Of interest, the BTU content of wood varies significantly by type, but not by weight. Seasoned hardwood is considerably heavier than Sitka Spruce by size, but by weight they have similar BTU output when burned.
I would never ever burn spruce, pine or hemlock or ANY softwood. Only seasoned oak, cherry rock maple, locust or hickory
Ash is awesome to burn-if you can get it dry enough, any decent wood from a deciduous tree will burn and create at least some heat. I've been burning dried tulip/yellow poplar this winter, it's been fine, but ash lasts longer & generates more heat. With the Emerald Ash Borer epidemic in these parts, there is so much standing dead & recently cut down white ash available for free that I would likely be able to use it for years. Still have a dropped ash at my Mom's with a 4 foot diameter trunk waiting for me to cut up & split when the tulip is gone. As above, NO PINE, spruce, or other evergreens!