I brewed a porter on Wednesday evening, using White Labs Irish ale yeast that was washed from a previous batch 2 mos ago. I made a 2.75 L starter (per mrmalty's calculator) and chilled/decanted before pitching. I pitched the yeast at 66*, aerated, then put the fermenter in the closet I use in the basement. This closet with nothing in it will sit right around 60* and I have a tiny space heater with a thermostat set to keep it at 66*.
Thursday morning, about 10-11 hrs after pitching I had airlock activity and the fermometer on my fermenter said the beer was now 68*, but the closet was still 66*. When I got home that evening and checked on it ~24hrs in my fermometer said it was at 73*!! So I moved the fermenter to an unfinished part of the basement with concrete floors, and let it sit overnight. This morning it was back down to 68* and I put it back in the closet...
I guess my main concern is will the heating up to 73* will cause much in the way of off flavors?? in particular I'm concerned with diacetyl with this yeast.
My other concern is that there was basically no airlock activity this morning which makes me wonder if I caused a problem by cooling it back to 68* overnight? I know airlock activity can be a big variable, and doesn't mean anything, etc... its just that in my experience with my system the airlock usually goes a lot longer.
Overall I think this is a lesson for me in temp control and only monitoring ambient instead of actual liquid temps...
Any input would be appreciated, thanks!