old type heater metrix can be washed out with a mix of caustic soda and hot water, but dont wash your hands in it,easy and best way is to drain all the water or antifreeze out and top up again with caustic soda and water already mixed and then run engine untill normal temp then keep engine going for a few mins on half throttle, switch off and let cool before draining reflush with clean water before replacing fresh antifreeze.
Safer option is to use dishwasher liquid.
