Well it is getting close to winter and if you live up north it just as well be already. We are only getting in the 40 for highs here in Utah. Heat packs are a life saver for us in the shipping business. If it were not for them we would not be able to ship live animals or plants this time of year. I have been real impressed by all the businesses that use heat packs to ship thier products during the cold season. I have talked with several people on the phone about how many to use for what size box. My rule is to use at least two on a box over a square foot. Plus I always have hole in the box so air can get in too. It takes oxygen to keep the packs warm so good air is needed. After that I look as to where the box is going and how long it will be traveling. Then I decide just how many to put in the box. Personally I have never had it get too hot but I suppose it is possible. If you have never tested one you should. I did and I was really impressed by how long they will last. Any ways just thought I would say a few words about them. Thanks for visiting the site.