Fifth wire

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paulmars

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My HVAC thermostat is 4 wire system W,R,Y,G. I just bought a new thermostat that uses 5 wires, w/extra Common connection. I was considering running a 5th wire, no easy task. then i removed old thermostat and pulled the wires out of the wall and there is a 5th black wire that was not used. So, i go to the furnace in the garage and look at the control board and there are the same four wires hooked up to the board and the fifth black wire not hooked up. So, all i need to do is connect this wire to common or furnace ground, right?

Before i did that, I checked several things.

At thermostat end of the wires with the thermostat disconnected I get 28 volts AC between R and each of the other three, but between R and the black I get volts starting at 7, 8, or 9 volts then it climbs up rapidly at 1st then slowly and by the time it gets to 12.x it is increasing very slowly. Now i go to the furnace and I get the same 28 volts between R and each of the others, except black. Between R and Black I get zero.zero volts. Between the black and ground is also zero.zero volts.

ive gone back and forth several times and get the same.

How is this possible and what does it mean?

tks,
pa
 

Stuff

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It is called phantom voltage. The digital meters are overly sensitive and pick it up. When a "floating" wire runs in parallel with a hot wire it picks up some voltage due to electromagnetism. Normally nothing to worry about.

Hook up the black wire to C at the furnace and then you should see expected voltage at the thermostat end.
 

Jadnashua

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You'll need to find the wiring diagram which, if you don't have the installation instructions, is usually glued to the inside of one of the panels, folded up and stuck somewhere inside, or, you may be able to find it on the manufacturer's website. Basically, though, if you can find the 24vac transformer, one lead will be the R, and you want the other side of the secondary (do NOT connect to the primary!). Between the R and C at the transformer secondary, it will be somewhere around 24vac (often, it's higher, not usually lower).
 

WorthFlorida

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Go to the Honeywell site for good info on C wire. https://yourhome.honeywell.com/en/questions/what-is-a-c-wire

Honeywell has a optional c wire saver, "Wire Saver Module (THP9045A)". Further instructions are at the url.
http://yourhome.honeywell.com/en/qu...do-i-have-if-i-dont-have-the-c-or-common-wire

The Honeywell Lyric Round Wi-Fi Thermostat is the one exception to the Wi-Fi, smart, connected thermostat rule, as it does not require a C wire. https://yourhome.honeywell.com/en/products/thermostat/lyric-thermostat


The C wire is needed for the new WIFI thermostats that need power 24/7. Some older WiFi and just about all programmable digital ones uses batteries. You can run just one wire if you like, it is low voltage so there are no concerns running it in a home. But if you have to go through all the trouble just run a new thermostat wire with as many conductors as you can find. If you every replace your AC, do go with a heat pump and that will take one additional wire.

If you are really stuck there is items on the market such as add a wire. https://smartthermostatguide.com/c-wire-venstar-add-a-wire-adapter-review/

Another way is to bring the 24 v AC right too the thermostat with a wall plug in transformer. You wouldn't want the wiring running up the wall but you can snake it up the wall from the thermostat cavity to the bottom of the wall by drilling a small hole in the wall.

The internet is loaded with adaptations.
 

paulmars

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It is called phantom voltage. The digital meters are overly sensitive and pick it up. When a "floating" wire runs in parallel with a hot wire it picks up some voltage due to electromagnetism. Normally nothing to worry about.

Hook up the black wire to C at the furnace and then you should see expected voltage at the thermostat end.

thanks
 
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