several videos say you can do that, but I thought I'd check with the experts (you guys) first.

https://www.youtube.com/watch?v=yi5kfn3bmEk

Also, I do not have a problem running 3 wire thermostat cable up to new thermostat (old house with hollow walls). Can I put a self tapping screw into the metal box where thermostat wires hook into, and run a c-wire off that? Not sure if that would work.

Jamie Hall

If you use a wall wart type device, or an independent transformer in the basement with an extra wire running to the thermostat, and hook it up to R and C on the thermostat, it is, effectively, a completely separate circuit from the furnace. Shouldn't be a problem.

Trying to use the transformer already on the board, however, poses two problems: you have to be sure that it has enough power to power the Nest, which it may not have, and more important you have to be sure that you are able to correctly identify the hot wire and the common. With a two wire thermostat, this isn't a problem -- it's just a switch. However, there is a 50/50 chance that one of the two wires at the thermostat is actually the hot wire and the other is the switched wire -- and this can be a bit tricky to determine. Much better, if you are going to try to do that, to use a test light to help identify what's what.

Your idea of a screw as a terminal might work... and might not. That depends on whether the transformer on the furnace is grounded on one side of the 24 volt output. It might be. Might not be, too.

NoCwire

Thanks. My only concern with putting two wires in the R terminal of Nest, is will the voltages conflict with each other? In theory, they should both be 24V. However, if one is off by a couple of volts, does it matter?

i.e. Could the "wal wart" transformer feed the furnace transformer with a different voltage, and cause problems?

Jells

The way the devices I've seen, and the DIY setup I did work, is that they use a relay to switch the thermostat line from the boiler, so that the two circuits do not comingle. I'd have to study my nest documents again to see if there's a way to power it completely separately from its relay circuit for the boiler switch. There's a lot of terminals back there!

NoCwire

So I did a little more research on my setup. It appears I have one of these attached to my thermostat. Anyone have any ideas on how I can make this work with the Nest 2020?

https://customer.resideo.com/resources/Techlit/TechLitDocuments/95-0000s/95-6571.pdf

Jamie Hall

You'll notice in your .pdf that in every case the control is listed as a T87F thermostat or a low voltage thermostat. That is a very simple no digital two wire thermostat; an on/off switch. Those thermostat terminals are the ones you want to use -- nothing else.

Now if you connect a different transformer to R and C on the Nest, that is a completely different circuit, and the two won't interact, so you don't have to worry about the voltages being somewhat different -- provided that none of your 24 VAC wires are grounded. They must float.

NoCwire

Thanks Jamie. That's the best news I've heard yet. So, if the "wal wart" wires go to R and C on the Nest, where do the two wires from the furnace go on the Nest? Do I double up one hot wire from "wal wart" and one of the furnace wires into the R spot on Nest? If so, does it matter which furnace wire joins the "wal wart" hot wire?

Thanks for the info

Jamie Hall

Yes. You double two wires on the R terminal (or, better, splice them to a short wire, and hook that into the R terminal. The two wires for the furnace are on R and W, and it doesn't matter which one (assuming nothing is grounded)

NoCwire

Any way to test if they're grounded using a multimeter? If so, how do I test it?

Jamie Hall

NoCwire said:

Any way to test if they're grounded using a multimeter? If so, how do I test it?

Theoretically yes, but the results from a multimeter can be odd. I prefer a 24 volt test light -- try an auto parts store. With a fuse in it. Then clip one end of the light to a good ground, and probe the wires or terminals in question with the test light probe. It the transformer does have one side grounded, the test light will light when one of the two terminals on the transformer (or the wires coming from the transformer) are touched.

If one of the two terminals is grounded, the one which you lit when you touched it is "hot". That should go to R on the thermostat.

NoCwire

Got it. Thanks.

Just to clarify, if it is grounded, can I connect the hot furnace terminal (wire) to the hot wire from "wal wart" and put them both in R terminal on Nest?

Reason I ask is you mentioned "so you don't have to worry about the voltages being somewhat different -- provided that none of your 24 VAC wires are grounded. They must float"

If the furnace terminal are grounded, will that be a problem?

Jamie Hall

Say what? See above... if only one transformer or the wall wart output is grounded, not a problem.

NoCwire

Ok. Got it. Many thanks for the help.