Why metal granules from cast central heating boiler?

[pamela]

The heating engineer came today to clear a blocked condensate pipe in the central heating boiler. Seems that over time granules of aluminium from the cast block accumulate until they fill the trap in the condensate overflow pipe.

Where do these granules come from? They are sort of halfway between filings and swarf.

Can't the cast block be scrubbed clean during manufacture to prevent these grabules dislodging? Or are they formed by some chemical or mechanical process while the boiler is in use?

This is an Ideal Isar boiler. Heating engineers never tire of telling us it is known to be troublesome but that isn't the point.

[Alan]

That is your problem. They were a very poor design, with a cast aluminium heat exchanger. Unless the water in the system is kept scrupulously clean, both with inhibitor and regular flushes (at least every 2 years, preferably every year), it will corrode inside, and you'll get a hole in the heat exchanger eventually. The water in the system gets slightly acidic over time, and because you have iron and copper in the system with an aluminium HE, it will promote galvanic corrosion once the inhibitor starts to wear off.

[jrwalliker]

How would the use of inhibitor in the hot water system affect what happens on the flue side of the heat exchanger?

John

[Brian Gaff]

Well if its an alloy, you get some oxidisation and indeed even cold water running through an alloy pipe seems to generate some kind of bits eaten from the pipe, so maybe its a self destruction mechanism built in?

Brian

[pamela]

So these granules are the result of corrosion? The seem like little grits of pure metal which I thought were being physically scrubbed from a casting.

Are they something like aluminium oxide?

I don't know who installed the heating system but the circulating fluid looks like clear water. Does it now need corrosion inhibitor or is it too late and requires full flushing first?

If corrosion inhibitor is introduced could it cause further problems by dissolving plaques (if thats's the word) which may have been plugging small leaks in the pipework?