i need to change the DHW heat exchanger on my isar he 30?

drain system ,there must be drain out point out side property , or on lower radiator ,open air vent on rads when empty ,shut main stop cock cold water in house ,undo two tork screws hold hot water heat exchanger,you need no 6 or 5 tork screw driver to undo nut it look like nut but its not and not phillip screw ,replace new one or clean old one with spirit of salt acid ware hand glove if use acid,close air vent ,fill system let all air out

ltd
June 2015