Recently, I've had a streak of hot fermentations. My primary vessel nears 80º, and sometimes more.
With my latest batch, I've moved to a much cooler part of the house. I expect it to consistently stay around 69º.
However, out of curiosity, if it were to fluctuate, what would be too low? And if it is too low and fermentation is suspended, does this pose any further issue than a longer fermentation period (after bringing the vessel back to ~70º)?
Check the yeast maker's web site for preferred temperatures. Yeast can work in a range, many down to 60°F. Pick a yeast that can handle the temps, hybrids like California ale.
If fermentation is truly stopped, not just slow, you need to wake the yeast up again. When temperatures return to the preferred range shake the carboy up to rouse the yeast and it should pick back up again. When you shake it CO2 will come out of the beer and your airlock might sputter all over the place.
Optimal temperature ranges vary slightly yeast to yeast. So a check with the yeast supplier will help with that data. But for most Ale yeast getting below 60F is probably too low. Maybe you still get fermentation, but the lower the temp gets for most ale yeast the cleaner it ferments. So if your making something with an english ale yeast, you might miss out of some of the intended flavors that strain makes.
When temperature drops too much for the yeast this can signal them to start to flocculate out prematurelty. Again, certain english strains flocculate so well that reheating the fermentor may not be enough to get them active again. If you suspect that the yeast has significantly started to floc-out due to low temps you'll need you rouse them manually.
You can rouse yeast with a sanitize racking cane and stir a bit, you can literally swirly the fermentor some if you have enough head space to do it. Some brewers with access to pressurized CO2 (from a kegging setup) will actually blow CO2 through a racking can into the cake to get the yeast up again.
If you do need to rouse your yeast you should warm up the fermentor first, then do the rousing. Otherwise, the yeast will settle right back out and not "wake-up".
During the winter our house stays very consistent around 69 - 70 degrees. It sometimes drops 2 or 3 degrees and I have not had any issues as far as I know. I've never experienced huge drops in temperature though. I leave the carboy on our concrete floor and I'm assuming that helps maintain a consistent temp.
I'm amazed that your 80°F fermentations finished at all. That is pretty high for almost all the yeast strains I'm using. I try to keep mine at a maximum of 74°F which is the upper limit for most of my yeasts. Fermenting at these temperatures usually results in a high final gravity and a lot of diacetyl and residual sweetness. Depending on the style, this may be okay. If that space in your house is constantly around 69°F and not subject to a lot of airflow (doors opening etc), you will probably find the fermentation running 2-3°F warmer due the heat produced by the fermenting yeast. Still keep an eye on it, you may find it still running hot and you certainly won't have issues with yeast dormancy at that temperature.
