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Shipping in the Fall, Winter and early Spring is just about impossible without the use of heat paks. Even when using styrofoam containers and over night deliver, heat paks and the proper use of same can make the difference between arriving frozen simply arriving alive and arriving in superlative condition.

All heat paks contain pretty much the same chemicals, primarily an oxide of the mineral iron. When the iron oxidizes as it is expose to oxygen, heat is the resulting energy. The process is called an exothermic reaction as it give off heat during the oxidation of the iron (as opposed ot an endothermic reaction where heat is absorbed during the process). This is more than interesting to know (or maybe not interesting to know at all), by knowing how the heat pak works, you can pretty much figure out what needs to be present for the pak to work. If you use heat paks in a small box and seal the box "air tight" you just may limit the flow of oxygen to the paks and turn the chemical reaction off (or at least, slow the process).

With a small box, or to get the maximum heat release for the pak in a larger box, a few pencil sized holes can be cut through the top of a styrofoam box to allow air (and the oxygen contained in same) to flow through the heat pak and initiate the chemical reaction. The paks should be taped to the inside bottom of the lid of the styrofoam box (or inside a flap of a cardboard carton if you are not using an insulated box) with a good quality tape. We like to put a few layers of newspaper over the contents of the box so that they will be under the heat pak when the box is closed. The newspaper deflects the heat from being too intense above the product eliminating the potential for a hot spot. Also, the layer of newspaper will act as a safety device in case the tape holding the pak onto the lid fails and releases the heat pak into the product.

Heat paks all operate in a similar fashion.   They are initiated by opening the plastic packaging that the pak comes in. There is sometimes a mesh side to the pak and sometimes not...depending on the brand you are using. In all cases, the package is air permeable as oxygen is an essential part of the chemical reaction. If the pak has a mesh side, the mesh side should be facing the inside of the box. The air enters the pak through the mesh. The mesh needs to be unencumbered for the air to be able to enter into the pak. It takes some time for the pak to become warm. It may take just a couple of minutes for the pak to become noticeably warm and it could take 15 minutes...this is also a brand dependant issue. However long the pak take to begin reaction, most paks  that are used to ship fish and herps will reach their peak temperature at about 22 hours from initialization. The temperature builds to that peak temperature and then begins to taper off. The short acting heat paks used for handwarming and such will reach their peak temperature in a shorter amount of time or course and will "burn out" much more quickly. The amount catalytic chemical (frequently a salt of some sort) in the warmer will vary between rater use times. The real difference in the "rating" of the heat pak or the length of time which the manufacturer assigns to the unit is in the amount of time that the heat pak takes to come to to completely used state after it has reached it's peak temperature. That length of time may not be a true indication of the amount of heat that the unit provides during that tapering off period however...so don't think you will get maximum heat from a "60 hour" heat pak for 2 1/2 days...not going to happen. All heat paks begin to taper off after reaching their peak temperature. A long acting pak such as a 60 hour unit may actually feel only slight warm to touch at 60 hours. Depending on the temperature of the outside environment and the amount and effectiveness of the box's insulation, a long acting heat pak my even feel cold to the touch prior to it's rated time.

Two heat paks will bring the interior of the box closer to the rated temperature of the heat pak. If the box is large and a single heat pak is used and the outside temperature is extremely cold, two paks are recommended. Not that two heat paks will double the heat (they won't) but because large volume boxes have more surface area, the box will allow the allow the heat to pass more quickly through the insulation. Two heat paks will put more heat into the box more quickly...compensating somewhat for the heat loss of the larger box.

Contrary to some folks thinking (or lack of same) two heat paks will not prolong the process. In other words, two 40 hour heat paks started at the same time will not last 80 hours. We once had some fish sent to us by a inexperienced shipper. He sent the Angelfish using a 3 day service and put 6 handwarmers (essentially 6 hour heat paks)...and when the fish died he told us that he thought the heat paks would last. Silly person...and has since stopped shipping fish (a good thing for the fish world).

We recommend you use 40 hour heat packs for most situations. Using a 60 hour heat paks so that you can feel better about using a 2-day delivery service level is not a good idea. Heat and time work together in the shipping world...longer time is not very good with more heat. There are several things going on inside the bag that the heat will not help, but may actually hurt. Time is the worst of the shipping enemies...heat is a close second. Making the heat last longer still does not compensate for the chemical changes in the water created over time.

A sixty hour heat pak holds it's peak temperature longer because it generally has more material in the pak to oxidize. The challenge is that frequently the peak temperature is too hot for the critter in the box, and the additional mass fo the 60 paks can create an oven. Also, the use of a 60 hour pak is sometimes chosen so that a longer shipping window can be used...a false economic advantage as the best tool in the shipping process is the efficient use of time...speed is your friend.

Cooler water holds more oxygen than warmer water. No amount of heat can cure a lack of available oxygen. No amount of oxygen piped into the bag during closing will compensate for the warm water which is unable to physically absorb the gas. Heat applied for long periods of time (as in 2-day shipping) can be more harmful that good for most fish groups. There are a few exceptions (Killifish for example...Bettas being another) where the fish either have the capabilities to breath atmospheric are or have a metabolism such that low amount of oxygen do not have the same effect. However, most fish do not do well within a small closed system with heat applied. The only mitigation for this phenom is quick delivery of the fish, as in over night service levels.

Always use a heat pak rated for a period of time somewhat longer than the time the box will be in transit. A 40 hour unit also give the box some protection in the event that the shipping vendor has a malfunction/breakdown or weather related situation and the box is delayed an extra day.