My plumber will install my hydronic system with a "glycol" mixture.  The boiler will supply the hydronic heat to an indirect hot water heater (or sidearm).  Since the hot glycol coil is inside the hot water heater, I want to specify that he use propylene glycol instead of ethylene glycol.  Ethylene glycol is toxic if ingested, while propylene glycol is "relatively" non-toxic.  Any thoughts from the forum?

I agree 100%. Anything ethyl based is not a good choice if it could be contacted by humans (or pets). Automotive antifreeze (ethylene glycol) in a puddle will attract birds, cats, dogs, deer, etc. because of the sweet odor and if they drink from it they most likely will experience a painful death. The coils in your system could eventually leak into the heated water, and taking a shower may not be a big deal (unless you enjoy drinking hot shower water) , but, running hot water for a hot beverage often enough definately could be fatal. Low doses over a long period of time (chronic exposure) could cause symtoms (depending on general health, etc.) that may be extremely difficult to diagnose, thereby lengthening the exposure even more due to lack of finding the cause.

Go propylene. Cost is not an issue when it's life or death.