How do you clean a Presto Parabolic heater?

You don't clean a heater core. Once it's clogged, you cannot clean it out successfully. You need to replace it instead.

Shut off the gas let the tank cool THEN get a soft cleaning agent such as soft scrub or Fantastic , clean soft cloth and carefully clean the out side of the tank

If you live with hard water its beneficial to do this yearly and doesn't hurt in any water conditions. One may be suprised to see brown water coming out of a newish tank! Simply attach a short hose...

This is what I do to flush a heater core without removing it from vehicle, drain off aprox 2 qts of coolant from radiator. Remove both heater hose connections from engine. Fabricate a fitting to...

Take the hoses of and run water backwards & forward through it.