I would avoid any version with cold pipes inside the drain pipe myself. You'd soon have these problems:
- soap, hair, grease build up round the cold pipe, making for much reduced heat transfer.
- This muck-catching arrangement causes a pipe block
- It also causes a foul smell outside, if you have an open drain, since much muck is perpetually trapped in there.
- You cant clear it by rodding.
- And if Murphys law holds true, your drain rod will jam solid inside the piping.
If you must use a pipe-in-pipe, have the drain as the central pipe.
I would be hesitant to make them this way as diffrential thermal expansion could cause a lot of stress, and solder is a very soft metal. Wrapping microbore round the drain pipe eliminates these problems, though it does mean a fair amount of soldering. Its possible it may work ok unsoldered, eg with some grease and zinc oxide wiped where the pipes touch, and just some solder at each end, or maybe even large a stainless jubilee clip.
BTW theres no point insulating the thing once made. If you've got insulation to use, put it somewhere on the hot pipe.
NT