Home>Discussions>ELECTRICAL & LIGHTING>Replace Thermostat Wire from 2 to 3 conductors
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dcalabro
Replace Thermostat Wire from 2 to 3 conductors

I want to get a specific thermostat to control the boiler for my homes baseboard water heaters. It requires a 3 conductor connection for the C wire which im pretty sure is for power. Is it as simple as just to replace the 2 wire cable with 3 wire thermostat cable and connect the new conductor to the C terminal on my boiler?

sparky1
Re: Replace Thermostat Wire from 2 to 3 conductors

Im not a boiler expert, but if i had a diagram to go off of it would be a simple question to answer. Can ya pos it?

JLMCDANIEL
Re: Replace Thermostat Wire from 2 to 3 conductors

I answered this in another thread. Yes, just replace the 2 wire cable with a 3 wire cable. A 2 wire supplies power to the switch in the thermostat and a return. With a digital thermostat you need power on common from the transformer to supply power to the electronics and power to the switch and a return. The "C" connection is the common side of the transformer. It is much like adding neutral to a wall switch to use a digital control.

Jack

Brookworld
Re: Replace Thermostat Wire from 2 to 3 conductors
JLMCDANIEL wrote:

Yes, just replace the 2 wire cable with a 3 wire cable. A 2 wire supplies power to the switch in the thermostat and a return. With a digital thermostat you need power on common from the transformer to supply power to the electronics and power to the switch and a return. The "C" connection is the common side of the transformer. It is much like adding neutral to a wall switch to use a digital control.

Jack

Jack: this is interesting . . .
I have a 4-zone hydronic baseboard heat system (which probably everybody knows by now) controlled by old-style 2-wire Honywell-type round thermostats w/ mercury temperature guage (no timer whatsoever).
I found at Home Depot, their housebrand, electronic setback units for $10 !, compared to $45-$60 for other 7 day multi-period units. The HD units state they are compatible with nearly ALL systems and has built-in Energy Efficiency temperature cycles. I bought 5 (the 5th for AC) and installed 1, basically, following the same 2 wire configuration and it works ! (No way I can rewire a full brick & plaster house with a concrete floor). They are (apparently) separately powered by 2 AAA batteries (vs. from 24v transformer). (At a former home, I had a Lux setback unit which I think used batteries for memory backup & light the display, but I don't know).

I mentioned all of this just to say I was able to swap out & in easily, . . .

however, the installer left no cable slack and I need another 2 inches to connect to the top of the new HD thermostat so I have to patch the 2-wire cable, drill another entry hole in the plaster wall, to have the new unit at original sight height and avoiding painting over faded/unfinished spots. (Right now, it hanging by the 2-wire cable).

dj1
Re: Replace Thermostat Wire from 2 to 3 conductors

Brookworld,

1. what you have is "heat only" mechanical thermostat, costs $10 or less. Brand not important.

2. if you need 2" more (don't we all), you can splice a short wire to your existing wire.

JLMCDANIEL
Re: Replace Thermostat Wire from 2 to 3 conductors
Brookworld wrote:

Jack: this is interesting . . .
I have a 4-zone hydronic baseboard heat system (which probably everybody knows by now) controlled by old-style 2-wire Honywell-type round thermostats w/ mercury temperature guage (no timer whatsoever).
I found at Home Depot, their housebrand, electronic setback units for $10 !, compared to $45-$60 for other 7 day multi-period units. The HD units state they are compatible with nearly ALL systems and has built-in Energy Efficiency temperature cycles. I bought 5 (the 5th for AC) and installed 1, basically, following the same 2 wire configuration and it works ! (No way I can rewire a full brick & plaster house with a concrete floor). They are (apparently) separately powered by 2 AAA batteries (vs. from 24v transformer). (At a former home, I had a Lux setback unit which I think used batteries for memory backup & light the display, but I don't know).

I mentioned all of this just to say I was able to swap out & in easily, . . .

however, the installer left no cable slack and I need another 2 inches to connect to the top of the new HD thermostat so I have to patch the 2-wire cable, drill another entry hole in the plaster wall, to have the new unit at original sight height and avoiding painting over faded/unfinished spots. (Right now, it hanging by the 2-wire cable).

Yes battery operated units are available, however the OP stated he wanted a particular unit that requires 3 wire. The new wire connected to "C" which is the common side of the 24 volt transformer.

Jack

sparky1
Re: Replace Thermostat Wire from 2 to 3 conductors

2. if you need 2" more (don't we all), you can splice a short wire to your existing wire.

hahaha

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