A common misconception I hear often is the idea that if a mixture of two liquids with two different boiling points is held at a temperature above the lower point of the two, the more volatile liquid will be boiled off, leaving only the higher boiling point liquid behind. Or, more specifically, if a pot containing a mixture of ethanol (the alcohol some of us drink) and water is heated to somewhere just above 173.1ºF, the boiling point of ethanol, that in a short amount of time the ethanol will be driven off. Unfortunately, in reality you’ll be left with a hot pot of boozy water. It’ll stay that way until it slowly evaporates (not boils) away completely. At no point will you have a mixture which is alcohol-free. 

The boiling point of an ethanol-water mixture is determined by the ratio of the two. A mixture of 50% ethanol and 50% water (by volume) has a boiling point of about 180ºF. This is higher than pure alcohol, and lower than pure water. If this mixture is heated to 175ºF, it’s hot, but not boiling. Because it’s not boiling, there isn’t a considerable amount of anything being driven off. If we heat it further, it will reach its boiling point, but will not get any hotter. Unless we change the ratio of water and ethanol, the temperature will never rise above 180ºF. 

The interesting thing, though, is that when the mixture is boiling, and we do have a considerable amount of vapor being driven off, that vapor has a concentration of alcohol greater than 50%. It will in fact be somewhere close to 80% ethanol by volume. If we were to take that vapor and condense it back into a liquid, we’d have a mixture that’s still 80% ethanol by volume. And that, in essence, is exactly how distillation works!

If we look back at our boiling pot of alco-water, since the vapor being driven off is higher in alcohol concentration, it has the effect of reducing the ethanol concentration in the pot. Thus, it slowly becomes less alcoholic. And, in turn, its boiling point increases. The temperature of the boiling mixture will asymptotically approach the boiling point of water, but it will never quite reach it. Similarly, the concentration of alcohol will asymptotically approach 0%, but won’t quite reach it before the mixture is completely boiled away.

Pretty neat, huh? So what about that cheese fondue? First off, kudos for being the kind of parent that shares their fondue with their children. But be careful– even though you’re probably boiling, say, two cups of wine down to one cup, some of the alcohol will have been driven off but there will still be a non-negligible amount left.

A note to “cover my butt”: Distilling alcohol without a license is illegal in the US, as it is in most countries. Learning about distillation is perfectly legal, but practicing is not. I help legally licensed commercial distilleries measure and control their equipment, and I find it interesting, which is why I share distillation-related information here.

tl,dr: 36 ºF + 10 PSI + 10’ of 3/16” ID hose + patience = Perfect kegerator pour for most beer styles

Here’s a simple process I recommend for setting up kegerators at home:

1. Choose a temperature for your kegerator. I usually turn the temperature pretty low, to 36 ºF, because it makes some of the next steps a bit easier. If you prefer to serve at a warmer temperature, go for it.
2. Set your CO2 regulator so that your beer will be carbonated to the desired level, based on your preference and the temperature you’ve chosen. I usually keep mine set to 2.5 volumes of CO2, which works as a good average for the beers I make. Given my 36 ºF temperature, that means I set my regulator to about 10 PSI. I have the following chart from kegoutlet.com taped to the side of my fridge, next to my gas cylinders, if I ever want to change that: 

3. Choose your beer line size and length so that the beer pours at a reasonable rate. Using a line that is too wide or too short will cause beer to flow into the glass too quickly and foam, making it flat. Choosing a line that is too narrow or too long will cause beer to pour slowly. I like to err on the side of caution, and use about 10’ of 3/16” ID line. Normally it pours fairly slowly, but if I raise my pressure in order to carbonate a beer to a higher level, it means I don’t need to change my line to maintain a flow rate that keeps the beer from foaming when it hits the glass. The basic rule of thumb I use is to use 1’ of 3/16” line for every PSI the regulator is set to. I’m at 10 PSI with 10’ of beer line.

