Diesel fuel becoming stale is a common problem in boats and the Boat Owners Association has posted a warning not to use bio-diesel as it is more prone to this occurring. Far more serious at sea than simply a heater that does not work. Many marinas have an agreement with their oil company not to supply bio-diesel or ethanol/petrol blends. Maybe use of bio-diesel could contribute to heater failure because of extended storage times. Regards, Peter.