Thermostat wiring question

Yail Bloor · 09-11-2009, 06:50 PM

First just a little background: Years ago I replaced electric baseboard radiators with an oil boiler and hot water baseboard system. All the line voltage wiring was, of course, disconnected from the electric panel and made dead. Thermostat locations stayed the same and we used the pre-existing romex as control wire into the unfinished basement where we connected thermostat wire which we took to the boiler (we also pig-tailed thermostat wire to connect to the new thermostats). So I have a two wire thermostat wire rough-in.

Now I want to install new thermostats that require three wires (Z-Wave thermostats, for remote control) and my question is this: can I use the unused ground wire as the required C wire, a live 24V wire to power the new stat. My concern is it's a "bare" wire but not really as it's inside the romex housing. Any input would be greatly appreciated.

SD515 · 09-11-2009, 07:04 PM

Technically no. That bare wire is for grounding (bonding) purposes only. Never to be used as a normally current carrying conductor. One concern I will mention is, what if someone in the future works on that wire? They will most likely assume it’s a ground wire, and not know that there is actually voltage present on it. True 99.9999999999999% of the time 24 volts isn’t going to harm you, but it could. Remember, it’s the amperage that can kill you…not the voltage.

oilseal · 09-12-2009, 07:22 AM

Assuming that Romex line is carrying 24v for thermostat control, it's isolated from the 120v service via a 24v control transformer. The black and white Romex wires can be hot relative to the bare wire. The bare wire should be bonded to ground and the thermostat should operate normally.

jrodtaclet · 09-12-2009, 05:30 PM

I have a pellet stove that has a blower and a manual thermostat with two wires on 24v. When I set the thermostat to hotter then room tempeture the blower turns on. My question is, I purchased a digital thermostat to upgrade the old one, but the new thermostat has cool and hot temperature. I'm planning on connecting the hot 24v. wire from the pellet stove to the R letter in the termostat and the return RH letter to the 24v. blower motor. Will this turn my blower on when the temperature drops?.
Thanks a million

Yail Bloor · 09-12-2009, 06:01 PM

The new thermostat may require a C wire, or third wire. The config you described may in theory power the blower but unless the thermostat is battery powered it may not power the stat display. The directions for the new thermostat should indicate whether a third wire is required or if two wires and a jumper is sufficient. Now that that question is answered can future respondents to this thread please stick to answering the original question or post a new topic.

user62257 · 09-13-2009, 02:57 PM

SD515 "Technically no. _"

Most people would want to do it to Code, the question is does the Code
allow this, I say no. (until proven wrong, by a liicense electrician)

SD15 answer is Sound.

The question is do you feel lucky!

09-11-2009, 06:50 PM	#1
Yail Bloor Newbie Join Date: Sep 2009 Posts: 2	Thermostat wiring question First just a little background: Years ago I replaced electric baseboard radiators with an oil boiler and hot water baseboard system. All the line voltage wiring was, of course, disconnected from the electric panel and made dead. Thermostat locations stayed the same and we used the pre-existing romex as control wire into the unfinished basement where we connected thermostat wire which we took to the boiler (we also pig-tailed thermostat wire to connect to the new thermostats). So I have a two wire thermostat wire rough-in. Now I want to install new thermostats that require three wires (Z-Wave thermostats, for remote control) and my question is this: can I use the unused ground wire as the required C wire, a live 24V wire to power the new stat. My concern is it's a "bare" wire but not really as it's inside the romex housing. Any input would be greatly appreciated.

09-11-2009, 07:04 PM	#2
SD515 Electrician Join Date: Aug 2008 Location: Near Jackson Michigan Area Posts: 1,449	Thermostat wiring question Technically no. That bare wire is for grounding (bonding) purposes only. Never to be used as a normally current carrying conductor. One concern I will mention is, what if someone in the future works on that wire? They will most likely assume it’s a ground wire, and not know that there is actually voltage present on it. True 99.9999999999999% of the time 24 volts isn’t going to harm you, but it could. Remember, it’s the amperage that can kill you…not the voltage. __________________ Kyle Just because you can, doesn't always mean you should

09-12-2009, 07:22 AM	#3
oilseal Member Join Date: Aug 2009 Location: Maryland Posts: 61	Thermostat wiring question Assuming that Romex line is carrying 24v for thermostat control, it's isolated from the 120v service via a 24v control transformer. The black and white Romex wires can be hot relative to the bare wire. The bare wire should be bonded to ground and the thermostat should operate normally.

09-12-2009, 05:30 PM	#4
jrodtaclet Newbie Join Date: Sep 2009 Posts: 4	Thermostat wiring question I have a pellet stove that has a blower and a manual thermostat with two wires on 24v. When I set the thermostat to hotter then room tempeture the blower turns on. My question is, I purchased a digital thermostat to upgrade the old one, but the new thermostat has cool and hot temperature. I'm planning on connecting the hot 24v. wire from the pellet stove to the R letter in the termostat and the return RH letter to the 24v. blower motor. Will this turn my blower on when the temperature drops?. Thanks a million Last edited by jrodtaclet; 09-12-2009 at 05:53 PM.

09-12-2009, 06:01 PM	#5
Yail Bloor Newbie Join Date: Sep 2009 Posts: 2	Thermostat wiring question The new thermostat may require a C wire, or third wire. The config you described may in theory power the blower but unless the thermostat is battery powered it may not power the stat display. The directions for the new thermostat should indicate whether a third wire is required or if two wires and a jumper is sufficient. Now that that question is answered can future respondents to this thread please stick to answering the original question or post a new topic.

Thermostat wiring question