Two Questions on a Less Successful Start...

Lower efficiency can be due to a number of things but I doubt a temp drop during the mash is going to hurt it that badly, especially if you hit your temp at the beginning.

16 hours is not a lot of time, just make sure the carboy is in an area where the temp is good for the yeast. I had a beer take 2 days to take off just a few weeks ago. I agitated it and moved the temp up a couple degrees and had vigorous fermentation by morning.

 

1): What was your mash temp? It's possible a low temperature could affect efficiency, especially if you intended to be around 149 and it dropped down to 146 or below. Enzyme activity just takes more time at the lower temps. Do you batch or fly sparge? It's possible you had some channeling issues. And at the risk of making this way more complicated than it needs to be: you might look at your water and how that particular grist might interact with your water chemisty. It's possible your pH got a little out of whack, although that doesn't seem especially likely, given that this was a darker beer.

2): Could be anything. Not time to worry, yet.

 

For your consideration, below is an extract from a presentation Kai Troester provided at the 2010 NHC:http://braukaiser.com/download/Troester_NHC_2010_Efficiency.pdf

● Temperature: The hotter the better for efficiency (until 75-80 C / 167–176 F).
But temperature is needed to control fermentability. The lower the temp the
longer it takes for full mash conversion


● Time: the longer the better. But there will be a time when conversion
efficiency doesn't change or changes only little. Extended mashing extracts
more tannins and mash may spoil.


● pH: alpha amylase optimum is at pH 5.6 – 5.8 But: other benefits shift the
optimal mash pH towards 5.4 – 5.6 (room temperature sample)


● milling: the finer the faster and more complete the conversion. But: mash
tun design and lauter practice set a limit to how fine one can go. The
absence of channeling may allow finer grists for batch sparging than for fly
sparging. Malt conditioning can mitigate run-off problems

.
● mash thickness: thinner mashes convert faster and more easily. Thick
mashes impede starch gelatinization and inhibit alpha amylase activity. Even 6 l/kg (3 qt/lb) is not too thin.


● mash schedule: the more intense the better the conversion. Single infusion
-> least intense. Decoction mashing -> most intense. But: you should not
have to resort to triple decoction mashing to fix low conversion efficiencies.


● “Mash out”: raises temp to supercharge alpha amylase. Can convert more
starches (i.e. raise conversion efficiency) if conversion was not complete
before this point


● Diastatic power: The darker the malt the lower its enzymatic strength
(generally). Only a problem in Munich malt or adjunct mashes. Some light
malt helps as “enzymatic boost” in dark malt mashes.


Cheers!

 

Did you mill your own grain? What kind of mill? In my experience the first thing to investigate when you get a poor efficiency is the crush.

 

Things can happen at the LHBS, you never know who has been monkeying with the mill. What kind of mill do they have there?

I've had the gap change in my mill unknowingly and I didn't realize it until after the mash. You should get familiar with what a good crush looks like. So if it doesn't look fine enough for you, you can adjust mill and run it through again. I crush my malt fine, I get tons off flour, but depending on your system that isn't always a problem.