I was thinking about trying to condition an ESB in the keg. Is head space a concern? I tend to put ~4gallons in a 5 gallon keg. Are their any other areas of concern or is it as simple as adding the same priming sugar into the keg that I would have added to the bottling bucket then dumping the first pint or two.
I have with good results. Used a “belly-band” type heater to maintain temp. Also made a ball lock/gauge setup to watch pressure and reduce if needed. Top the keg with CO2 and you should be good to go.
I don’t think you’ll get sufficient carbonation with just 4 gallons in a 5-gallon keg. You need to fill it up all the way. The beer may carbonate slightly but I’ll bet it’s weak and perhaps sulfury.
In any case you will of course need to force it out with CO2. You might get a pint or three from the natural carbonation but that is all.
Assuming you fill the keg all the way, you also need much less priming sugar than you might think. 1/3 cup corn sugar in 5 gallons is adequate. I know it seems odd, but this is based on experience of the old timers.
I think you can calculate the head space pressure for the temp you are going to store the keg. Then pump co2 into the keg at that pressure right after adding the sugar.
For example, according to Beersmith Mobile you need 28.8PSI head pressure (gauge pressure) at 70F to get 2.5 vols of CO2. Where 70F is the temp of the room where you will set the keg to carb up. So, add the correct amount of sugar. Then, pressurize the keg head space to 29PSI and put the keg in the closet for a few weeks.
That’s a pretty solid idea.