Grant and Bob's recommendations are good, but perhaps I can add something.

Is everything stainless?  If so, your choices expand exponentially.  If not,
you need to be asking for something non-corrosive.  Also, keep in mind that
what you need worry about is 'freezing', not 'near-freezing'.  In other
words, if the liquid you use changes to a near-solid (or, 'almost' freezes)
that should be ok, but you don't want it to 'freeze', and change to a
solid.  That difference can make a huge difference in the cost of your
'solution'.  Both in terms of the volume of material you need (think volume
of antifreeze) or in terms of the material itself.

I once learned a valuable lesson in these regards.  We had a hot water
heater in a barn that we had faithfully been draining and had been also
blowing out the lines.  This was a pain in the neck as at least 2-3 times in
a winter we'd have to fill the heater and use the hot water.  On a chance
visit a friend was is an engineer asked 'why'?  He had noticed that the
heater ran on propane, with a pilot light, and proceeded to explain that the
water would not freeze as long as the pilot light was lit.  He explained
the water might change to a slurry (or, near freeze), but would not
freeze.  As long as there was a smidgen of liquid, there would be enough
room for expansion so that the tank would not burst.

He also explained that it was not necessary to use air to blow out the
lines, and all we needed to do was to get out 'most' of the water so that if
the remaining amount froze there would be empty pipe enough to handle
the resulting expansion of volume with rupturing.

With some trepidation, we followed his advice and never had a problem.

-- 
Lloyd Spear
Owner Ross Rounds, Inc.
Manufacture of equipment for round comb honey sections,
Sundance Pollen Traps, and producer of Sundance custom labels.
Contact your dealer or www.RossRounds.com

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