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January 14, 2007 12:24 AM

Not-My-Walls-To-Cut-Through-Filter: Anyone have experience running Cat5 through HVAC ducts?

I just moved into a space-limited Row Home and would like to keep my rack of various equipment (dvr, X10, security, nas) out of sight and earshot. The basement looks to be ideal for this purpose - but with plaster walls and new hardwood floors I can't in good conscience drill to run cable to the first and second floors. New windows eliminate the possibility of external-house runs, and the existing cableTV drops were drilled through the houses external brick and grometted. The only option seems to be the existing heating duct. There is a straight vertical duct-work run to all floors that terminates in the basement roughly 8' from the furnace itself. My non-plenum rated cable says it has a max operating temp of 140°F.

My question(s):

- Has anyone ever done this?
- Did the cable (or even cable jacket) melt?
- Would a 70+ year old duct run have built-in obstructions like a grate or something to stop urban rodents?

Thanks in advance!

posted by datacenter refugee to home & garden (15 comments total)

The problem isn't melting, but that non-plenum cable gives off toxic fumes as it burns. You don't want that in your air ducts in case of a fire.

Also, and this may be urban legend, I've heard that non-plenum cable can act as a fuse for fires.
posted by sbutler at 12:42 AM on January 14, 2007

Just get some plenum cable or pull off a phone jack, snake down the wall cavity into the basement, and put a combo phone/cat5 jack in the existing hole.
posted by wierdo at 1:00 AM on January 14, 2007

A Google search turned up this result (the first web search result I've seen that links directly to their Book Search service.) Sounds like lots of folks do cable through HVAC.
posted by dendrite at 1:02 AM on January 14, 2007

Similar, previously
posted by !Jim at 1:22 AM on January 14, 2007

I did it when I lived in a condo. Worked fine for me and it was WAY more convenient. Just don't get too close to the furnace! :)
posted by Thrillhouse at 5:33 AM on January 14, 2007

I have done it with no problems, as long as it's away from from the furnace. If your duct would get hot enough to melt cable, then I suspect you would have bigger problems anyway.

Also, I have fed cable from the basement then up next to the ductwork through a slight gap where the duct meets the wall or floor. There was enough flex between the nails to feed the cable through. I just duct-taped it along the outside of the exposed ductwork in the basement with, then out the gap. I have also done this with speaker wire, with no problems.
posted by The Deej at 6:23 AM on January 14, 2007

When I lived in a Pittsburgh row house, we ran our CAT5 in the ductwork. I pulled it out when I moved and examined it for damage, but found none. It had been there for almost a year, and survived a cold winter with the furnace running.
posted by odinsdream at 9:02 AM on January 14, 2007

Thanks everyone for the answers. Looks like I wont do this until i have plenum-rated cable. Wish I hadn't been so cheap when looking for cable with my home depot Gift Cards :(.
posted by datacenter refugee at 12:04 PM on January 14, 2007

Can you not return the box of cable you bought?
posted by wierdo at 12:38 PM on January 14, 2007

Even if you don't have the receipt, Home Depot should give you store credit if it hasn't been used.
posted by Good Brain at 12:48 PM on January 14, 2007

Weirdo / Good Brain - Good Point. I will try tonight!
posted by datacenter refugee at 1:01 PM on January 14, 2007

One other datapoint: even Cat 5, 5e or Cat 6 cable that is plenum rated may not be sufficient for use in HVAC ducts, where condensation can form. You'd need a dual rated plenum/wet locations cable, if you could find such a thing. And even plenum rated Cat 5, 5e, or Cat 6 cable can suffer performance problems if it lays directly against metal ductwork, ductwork support straps, or encounters very short bend radii, or other common installation physical problems. Few home DYI installers fully test and certify their installations after running the wire, preferring to "troubleshoot" links that fail to "work" later. But I've seen a lot of Cat 5 home wiring that worked very badly, and yet wasn't completely "dead."

Cat 5 installation guidelines exist for a reason; understand and observe them diligently, and Things Work.
posted by paulsc at 2:52 PM on January 14, 2007

I have lots of experience with professional telephone installation and repair. Don't worry about whether you're using plenum-rated cable or not. The amount of toxic fumes produced by a few cables is not going to be a big deal if there's a fire.

It is a different story in commercial buildings. But even then plenum cable is only required IF the entire space above the ceiling tiles is used to carry ventilation air, rather than dedicated ductwork. That's because ceilings are often loaded with dozens or even hundreds of cables. Plus code enforcement is likely to be much more strict.

Unless you're running the cables real close to the furnace, heat shouldn't be a factor, as others have said.

The biggest problem is reliability and aesthetics. I've seen lots of cables fail from crushing or from metal edges where people run them in or out of the ductwork. Believe it or not, houses/ductwork/vents/cables vibrate and move, and that can cause cables to break or short out. Usually in such a way that it can be very hard to troubleshoot and locate the problem.

BTW, it's usually pretty easy to run cables to the first floor if the basement ceiling is open. Just stick a screwdriver between the baseboard and shoe molding (quarter-round), and push them apart a little bit so you can drill down between the two moldings. Then use a surface mount jack attached to floor molding, so that the jack is mounted on the baseboard. The professionals usually use a quarter inch drill bit about eighteen inches long that you can buy at the larger hardware stores.

It may also be easy to fish a wire up through a first floor wall if you want a flush wall-mount jack (except on outside walls with insulation). Either way, just make certain that you know where you're drilling so you don't hit any other wires.

In older homes it is also often pretty easy to pull wires up to the attic through the opening for the soil pipe and then drop them down again through the walls or closets to your second floor locations.

One final tip, make certain you use "real" data or telephone cable. When you strip the outer casing off, the inner pairs should be twisted together. That's why they call it "twisted pair," and the main difference between different grades of cable is the amount and kind of twist.

It's not likely you're going to get bad Cat 5, but I've seen lots of places wired with "four conductor" telephone cord, thermostat wire and other junk, and it can cause noise on your telephone and have a HUGE affect your DSL speed.
posted by 14580 at 4:02 PM on January 14, 2007

Thanks for everyone's help - I returned my existing cat5 for plenum + indoor/outdoor rated stuff and it's working well. The run in question is flanked by cisco switches so I've got an snmp poller watching it like a hawk for any errors.
posted by datacenter refugee at 11:05 PM on January 14, 2007

Probably not sufficient throughput for the DVR, but I've been bridging my LAN with Netgear's new line of 200mbps powerline ethernet hardware. It's not cheap, but the speed, reliability, and range beats wireless.
posted by Caviar at 7:47 PM on January 15, 2007