donelurkin, I'd be hard-pressed to explain chemically exactly how an FRH works (but that's a good idea for a future website page) but my understanding is that salt and water are two of the ingredients that need to be in there for the exothermic reaction to occur. Standard military FRH's have the salt included in the FRH so all you have to do is add water.
In order to make civilian MREs less hazardous to ship, the FRH developer (Innotech/TrueTech/Zestotherm) removed the salt from the heating element.
So my guess is that if you only added plain water to one of these heaters, you just get a cold, soggy heater.
Don't forget what happens when you ship a cargo container full of FRH's and get a moisture-leakage problem:
Meals: Ready to Explode