Diluting with water immediately prior to fermentation

[Indiana Red]

My boil kettle can only hold 8G but I am making a 10G batch. It happens to be a low gravity Berliner Weisse so I dont have capacity issues with the mash tun.
But if I run all the hot side process as 8G and then split it into two carboys, I can top off with water at the same time I pitch.
I'll have a big starter so Im thinking the slightly higher risk of competing beasties in my top off water will be minimized.
What do you think is my risk level here or how can I best limit this?
What would be the safest source of top off water, (boiled and chilled would be best im sure)
Bottled spring? Distilled ? Does it matter much ?

Note: What will be going into the fermenters will be a boiled and chilled kettle sour (lacto) with a PH of around 3.4

 

[weizemanner]

If you can boil 8 gallons without boilover, I would do it and add 2 gallons of distilled water for 79c apiece. Wipe the cap with starsan. It's boiled and ozonated shouldn't need to do anything else.

Depending on your setup boiling more wort can drive off up to 2 gallons in steam and it's more likely to burn the bottom and darken the beer in my limited experience.

Why not just do 2 batches?

 

[schematix]

If you have an 8G BK the most you'll be able to boil would be about 7G with fermcap. 7G pre-boil is no more than 6G post boil. I think you're looking at 4-6 gallons of top up water.

 

[Indiana Red]

If you can boil 8 gallons without boilover, I would do it and add 2 gallons of distilled water for 79c apiece. Wipe the cap with starsan. It's boiled and ozonated shouldn't need to do anything else.
...

 

[dwhite60]

Make a smaller batch. Do it twice if you HAVE to get 10 gallons. This eliminates all your problems except maybe the extra time.

All the Best,
D. White

 

[chickypad]

I sometimes increase my batch size on low gravity beers by making 3 gals of wort on my small system and topping off to 5 gals in the fermenter. Even if you can only get 6 gals finished that's about the same ratio. There's no problem with this as long as you account for potential efficiency losses mashing a higher gravity wort, it's really no different than doing a partial boil extract batch and topping off. I just use water straight out of the tap and treated with campden - never had an infection. I assume the chloramine in the city water takes care of any nasties. I do spray the faucet head with star san to make me feel better.

 

[SleepyCreekBrews]

I occasionally make 15 gallon batches by topping off with 3 gallons of filtered water, you should be fine. ( my BK is a keggle)

 

[jlvgolf]

You don't need to do anything to the water if it's drinkable straight from the tap. When I started brewing I did partial boil extract batches and had to top up 2 gallons (out of 5) and now when I do full boil AG batches I sometimes overboil and have to top up. I've never had an infection and you shouldn't have to worry about topping up with untreated/unboiled/etc... tap water. Unless your water isn't normally safe to drink and then that's another story.