Chemical heater packs for boots

I have been using Grabber Toe Warmer chemical heater packs to keep my toes warm in my AT boots, not very effectiveyly. The package says they should last for 6 hours. When I tried them out, they only last for 3-4. My packs are about 3 years old. Would fresher packs last longer? The ones I have been using have not been activated previously, the package has not leaked and they are fresh. Do other brands of heater packs last longer? Does anyone have other suggestions?

I am thinking of using my hotronics in my AT boots, but the battery is bulky and heavy.

I just finished my box from last year a few weeks ago and grabbed a new box from Costco.
My unscientific analysis from the last few days say the new ones are warmer and last longer...

In similar unscientific experiments with handwarmer packs, I've found that older ones are usually less effective. I imagine that the packaging is not 100% impermeable so over years a tiny bit of air will leak in diminishing the effectiveness.

how long do your new ones last?

These dry heater packs require oxygen to work. They rely on a relatively rapid oxidation process that release heat as a byproduct of the oxidation. In a coat pocket, there is plenty of oxygen available. In a boot, there is very little oxygen available, especially at the toes since your boots should be tight and prohibit air from exchanging.

Shelf life is dependent on oxygen permeability of the packaging. Generally, foil (Mylar) is better than just plain plastic. Thicker plastic is better than thinner.

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Originally Posted by harpo-the-skier

how long do your new ones last?

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thru a day of lift riding which up here is 6 hrs max

I find I generate enough heat touring in AT boots to not need chemical heaters but if I run the AT boots at the hill I use the paks and a bootglove which will work even with tech fittings

tele babe got me a forced air boot dryer for Xmas so I crank that thing up in the AM to warm the boots up before hitting the hill you want the boot warm and the boot glove stops heat loss