can anyone out there think of a reason not to use 50’ of FDA approved vinyl water line, at 19 cents a foot, instead of a 50’ stainless coil when making a jockey box? I’ve been racking my brain trying to think of a reason not to do this. Haven’t come up with a reason so far.
Heat transfer is the biggest reason. I would avoid copper even though it has the best thermal transfer. Stainless is a lot less reactive and cleans up nice with PBW. Copper can get nice, bright and shiny, that's when it reacts to beer. It needs an oxidized layer and then it turns a dull brown. When I use to have a copper heater in my RIMS, the water would turn green when I did a CIP (clean in place) with PBW. Plastic may not transfer heat as well and it may work, but not as good as stainless.
Triple the length means triple the pressure... It takes a lot to move beer through a narrow channel. At some point the fittings appropriate for plastic won't handle it. If you're hankering for a jockey box, find some way to adjust your budget and swing the proper equipment. There are used set-ups and there's surplus stuff occasionally. Don't cheap it out...your beer will thank you. I've never regretted spending a little extra for something that works as it's supposed to rather than being a works-pretty-well solution that requires constant baby-sitting...and I'm a cheap SOB. And, yes...the biggest draw-back is heat conductivity. You have to chill beer instantly going through the tubing. Plastic is an insulator, not a conductor.
I do that for my 6 and 10 litre kegs. I've seen a bunch of examples of wheelie bin based solutions for multiple 19 litre kegs.