to sandman: a pound of water is a pound of water is a pound of water. while its true that the density of water at 34 degrees is greater than that of 150 degree water, the number of molecules in the water would be the same. the only thing that changes with heating is volume. also, another point that can be brought up, is that in an open system water evaporates, meaning that it takes less energy to heat up the quantity of water left. but thats a whole other concept.
also, would btu's be converted to joules or calories when converting to SI? im guessing joules, but calories would be more useful in similar applications.