[Steve Ruch]

I'm planning to do a 1.100 three gallon batch then adding water to get a 1.050 six gallon batch.
Can I just add distilled water straight or do I still need to boil the water before adding it after the mash?

[Stevie]

In theory, bottled water is sanitary. I have added purified and distilled water without ill effects in the past.

[davidgzach]

I used tap water for years back in my extract days.  I have good municipal water though not well....

Dave

[mabrungard]

It should be fine to add distilled water to the kettle. It should be free of significant contaminants. I would be cautious if you were added that water directly to the fermenter since there is a remote possibility that it contains microbes.

[brewinhard]

What about dissolved oxygen levels in bottled water (distilled or spring) when adding it post boil?  Is there any in there?

[Steve Ruch]

It should be fine to add distilled water to the kettle. It should be free of significant contaminants. I would be cautious if you were added that water directly to the fermenter since there is a remote possibility that it contains microbes.

It would go in post kettle, pre fermentation, perhaps chilled to help get the whole batch down to yeast pitching temperature.

[santoch]

I know that you were saying you were considering using distilled, but for others who might be thinking about it, topping up with unboiled tap water is a prime cause of chlorophenolic off flavors. 

Make sure you remove the chlorine by boiling, using metabisulfates (powdered or campden tablets), or charcoal filtering (very slowly - too fast and the chlorine makes it through).
Chloramine is even harder to remove - boiling doesn't volatize it out.  filtering must really be slow but metabisulfates work better.  1 half of an aspirin sized tablet will treat about 10 gallons of water.

HTH-

[euge]

What's even better is if you can keep your jugs of distilled water chilled. You can even freeze one and cut it out of the jug to help cool the wort.

I'd say the risk of infection is minimal, less than exposing the chilled wort to the surrounding air.

[Steve Ruch]

What's even better is if you can keep your jugs of distilled water chilled. You can even freeze one and cut it out of the jug to help cool the wort.

Just what I was planning.