Effect of heat on CAT5 Ethernet Cabling

Solution 1:

The spec says -55C to +60C so you may be exceeding that rating.

http://en.wikipedia.org/wiki/Category_5_cable#Electrical_characteristics_for_Cat.5e_UTP

Personally I think it will be ok if you lag the cable and possibly by a good quality Cat5e or Cat6 cable.

Solution 2:

Personally I wouldn't run a cable like that without some kind of insulation/heat shield, and I would only do that after looking for alternate ways of routing the cable.

Problems I see are:

Electricity doesn't work well with heat (heat negatively affects conductivity) - Dave Cheney already pointed out the specs regarding acceptable temperates for Cat5e.

The cables plastic jacket is likely to melt over time (or rather it will soften up just enough that it begins to fray apart under it's own weight), exposing the wires inside. This will likely result in packet loss over the cable.

And most importantly, you could start a fire. Either from the plastic jacket heating up enough that it catches fire (burning wires are great - it spreads through walls, and releases toxic gases) or after the cable frays, the exposed wire could touch the metal and short, resulting in an electrical fire (the fact that there is water involved isn't great either).

Don't take all this as overly dismissive - I've personally taped RJ12 to the outside of building ducts as a quick way to route in an extra phone - however an old style cast iron system does not sound good. A simple test would be, with the heating jacked up, can you hold onto the pipe for long without getting a burn?