Relatives combi has a blocked plate heat exchanger on a Microgenus 28MFFI, non condensing model about 10 years old. I diagnosed this by searching in this forum. Does anyone know where the drain off is on this boiler? Obviously I do not want to drain the whole system down if I can avoid it. On the CH return to the boiler the integral isolating valve has a big cap under it, while the flow isolator valve does not. Is this isolator and nut the drain for the boilers heat exchangers? If so how does it work?