Store it in your bladder. L
1.I need to keep it at the same temp as hot tea 2. -20c 3. one bottel, but just wonder how to keep 1 gall as well
. 42 . . How much water? A pint; a gallon; a firkin? . How warm is warm? Body temp; just below boiling? . How long do you need to keep it warm? . What power source(s) do you have available? . Is that -20ºF or -20ºC? . Enquiring minds want to know!
The most effective way to do that is a thermos flask.
+1. Nothing (in my experience) beats a thermos for maintaining temperature of heated liquids without injecting outside power. So like packing your cooler with frozen foods in the summer months when you go car camping to save on ice costs, packing a thermos with hot water during the winter is a great way to keep it from freezing...(if you want cold water once you're wherever it is you're going, just pour some in a cup and let it cool) for a while anyway. eventually even the thermos gives way to nature
Keep a bottle or two under your coat.