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There are 5 wires coming from my furnace (no AC) in the basement. Two of them have been connected to the existing 2 stranded wire (black and white in photo) running between the mechanical room and the thermostat on the 2nd floor. 3 are just cut off. I assume this was easier during the install than running a new wire.

I'm assuming if I had five wires running between them I could have control of the furnace fan. But instead, can I just wire a switch where the remaining 3 wires come off the furnace?

If I do manage to run the wire, is it true that I would have the additional benefit of powering the thermostat
without batteries?

Thanks

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I would just run new thermostat wire 18/8 from furnace to thermostat. Do it once do it right. When you sell your house nobody going to poke at your creative switches in the middle of the wire. I see a lot of weird stuff homeowners came up with. Another option is to buy wireless thermostat and it will do all
the stuff you wana do and more. Will cost more money.
 

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Hi, pull a new 5 wire from that location or from the furnace and install a thermostat that has a Fan On / Auto function, don’t mess around with a switch, what kind of Stat is there now ? you may be able to add a sub base on it.
 

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There is no problem with wiring a switch at the furnace to turn on and off continuous fan -> connect one of the unused conductors to the R terminal on the furnace board, the other on the G -> wire in a single pole switch.

Leave the other wires on the terminals as is -> you can have more than one wire per terminal.

Of course, it's better to run a new t-stat wire.
 

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I find 18/5 is the most commonly available to the diy'er retail - 18/8 only online and gets very expensive.
Up until covid it was available in all of the big box stores. Now you never know what you'll get. If 5 is all they can get, so be it. I've had several people recently that could only get 4 wire and they wanted to go out DIY, so it was just doubled up.
 

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You can buy on Amazon or other online place. Even Walmart sells online. I just bought on OfferUp 3x 250 ‘ rolls 18/8 for 60$ each. If I was diyer I would just go to Hvac guy on the street and ask for 18/8 wire. Tell him I need 30 foot or whatever. Give him $50 and that’s it. Some might say no . So ask another.
 

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18/5 is plenty being that there is no AC on the system, and 5 conductors would be enough even if there was.
There may be at some point in the future. A 2 stage furnace upgrade when this one dies with a humidifier and ac would need it. A 6 wire isn't much more than a 5 wire. When in stock and no price gouging, even 8 wire wasn't that much more.
 
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