They're no more dangerous than any other heating source as long as the tenant uses/maintains it properly.

My thought process for not including the major and minor repair savings, is because ideally I have already replaced all of the major long life common home expenses (roof, appliances, heating and air, etc).

Just to get the heat, water, gas and electric back on can have a lot of unknowns involved.

Block constructed houses also take less to cool/heat and the insurance savings is significant.

I prorated rent for the days they had no heat and had to use electric (48 hours) and credited for cleaning.

It sounds like you are paying the heat bill?  

My contractor realized they are having issues heating the house to their liking.

The heating was not cited.

Do they unit have central heat and air.

@Matt KeltyIn a normal 12 month student rentals I include as few utilities as possible but in cases of shared heating or something similar I cover that portion of the utility bills.