For heating, I've found that reptile tank heating pads work well for mainting
CO2 bottles at a nice warm temperature. I put them in a box under the bottles
and stuff insulation around the bottles.

Why your CO2 is fizzling out so soon, I don't know except, perhaps the type of
yeast. I mix regular bakers yeast with a Canadian wine yeast. Bakers yeast has
a quick take off; the wine yeast last longer. If you need the brand name of
the wine yeast, let me know.

Hope this helps.