Rodney, thanks for your detailed feedback, I am sure it is helpful to all. As clearly stated, the wall heaters do a fine job of keeping the rooms toasty in the basement even in the coldest nights of the year, as those you were staying for, if you follow the simple instructions, that are clearly stated, to keep the doors closed. The wall heaters are indeed safe to leave on 24hrs a day when required, but are ineffective if the doors are left open (simple physics that heat rises)
Regarding the dirty sheets, we never did find the supposed dirty sheets that your guest left, I can assure you 100% that the sheets were clean out of the bag and fresh on the bed before you arrived.
It was a mistake that both space heaters were left for you, I only leave one out for the reason that you describe.
This condo is rented almost every night of the winter, and I have not had issues with guests reporting problems as above. I think it was an unfortunate situation of late night arrival, overtired company and not reading the instructions. I am sorry you and your friends had a less than perfect experience.