Some suggestion: 1),Do some insulation work between ground and bottom of tub. 2),You can consider to put the water into a pot, put the pot on the grille when wood burning. After the water in pot boils,just pour them into the tub. By this way,the speed of water heating in your tub can raise by 20%, means you can reduce 20% time to heat your tub! 3),Important is to immerge the outlet port under the water surface level.By this way,all hot vapor will blow into water and therefore to speed up heating tub.