Back to the Beer Simple Mail Bag this week! Recently, a brewer asked whether he/she could use a kegerator as a fermentation fridge, and vice versa. The answer, of course, is "of course." But just because you can do something doesn't mean you should do it.
Quick answer: yes, it can be done, but it's probably not worth the time, effort, and beer impact.
I feel like we need to get this out of the way first: if the choice is between "no temperature control for fermenting beer" and using your kegerator to do the job, then this is a no brainer: use that kegerator.
I don't know that there's anything more valuable to beer quality than temp control during fermentation, so if you're choosing between leaving your fermenting beer in the garage and letting your kegs warm up a bit, then by all means, choose the latter.
If you're going to do that, though, you need to be aware of a few things.
A Question of Pressure
First, when you dial that temperature up, you're messing with the CO2 balance on your draft system. You can either adjust the CO2 to rebalance it (and serve your beer warm, temporarily), or (probably the better idea) pop the gas lines off of your kegs and just let them maintain the pressure they already have in them. As they warm some CO2 will come out of solution into the head space of the keg, but it'll re-absorb when you chill your kegs down again. Once they cool off, re-connect them to their gas lines, and they should serve just as smoothly as before.
When in doubt, don't adjust more than one thing, and since you're already adjusting temperature, probably best to leave the CO2 regulator out of it. When you cool down again, it'll be right where it needs to be (as long as you return to the same temperature!). Disconnect from the CO2, ferment at whatever temps you want, then reconnect your kegs when you re-set the temperature back to serving temps.
A Question of Stability
You should also be aware that you're increasing the level of flavor instability you're likely to see in your finished beer. Letting those kegs rise and fall in temperature is a mild form of beer abuse, and when you let them warm, you're increasing the rate at which staling reactions occur (and, assuming you have some small degree of contamination, the rate at which your beer is developing off-flavors).
Don't get me wrong - it's still better than letting your beer ferment at "whatever temperature it is at that moment in the basement or guest bathroom," but it's a cost you should be aware of. If you have lots of pull-through and your kegs are rarely on for more than a month or two anyway, then don't worry about it. If you take a little longer to work through your kegs, though, it's a risk.
A Question of Time and Money
What's your time worth to you? This is where the multitasking argument breaks down for me.
Coordinating and choreographing all of this not-hot keg-and-fermenter action is a real time suck. It'll require you to plan ahead so you have cold beer ready to serve at a time when you don't need temp control for fermentation. If you don't have a massive kegerator it'll probably involve removing and re-inserting kegs. It certainly puts more strain on your draft system and its connections (changes in temperature can cause CO2 leaks as fittings change in size and fit and work loose).
Is it worth it? Honestly, probably not.
No-frills refrigerators are inexpensive, and depending on where you live they range from "free" (watch the curb, especially for discarded freezers that can't freeze anymore - they may still have plenty of cooling power to hold your beer at lager-fermentation temps!) to $50 (off of something like Craigslist) to $150 (new and delivered from a big-box or mega-hardware store). It's a small price to pay for flexibility.
A Question of Tolerances
At the end of the day, the answer to this question is, "yes, you can multitask with your kegerator," but only you know if it's worth it to you.
Done deliberately and conscientiously, sure, it can be done. But for me, it's worth the max cost of $200 (for a decent-sized new-and-delivered chest freezer and a decent temp controller) and 5-6 square feet of floorspace to run a separate fermentation fridge.
So, your call. Keep those questions coming to firstname.lastname@example.org. Always happy to hear from you all!
Keep it simple.