Glowworm 18hxi boiler - drain question


We have a Glow Worm 18hxi condensing boiler fitted several years back. During this cold spell, we've had some trouble with the drain for it freezing up (the boiler is the in the garage and is teed into the waste for a sink I have in there).
Not a problem in itself - there is a just a section of waste pipe that goes through the wall to an outside drain which is prone to freezing as the pipe is shot and leaking a little. Bit of hot water poored on it soon unfreezes it and I'll repair it properly when I get chance.
However, when I was diagnosing the problem initially, I noticed a large blue plastic screw cap adjacent to the drain point on the underside of the boiler which I took to be a means of manually draining it if the pipe became blocked. Sure enough, on removing it, a cup full of water came out - but also what I can best describe as a load of grit, a bit like iron filings in appearance. Any idea what this is and whether there is anything abnormal here?
TIA, Midge.
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some residue left over from the manufacture of the heat exchanger (if the filings are bright) Failing this, they are probably the result of internal corrosion of the heat exchanger, which is an inevitable result of the modus operandii of condensing boilers i.e. warmth, humidity in a mildly acidic soup. To make a heat exchanger to be impervious to this environment would be prohibitively expensive, so they suffer from slow rot. The philosophy being that something else will fail before the heat exchanger, when you will be advised to cut your losses and mend it with a new boiler.
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Thanks - it is on contract so I might get it looked at during the next service. The grit is dark BTW.
Midge.

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