Double decoction temperature problems

[stadtbrau]

So I brewed a Czech pilsner this weekend using the hochkurz mash schedule detailed by Kai on his wonderful website.
I mashed in at 1.5qt/lb and pulled my first decoction at as close to that consistency as possible. I gave it a quick stir before I started scooping. After about 20 minutes at 158 I got it up to a boil and let it go for another 20. I dumped it back into the mash and the temp went from the temp I was holding of 144 and only reached 150. I was hoping for 158. I changed the temp on the controller and got to 158 in a few quick minutes but still want to know what happened.
I used the same calculation for my next decoction to go from 158 to 168. I hit that temp exactly which was nice. The second decoction was just wort pulled from under the screen.
Any ideas why my first decoction didn't raise my temp enough? Maybe I should have been checking the temp of the first, thick boiling decoction instead of just visually seeing it bubble. That is really all i can think may have happened.
Thanks for the help

 

[BigEd]

Here are a couple of possibilities:

Decoction novices often don't pull enough mass from the main mash for their boiling fraction. The old r-o-t with decoctions is 1/3 for your pull. That means 1/3 of the mash's total weight. Let's assume for simplicity your grain bill was 10 lbs. With 1.5 qt water per lb you added 30 lbs of water for a total mash weight of 40 lbs. One third of that is 13.3 lbs. To compound this problem 1/3 is not always enough I wind up using 40% as my default. So, how much did you pull from the mash for the first decoction?

A second common problem is technique. Just because the liquid in the decoction vessel is boiling doesn't mean the solid material (which is, or should be the majority of your pull) has reached the same temperature. It takes a while for all of the solids in the decoction boil to actually reach boiling temperature. You also need to stir well, not fast or vigorously, but just about constantly to help the distribution of heat as well as limiting the chance of scorching.

These are not the only reasons you may have missed your temp, just the most common.

 

[Yooper]

I mashed in at 1.5qt/lb and pulled my first decoction at as close to that consistency as possible. I gave it a quick stir before I started scooping.

I think this is it- first, you have to stir well so it's thoroughly mixed. Then, pull only thick wort- mostly grain with little liquid- for the decoction. That makes a huge difference, and like @BigEd said, you want to pull about 1/3 of the mash.

 

[stadtbrau]

I didn't pull anywhere near 1/3. I used a formula to come up with a volume. As memory serves it was slightly more than a gallon. So that is more like 1/5.

I used the same formula to come up with roughly a gallon for my second, wort only, mash out decoction and that temp rise was right on.

I will be trying this again pretty soon. I will take a larger volume as well as check the temp of thick decoction boil. I'm pretty curious what that temp was even though it appeared to be gently boiling.

 

[BigEd]

Next time try this:

Mash in thick, 1 qt water per lb of grain. That will mean that 1/3 of the weight of your mash will be the same as the weight of your grist bill. When you pull the decoction it should be thick, let most of the liquid from the grain fall back into the mash before adding the pulled fraction to your decoction vessel. Pull at least 1/3 of the mash's weight, don't skimp and don't be afraid. You can also add a quart of water to the decoction vessel and get it boiling before the pull. That will give you both some insurance against scorching and a head start on temperature. YMMV but if done correctly a decoction step should add 20F to the mash temp when it is mixed back in after the boil.