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. 

 If you need a high air flow with moderate heat, like for a green house or drying then its ok, shouldn’t be so mean about the grade of the heat, we don’t know what can be done with it… compressed steam in hot pipes also has issues.