Although heat packs are something I still keep in my bug out bags and even vehicle kits, I’ve never bothered to test them until recently. Unfortunately, I’ve found I had that problem with some of my equipment that I have purchased solely for survival; obviously not the smartest plan of action… I don’t recommend you be as complacent as I was.
Anyway, a while ago I was sitting on the couch watching a movie while I was going through my bug out bags (something I like to do regularly) when I came across a few extra heat packs I was considering throwing out because I simply didn’t see a need for so many. Rather than just tossing them I decided to try them out. They’re easy enough to use, just break the bag and shake; doing so exposes iron filings and the reaction with oxygen in the air produces heat.
Unfortunately, I was not impressed. After messing with them for almost an hour (including periodic shaking to remix the contents) I found that the heat packs produced only minimal heat at best in a relatively warm climate-controlled environment. I would not want to rely on them when it was truly cold and my fingers and toes were numb. Granted, I only tested one brand of heat pack that was rated at 10-12 hours, so, maybe another brand and/or a larger heat pack would have worked better. I don’t know. Regardless, I would imagine they’re made with similar ingredients or perhaps they really did expire. I don’t know the answer for sure.
My advice would be to purchase better clothing, gloves, shoes, etc and don’t cause yourself to find the need for an inferior tool. Of course, you could also find an alternative method to heat your extremities such as carrying a small Sterno can and lighter that would warm your hands faster albeit for not as long. You be the judge.
Incidentally, there are reusable heat packs that I’ve heard good things about.