the hand warmers you are using start heating to their max temp as soon as you open the package. they usually reach the max temp in about 10 minutes or so. but be careful, some of them have a max temp upwards of 140 degrees! check the back of the pacage to see what yours does. most of them will hold the temp for about 5 hours.

i have been using the (toastie toes) heating pads that came with the PISSINATOR. their max temp (during peak) only goes up to 100 but the average temp is about 96; so i don't have to worry with it overheating. and they sell replacement pads on the website too.

hope you get this in time. good luck!