I have a 5 year old glo-warm ventless

Since the thermocouple body is made of metal, and often these have a copper or brass sleeve, that *should* provide good grounding for the thermocouple.

Obviously some engineer at the heater company did not agree with this, so he added an extra ground wire.

Most gas valves are not all that sensitive to exactly how many microvolts the thermocouple feeds to it.

My guess would be that if there is a cheap metal bracket that holds the thermocouple, such as unplated steel or tin, the extra ground wire counteracts the effects of corrosion over time.

So if the heater were stored in a damp garage, the thermocouple connection would get corroded, and the heater would not stay lit. The fix is the extra ground wire.

There are two types of failures with thermocouples: the safe-failure is when the gas is shut-off, even when flame is present, and the unsafe failure would be when gas is not shut off, even when no flame is present.

If the thermocouple somehow was not adequately grounded, then the most likely failure would be a 'safe failure' , such that the burner would not stay lit.

You could rig up some sort of extra grounding wire, but unless the part is subject to extreme vibration or is in an extreme (wet/salty) environment, it's probably not needed.

Of course, to be safe, do a flame blow-out test after you install the new part. While it's highly unlikely that an unsafe failure could occur, better safe than sorry.