What would cause my efficiency to drop for this batch?

This is my 4th all grain batch that I've made, an AHS Belgian Trappist Dubbel
Malts are as follows

7.5lb Belgian Pale
3lb Munich
1lb Cara Munich
.25lb Special B

I mashed at 150 for 60 min with 1.25qt per lb of grain.
Then batch sparged at 175 with two infusions to make a 7gal pre-boil volume (7 gal because my BK loses A LOT in an hour). Gravity pre-boil was 1.042.

The grain came from the same shipment as my last couple of batches, which all had identical grinds. So I don't think it was the grind.

The past three, an American Cream Ale , an American Amber Ale, and a Belgian White beer all came out with efficiencies in the 80% range. The lowest being about 81% and the highest being 88%. All using the same methods.

This one however was only 66.74% on the efficiency scale.
Due to the low efficiency, my FG was low also at 1.061 when it should have been 1.069 (the recipe has 1.5lb dark Belgian candy syrup also, if the 1.069 doesn't make sense to you)

Did I not mash long enough for these particular malts or something?

What this latest batch the highest target OG yet? I understand that it is not uncommon to get lower efficiency with "bigger" mashes. Did you happen to test for full conversion (iodine test)?

Yes, it was the biggest batch yet... I didn't do an iodine test either. Is it normal for bigger mashes to take more time to convert?

Just thinking last night... How long does one generally let the grain bed settle when doing a batch sparge? Once I put in the 175 degree sparge water, I only let it sit a couple of minutes and then vorlaufed for about 5 minutes. Is that maybe not long enough?

krazydave said:

Just thinking last night... How long does one generally let the grain bed settle when doing a batch sparge? Once I put in the 175 degree sparge water, I only let it sit a couple of minutes and then vorlaufed for about 5 minutes. Is that maybe not long enough?

Click to expand...

For batch sparging, you don't have to wait at all. Just stir well, vorlauf, and drain.

I have lower efficiency with larger grain bills, myself. But not with only 12 pounds of grain. Generally my lower efficiency with larger grain bills comes from undersparging. What I mean by that is, say I have a 10 pound grainbill. I mash with 15 quarts of water, and then get out 3 gallons. I then sparge with 4 gallons, to reach my boil volume. But when I use 20 pounds of grain, I use 30 quarts of water, and get out 5.5 gallons (for example). I only need 1.5 gallons of sparge water to reach my boil volume. So, when I do a "bigger" grain bill, I often get 68% efficiency.

In your case, that's not a big grain bill at all. How was your crush this time? Edt- I see you say it was "the same". How did it look?

Yooper said:

In your case, that's not a big grain bill at all. How was your crush this time? Edt- I see you say it was "the same". How did it look?

Click to expand...

The crush looked good, the same as it did in the other grain bills prior to this one. I have two more kits also that came in the same shipment which all look the same. I can possibly post a pic if you think that would help?