I just filled my barrels with boiling water, sealed them and let them sit overnight... then hit them the next day with potassium sorbate at the recommended levels. The barrel will insulate against the heat loss - mine were still warm to the touch 24 hours later.
The boiling water will strip some of the barrel character out so if you expect a bourbon note - you might hit it with a 5th of cheap stuff - like Ancient Age and let that sit, rolling it to cover the interior for at least a few days, maybe a week. I like to stand the barrel on each head for 1/2 day with the booze in there...
You can always use the bourbon in a mixed drink later!