A 60K BTU heater doesn't run 24/7.  It doesn't even run 20% of the time.   Buying 12 rigs to run them 20% of the time is a incredibly expensive idea.  The only way to be more wasteful would be to burn money to produce BTUs.

You are still going it backwards.  Find out how much heat you need FIRST (i.e. we need x BTUs per day peak) and then figure out how many rigs it will take to produce that.  Note this is academic because GPU mining is mostly dead. 

Do you CURRENTLY have heat in your house?  If so did you have it all last winter?  If so do you have a copy of your utility bill?  You can estimate the fuel (if natural gas/propane) used and thus the amount of BTUs used and work backwards from that.