its ok to put in main sewage system as the bulk of water there will dilute any acidic effect, however over time in an old drainage system in a home, it can eat away and destroy any cement joints as I have sen happen in my place. so limestone chippings and in line neutralizers are a good idea.
As to the other queries, feeling better thanks, just been and replaced a halsead ace he 35 fan after it broke at 2 years old. Noted that there are no burner chamber seals availalbe fm halstead or merchants at present, makes life easy!!!!!!!!!!!!!!!!! and that a halstead fan costs around £180 plus vat!!!!!!!!!!! poor old custgard had to fork out almost £400 to repair his boiler, could have almost bought a new boiler for that. And it only had a 600x600 rad on the heating as they had put in any central heating yet, but they had that rad in the airing cupboard, and the heating on all the time, the poor old boiler was cycling on/off almost constantly, no wonder the fan burnt out. Wanted to take a photo of his diy plumbing but he was looking over my shoulder to much. Still the apprentice with me spent a happy 10 mins counting the cock ups.