hi there. i wast just woundering before i get the filling loop assembly for my bosch worsester 24 cdi rsf becouse its not allowing me to fill the c/h system is there anything else that could cause the system not to refill at all? have inserted the plastic key and opened the grey water tap to let water in, i here the water get to key but no further, surley it cant be anything esle but that or can it?