Can this be used for "C" wire to the digital thermostat?

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JasonM

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I have more than 30 year old gas furnace made by "Day & Night". I need to replace the old Robertshaw thermostat, which controls gas furnace and air conditioner.

I ordered Honeywell digital thermostat "Focuspro 5000 Non-Programmable Thermostat", and I want to use C wire to power the thermostat instead of the battery.

On the gas furnace control panel, I have terminals like
C/X R/4 W G Y.

transformer.jpg


My question is Can I use the C/X terminal on the gas furnace for C line to the thermostat to power the thermostat? I do not know yet if there is a C line on the old Robertshaw thermostat. If there is no C line, I can run a cable myself from the this C/X to the thermostat if this can be used for C line.

Thank you very much.
 

JasonM

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Thank you very much for the answer.

BTW, what is the X on the C/X terminal?

Thank you.
 

Reach4

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I don't know. It seems to be some kind of eXtension designation. But do expect to have 20-something volts AC between C and R all of the time.
 

JasonM

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Thank you very much. I will double check the AC 25volts between R/4 and C/X terminal using the volts meter.

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JasonM

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I checked the AC voltage between R/4 and C/X terminal using the volts meter, and it is about 27.5 volts.
Thank you.
 

Jadnashua

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The only issue you might have, and it probably won't be one, is if the transformer in the furnace can't put out enough power to make the new thermostat work. As it is today, your old thermostat is just a simple switch and does not draw any power of its own. The new one will, but it won't be much...whether that will be an issue is unlikely, but still possible.
 

DonL

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I do not understand the "to power the thermostat instead of the battery"

The Battery operates the relay, that closes / controls the 24VAC.

My guess is no you can not.

Are AA batteries that expensive ?


Good Luck.
 
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JasonM

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jadnashua, thank you for the comment. I hope it works fine. I will install the new one this Sat.
DonL, if I have a choice between the electricity and the battery, I would choose electricity. Thank you.
 

DonL

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jadnashua, thank you for the comment. I hope it works fine. I will install the new one this Sat.
DonL, if I have a choice between the electricity and the battery, I would choose electricity. Thank you.


I hope it works for you. Seems like your set-point will be lost on power fail. That may work for you.

Please check back, after you try it.


Good Luck.
 

Jadnashua

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Some of the fancier thermostats, especially those that are WiFi enabled, would drain a battery in no time, so rely on an internal power supply powered by the low-voltage transformer (thus the need for the common)...the battery is only a backup for the memory in most cases in that type. Most of the other electronic thermostats use the batteries, and on most I've dealt with, those last about a year or so to power the whole thing. Maintaining a WiFi connection and a more sophisticated processor just takes too much power for the typical battery to last very long.
 

JasonM

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BTW, the new digital thermostat requires 24Volt, but my transformer that I measured between C and R terminals gives about 27.5 volts. I hope this 27.5 volt is not too much and not going to cause problems to the thermostat.
Am I correct?

Also, I am going to run a 14 awg single wire to the existing 4 wires for the Common line. The Home Depot did not have 18 and 16 solid awg wire so I bought 14 awg. The old(about 35 years old) existing 4 wire does not degrade over time and need replacement. Is this also correct?

Thank you very much.
 
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Reach4

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Voltage is fine. Your wires described are OK too.
 

JasonM

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I successfully replaced the old thermostat today with the Honeywell digital thermostat "Focuspro 5000 Non-Programmable Thermostat. I ran the 14 awg signle solid wire from the fire furnace to the thermostat area using the fish tape.
I tested AC and the fan, and they work fine. I did not test the heater since I turned off the pilot lamp.

In retrospect, I should have installed the 18 awg 1 or 2 line solid wire. I noticed while installing the 14awg, it is kind of not pliable and unyielding.

I also connected the 1 outlet Belkin surge protector to the furnace plug, hopefully to make the thermostat electronics last longer.

All in all, it is a small accomplishment for me that makes me feel good.

Thank you all for your help.
 

JerryR

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Good Job.

Did it work without the battery ?

Enjoy
Don,

Here are some snapshots from the manuals.

Looks like with the c wire connected it should operate fine and retain settings when power is lost. The only issues would be without the battery the display would go blank when power is out and when power is on the nag "REPLACE BATTERY" would always be displayed on the screen.
 
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DonL

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Don,

Here are some snapshots from the manuals.

Looks like with the c wire connected it should operate fine and retain settings when power is lost. The only issues would be without the battery the display would go blank when power is out and when power is on the nag "REPLACE BATTERY" would always be displayed on the screen


That is great Jerry.

I did miss that.

I get about two years out of my Honeywell Programmable T-Stat. It uses 3 AAs.

I think I would install the batteries, but that is just me. Then you know it will take off where it left off when power is restored, and a Power glitch can not scramble the memory. The batteries should last a long time, and only be drawn from in a power outage, If C is connected.


Thanks for the post. That was nice.


Have Fun.
 
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