For a 225 liter barrel you'd need 3 to 4% per year to cover evaporative losses, plus ?% to cover tasting losses.
For a smaller barrel, I'd estimate a minimum of 6% to 10% or more if you taste with friends frequently.
Making an offdry wine. 112CJ Been there, done that. Cold. Definitely not guaranteed to work. At best, the SO2 will slow the yeast down...
As with many other things, bigger is better.
A couple of other caveats: (1) Always keep your barrel(s) topped completely full. Headspace is where bad things happen. (2) Be sure to maintain your free SO2 - and remember that free SO2 slowly drops over time in a barrel. That's not the case in carboys. You need to check the free SO2 periodically (every 2 or 3 months) and adjust it as necessary. (3) No matter what anyone tells you, DO NOT do any chemical "pre-conditioning" of a new barrel! You'd just end up dumping the best part of that expen$ive oak flavor down the drain. When you first go to fill the barrel, fill it with cold water to see if it's tight. If it leaks, let it sit until it stops leaking. When you see that it isn't leaking, empty the water immediately and fill the barrel with wine (or juice if you're doing a barrel fermentation). (4) If at all possible, keep your barrel(s) full of wine. If you have to empty a barrel, you need to blast as much sediment and tartrates from it as possible (hot water through a pistol grip nozzle into a bunghole-down barrel works pretty well). When the water runs clean, drain the barrel, burn a sulfur disk in it and bung it tight. Before bunging the barrel, put a couple of layers of Saran Wrap over the bung to protect it from the SO2 vapors. (SO2 is rest on rubber.) When you need the barrel again, it'll be sweet, but it may have loosend up enough to leak. Repeating the rinsing and gbutting process every couple or three months can prevent that from happening. Storing the empty barrel in a cool, relatively humid place helps too.
HTH
Tom S
