Adding dispersants to circulating water?

We have a Vaillant Combi, whic is persistently (at 2 year intervals) having to have the hot water heat exchanger changed as it has clogged with iron oxide. This is despite the system having been flushed and filled with Fernox-MB1 AND a Magnaclean magnetic filter being incorporated into the return.

The boiler itself just seems to attract black iron, where it aggregates into lumps which block the works. Is there a suitable dispersant available that could be added to the circulating water to keep the iron dispersed thereby preventing the clogging and allowing the Magnaclean to work effectively?

B. McHugh
October 2010