I have the truma boiler in my hymer, lately the hot water side of the system needs bleeding if the mh has been sitting for a while( a day or so). Looked on the truma website and it shows anon return valve on the schematics , but not where, the price list says it is essential for immersion pumps. I have a nrv on the cold water side and have no problems , or is ther something else wrong. run the taps to bleed the system and everything is ok