@Tami223 You have a lovely home that accommodates 6 and so far only 2 reviews which are quite positive. Hot water heaters do not give warnings before they quit, unless you know you have an old heater that will break down in the near future. Six people without hot water for more than a day is grounds for cancellation so it would be better guest relations for you to offer the 100% refund and let Air BNB help them find an alternative place. You will lose money of course but you are too new to the business to have really bad reviews. Good luck!!