Ethernet Wiring

Use Cat 6 Plenum rated wire and you'll have no problems. I used a hardwire backhaul for my mesh routers and have no issues.

Invest in a good punch down tool for the terminations and keep them neat and clean. Get or make a holder for the jacks & plugs when doing the terminations, it makes life much easier. No magic, just good workmanship. 

@elliottbnewcombjr recco for sealing the penetrations is solid. 

Avoid repeaters - by their very nature they halve the available bandwidth. Use mesh routers instead.

Worries about about static or thermal concerns are unfounded. Of course I only have 40 years experience with Ethernet cabling - Coax, Cat 5, Cat 6 in homes, offices, schools, and enterprise-class data centers, so what would i know? 

Interesting...

CAT5,  4 pair UTP Ethernet cable was first introduced in 1995.

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  CAT3,  4 pair UTP Ethernet cable was first introduced in 1991. Good for voice, not so much for data though, as I recall. 

Installed miles of RG62/U coax cable in the 1980s for desk top dumb terminals. 

Also installed some RG58 coax cabling in the 1980s.

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@lovehifi22 ,

Project status?   

The cable would be inserted into the duct about 15’ away from the furnace plenum.

I assume a straight shot through a 6" dia round supply duct to a ceiling mounted outlet vent register? (Naturally the ceiling register would be remove when installing a fish tape to pull in the data cable.)

Good luck if you first have to travel through a main supply duct to the tap off for the 6" dia round branch duct that feeds the ceiling register.

FWIW, I would recommend you purchase the data cable from Blue Jeans Cable. They will make you a cable cut to length and install RJ45 plugs on each end of the cable. Cable will then be tested to make sure it meets specs standards. Not just a wire mapping test.

Protect both connectors so they are not damaged during installation. 

I’m retired now but at my last work place they ran cat 5 through the building ducts and it was a huge improvement over the previously horrible WiFi that we had.  There were no issues over the year and a half of use before my retirement.  I chatted with installer who said that they do this all the time.  He said that he had never encountered an issue with warping due to HVAC, which was the first question that I asked.

  Ethernet cable is inexpensive.  Even if something did warp after years of use, replacing it should not be to painful 

Thanks to all for your helpful comments and suggestions. 
Greatly appreciated.