Iron filings in underfloor heating system

[Ron6a]

I have a fairly modern house with underfloor heating throughout. All plastic pipes, no radiators or metal parts, other than the parts in the vaillant ecotec plus 630 boiler. It has a Magnaclean filter fitted, which I check at least twice a year. Every time I do this there is a build up of metallic particles, some large enough to see. Never a lot but always some. This deposit has also caused the F75 fault to come up 3 times in 4 years. Each time an engineer from Vaillant has changed a part that has been clogged with metallic/magnetic particles. I am totally mystified as to where these particles are coming from, as the system has no metal components, other than the boiler components and perhaps pump parts and expansion tanks. I cannot imaging it is in the mains water supplied by Affinity water.
I would be grateful if anyone has any ideas, as the F75 fault (pump or low pressure) has come up again.

 

[Nozzle]

When you clean your magnaclean filter twice a year - do you also re-dose with inhibitor?

I suggest your guess about pump parts might be spot on, but there is one other metal part - your DHW cylinder.

Nozzle

 

[Gasguru]

No towel rails?

 

[Madrab]

Mixed metal systems can always create some, albeit small amounts of corrosion. The key, as suggested, is the system is clean and inhibited.

Sounds like your filter is catching anything before it gets to the boiler ( as long as it's the last thing on the return) so that's working as designed. What's being replaced when it shows F75?

It may be time for a clean, flush and re-inhibit. Did the engineer not suggest any possible causes?