E J Gumby
I did another Quick and Easy 10 litre BIAB brew last weekend.
Mashed for half an hour, then took my eye off the ball whilst  heating up for the boil.
Disaster! - I suffered a boil-over whilst my back was turned. I don't think the Hot Break had fully passed.
Had to switch off the heat to clean up the cooker, which took about 20 -30 mins.
Got the heat on again afterwards and got up to the boil, but didn't get the foam - up again.
Anyway, boiled for 30 mins, cooled it in the kitchen sink and got 10 litres in to the FV.
Now, when I've done these brews previously, I've been getting OG readings of about 1.044.
This time it was 1.039.
So the question is; was this low OG down to the interrupted boil or because I'd used a new batch of malt (Irish Minch Pale malt ) which I got from The Homebrew Company and wasn't as finely crushed as the Pale Malt I had been using from Rob at The Malt Miller,who puts a finer crush on it for BIAB brews.

If it was because of the Boil - over, I can just chalk it up to experience, but if it was beause of the coarse
crush how much extra malt do I have to use to overcome the lack of efficiency. 200-300 grams maybe?
(I was doing a Single malt - Single Hop - Comet using 2kg of Malt)

Any suggestions welcomed.

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Boilovers suck, dont they ? 😉

First question, did you top up your wort after the boilover to maintain the 10L final batch ? that would be the most obvious reason for a low OG - dilution.  1.039 vs 1.044 isnt much of a difference tbh - I doubt you'll be able to tell in the finished brew.

To know whether your efficiency is lower will require repeat batches - if you're still consistantly low after 3-4 batches, then adding in a few 100g's of extra grain will cover it.

Beer is like porn - you can buy it easily enough, but its so much more fun to make it [wink]
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Low og may have been boil related as a 30m boil won’t concentrate the wort that much. The interruption may have exacerbated this as you might not have had a vigorous boil for the full 30m,
Could be mash related too, if you had a few clumps etc. Or the crush. Or more likely a combination of minor problems adding up to 5 lost points.

As pinto says it’s not disastrous, make notes on each brew and adjust grain accordingly when you have a sense of your typical efficiency
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My experience with the Minch malt has been very positive but I found on several occasions that the HBC crush was not what I wanted. After getting whole kernals left in brown malt I ordered I chose to buy a 3 roller mill. Has hugely improved yield and I've reduced amount of pale malt going into all brews.
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Yeah I've used Minch quite a bit, got a few kgs in stock never had a problem with it, but I do mill my own grain and crush it fine.
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Boil over - bad luck, not nice!

Loss of efficiency was likely to be a combination of grain and loss of wort I guess. A finer crush does improve BIAB efficiency. 
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