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Hi all, I am wondering if my low apparent attenuation levels are normal or indicative of a problem in my process.

The following is true for each brew:
- I brew using Briess liquid extract with a partial mash, partial boil (11L boil volume diluted to 21L in the fermenter)
- I aerate using a KegKing Vortex aerator for 2mins
- I make the recommended starter size, using a stir plate
- yeast is usually 2-4 weeks old, stored refrigerated
- my fermentation temperatures are controlled using a fridge and an external temp. controller. I pitch 1 - 2°C below fermentation temp and raise it up a few degrees half way through fermentation
- I add 1/8 tsp yeast nutrient to the starter, and another 1/4 tsp to the boil

Some recent brews:
Oatmeal Stout, Wyeast 1968, I got 64% attenuation (Beersmith estimated 71%, Wyeast advertise 67% - 71%)
Foreign Extra Stout, Wyeast 1028, I got 63% attenuation (73 - 77% advertised)
Wee Heavy, Wyeast 1056, I got 71% attenuation (73 - 77% advertised)
Irish Red Ale, Wyeast 1084, I got 69% attenuation (71 - 75% advertised)
Dusseldorf Altbier, Wyeast 1007, I got 72% attenuation (73 - 77% advertised)

Is this normal? If not, where could I be going wrong? Thanks in advance!

I'm not familiar with Briess extract but some extracts are less fermentable than others.
Also, while attenuation in the mid 60's to low 70's is lower than expected, the manuf and Beersmith estimates are just that, estimates.
Try doing a fast ferment test of your wort. That will tell you exactly how much attenuation you can expect from your wort and yeast combination.
To do the test, after you pitch the yeast, take a small amount (80z or so) of the wort and place it in a covered container. Keep it warm, shake it up whenever you think of it. The sample will ferment out pretty quickly and then you can measure the final gravity. 

I'm not familiar with Briess extract but some extracts are less fermentable than others.
Also, while attenuation in the mid 60's to low 70's is lower than expected, the manuf and Beersmith estimates are just that, estimates.
Try doing a fast ferment test of your wort. That will tell you exactly how much attenuation you can expect from your wort and yeast combination.
To do the test, after you pitch the yeast, take a small amount (80z or so) of the wort and place it in a covered container. Keep it warm, shake it up whenever you think of it. The sample will ferment out pretty quickly and then you can measure the final gravity.

Thanks for the reply. Yes I've tried a few different manufacturers for the extract, and they all seemed just as fermentable as each other. And yep I definitely plan to do some a forced ferment and diacetyl test on my next brew - that will at least help me narrow the problem down a little.

When I was extract brewing I had the same issues with attenuation. I started subbing in some plain old table sugar for a portion of the fermentables which made a huge difference. It looks like that tip is included in dmtalyor's post...

Are you using dark malt extract?  That could be an issue.

When I use extract, I use Breiss Pilsen DME.  It's the lightest and most fermentable extract I have encountered.  To make a stout, I simply use all the dark grains you would normally use to get the color I want.

I've had no issues with attenuation with Breiss Pilsen DME.

Yes, this is your problem:

- I brew using Briess liquid extract

Try dry malt extract instead.

I'll just leave this here as well:

Thanks! Why is DME more fermentable?

Many roast malts contribute low fermentability that is in the 40 to 60 percent range. So you can expect to have your dark beers not attenuating quite as well as a pale beer mashed at the same temp and fermented with the same yeast.

True, but I'm experiencing low attenuation on beers with no specialty malts at all...

Thanks for all the helpful replies, I'll try using DME on my next brew and do a forced ferment test and let you know how it goes.