We’ve just built a new Keezer (new to kegging) and I’m wondering how full we should fill the kegs.
The Keezer will hold three 5 gallon and one 2.5 gallon corny kegs and I’m thinking that they shouldn’t be filled all of the way to the top (i.e. 2.5g in the 2.5g keg) to leave some headroom for the CO2. However, I’m not sure 
I just fill mine till my secondary is empty, minus the gunk. I still have headroom for CO2.
There is a seam near the top of the keg that I think is the 5 gallon mark. I typically try to fill it up to there, and keep it below the level of the gas dip tube.
I try to fill them between the top seam and the bottom of the IN tube.
I get some satisfaction when I have exactly enough beer to fill the keg to the top.
I’m not sure how much it matters that you have any headspace in your keg.
For me, having some headspace is a necessity. I don’t keep the kegs constantly hooked up to CO2, so it’s the CO2 in the headspace that gets absorbed into the beer.
Still using those cobra taps, eh Denny? 
I use the high pressure and shake method. I don’t think the head space makes much difference this way.
definitly leave some some headspace if you are force carbonating. the more headspace the faster it will carb.
Why?
Let’s say I fill my keg to the top, then put 35 psi of CO2 on it and shake the keg for a minute or so until it stops accepting gas. What difference would the headspace make?
I don’t think it would matter with a regulator set as long as the surface area is the same.
I fill to just below the gas-in dip tube. Getting beer sucked into your gas lines is a pain in the rear.
The beer will continue to absorb CO2 until it has met equilibrium. If you apply 10psi of CO2 to the surface of the beer, it will absorb CO2 until the volume of CO2 in the beer is equal to the applied pressure of CO2 on the surface of the beer.
The rate of dissolution of CO2 in beer depends on:
Temperature and Pressure
Degree of under or over-saturation
Presence of mixing
Interfacial surface area between the CO2 and the beer (the more surface area…the more transfer of gas)
When the partial pressure of the undissolved gas meets the partial pressure of the dissolved gas, equilibrium has been met and the beer is then carbonated. This is all assuming that the correct amount of CO2 pressure at a given temperature has been met.
+1
My preferred method:
Beer should be chilled to 31-32 deg.
Fill keg to just below IN tube.
If beer comes out of the IN tube while filling then pressurize and draw a pint off.
Set regulator to 15lbs, connect the gas in tube, shake on side with gas tube toward the floor.
Shake for 3 minutes.
Put back in 31deg cooler for 1/2 hour to allow foam to settle.
Vent keg to 2-3lbs(ish) Put keg in keezer (still at 32 deg) and hook up to gas.
The beer will be drinkable immediately and will reach optimum carbonation in 2-3 days.
My $.02
Yep. I tried to figure out an alternative, but with a small house I just don’t have the space to do it any other way.
Taps on your fridge wouldn’t require any additional room. Cheap it ain’t though.
No, but a place to put the CO2 bottle does. Believe me, it’s not like I haven’t thought about it!
Tell you what, try it for yourself. Set aside one keg with 1 in of head space and one with 4 inches and see which one carbonates faster. The more headspace for Co2 to fill the faster it will absorb if you are going with top pressure. This goes for the shake method too, the more headspace the less amount of time you have to shake. If you leave NO headspace the beer won’t carbonate.
My guess is that what you’re seeing is the effect of a smaller volume of beer carbonating faster. In other words, the volume of liquid rather than the volume of the headspace is the critical factor.
The appropriate test would be two different size kegs (a 5 gallon and a 3 gallon or whatever) with the same amount of beer in them. Identical volumes of beer should carbonate at the same rate. At least that’s what makes intuitive sense to me.
As for no headspace, while I’ve not tried it, I have to think it would carbonate eventually if left hooked up to CO2 at constant pressure. However, if it’s completely full the surface area that the gas would be in contact with might be as small as the dip tube.
As to the shake method, no disagreement. There’s more surface area while you are shaking.