In bar/restaurant applications things tend to get a little trickier, because not only are the beer lines typically much longer, but keg coolers can be located on a different floor of the building than the tap, which affects the beer flow rate. With long beer line runs that travel a large vertical distance, it’s common to use “beer gas” instead of pure CO2. This is a mixture of CO2 and nitrogen. Because nitrogen is essentially not soluble in beer, it allows for higher pressures to be used without over-carbonating the beer. There’s some science behind all that, which we’ll save for another day.

In summary, a perfect pint requires the beer to be carbonated to the desired level and for it to flow out the faucet at a slow enough rate that it doesn’t foam in the glass and lose too much carbonation. This is a balance between temperature, CO2 pressure, and line length. Temperature is based on preference, CO2 pressure is based on desired carbonation level and temperature, and line length is based on CO2 pressure.

Here are a few common problems that can be found in home kegerators and how to correct them:

• Beer rushes out of the tap too fast, leaving a glass full of foam with little liquid: Usually either the CO2 pressure is set too high or the beer line is too wide or too short. I’ve seen a lot of commercially-available kegerators that come with about 3’ of ¼” line. That almost always needs to be replaced with a longer length of narrower line. If CO2 pressure increased for some reason (looking at you, children-who-shouldn’t-be-touching-dad’s-beer-stuff!), it’s a double-whammy, because not only will the beer flow faster, but it’ll be more highly carbonated if it had sat that way for a while.
• Beer pours out slowly, but foamy: This usually means there are bubbles in the beer line for some reason. A common cause of this is a kegerator with a tower on top where the taps are located. If the space inside that tower is warmer than the rest of the kegerator, the beer will warm up as it sits there, causing CO2 bubbles to form, since warm beer doesn’t carbonate to the same level as cold beer, given the same CO2 pressure (see that chart above). There’s no quick fix, but sometimes using a computer fan to blow cold air into the tower space can greatly reduce the problem. Insulating the tower can also help. If all else fails, just pour pints in quick succession to reduce the time bubbles can form.    The problem could also be that the beer was overcarbonated before being connected to the tap line, which can happen when trying to quick-carbonate a keg or connecting a keg which was carbonated by someone else (perhaps another homebrewer or a commercial brewery). Essentially the beer in the line has more CO2 dissolved in it than the exerted CO2 pressure can contain, and the bubbles come out of solution. There’s not a real quick fix here, unfortunately. The best thing to do is disconnect the beer line and release the pressure from the headspace on the keg every few hours until the problem is gone. If temperature differentials aren’t the problem, and the keg isn’t overcarbonated, and you can’t see any bubbles forming in the line, it could be a bad seal on the liquid line allowing CO2 to enter directly from the headspace. This usually means the o-rings on the keg fittings need to be replaced.
• Beer is pouring at a reasonable rate, but is flat: Either the temperature is too warm (check the chart above), or the beer needs longer to carbonate. Resist the urge to raise the CO2 pressure, as this will cause the beer to flow faster.
• Beer is pouring too slowly: When you just don’t have the patience to wait for a pint to pour, you’ve got two options: either shorten the beer line, or increase both the CO2 pressure and temperature. I usually recommend the former.

So what about actually pouring? This one is a bit straightforward. If everything up to this point is set up properly, pouring the perfect pint is as easy as this:

1. Hold the glass at a 45 degree angle, and place it just under the faucet so the faucet is in the glass, but not touching it.
2. Open the faucet fully and quickly. Any time the tap is partially open it will cause the beer to foam.
3. Allow the beer to fill the glass, tipping it upright once the beer approaches the faucet.
4. When the beer gets about 3/4″ from the rim of the glass, quickly close the faucet fully. The head should form and fill the rest of the space to the rim.
5. Top off if needed. If there isn’t enough head on the beer, crack the faucet slightly to add some foam. If there’s enough head on the beer but the level is low, open the faucet fully.
6. Clean your faucet. Nasty things happen to faucets left uncleaned.