Thanks to everyone who responded.  Turned out to be a great thread.

After attending a workshop on hydronic radiant heating this weekend, I'm
somewhat interested in the possibility of using this.  I'm shooting from the
hip here - so if I'm missing something. Please let me know.  Humor me though
- this could have potential.

What if there was a submersible water heater/pump that would heat your water
to say.... 130 degrees.  Not hot enough to melt your tank, but enough to
keep it really warm.  This (130-140 degrees) is roughly  the temp that goes
through the radiant piping in a home.  Then you could use this pump to
circulate the hot water through KITEC tubing that you lay under your carpet
in the cabin.

This is basically the concept behind a hydronic radiant heating system in
your home.  If you did this in the van, it could be a permanent
installation, and all you have to do is turn on the heater and you serve two
purposes: keep your water warm and unfrozen, and heat the van safely at the
same time.

Drawbacks I can see right now:
1. The water you get from this sink would be really hot and I don't know if
you can get a pump/heater that you can use on potable water.  (But you don't
really want cold water in the winter anyway.)
2. To install the piping correctly, you would fill in the spaces in between
the pipes with plywood or something similar, which might suck in the case of
an accident.

Now that said - it's not nearly as simple as an electric blanket or a lil
block heater from WalMart - but could be a realy cool (er... Warm) way of
keeping your water handy and keeping the van warm.

Any thoughts/comments/noticeable problems with this?

--eric
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'85 GL Camper (Moby)
San Francisco, CA