Can they get too old even when left in the unopened package? I have some that are years old that didn't seem to work as well the last time I used it. Or maybe the heater on my unit doesn't get as hot as it once did. Has anyone ever been able to "wear out" a ThermaCell? Just want to be ready once the warmer weather arrives.

I have used unopened packages that were at least 3 years old and they seemed to work like new. My Thermacell is 7 years old and is a bit more prone to going out if it's not left to fully heat up before moving it. It's also a bit harder to light. But once lit it works as well as ever. I know about a half dozen other hunters who use Thermacells and none have mentioned any failures.

I'll be purchasing a new one shortly for this May's hunt and will use the old one as a backup.