Bitcoin miners give off a very low grade heat because it does not have a high temperature and when compared to quality heat sources, it gives off less heat as well. An engine for instance, has a very high temperature and is a high quality heat and gives off a boat load of heat. Heat reuse from a BTC miner is not efficient. There has to be a high heat differential between the heat that you are using and the ambient air for there to be a use case. That is not to say that you cant use some heat off a miner that could be useful when warming a bedroom or something like that. To justify that use case you would also need to have a high heat cost in the first place. There certainly are use cases, but to scale, its not an efficient heat source.