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[Abiton]

Ever wondered what your heater matrix looks like?

Well it's a lot like your main radiator, only smaller:




These are the connections where the coolant enters and leaves:
(ignore the teaspoon in the foreground, it's just holding it together)



This is the far end, minus its rubber end-cap:




With a subtle application of hacksaw brutality, one can see the innards:




So the coolant coming in has a choice of  12 small pipes to go through, all in parallel.
If you look at the plastic part I've hacked off, you can see the meandering central divider which keeps the 'ins' and 'outs' separate.  
This kind of design is necessary, in order to get maximum heat out of the coolant, and into the air fins in a relatively small unit.  It has disadvantages though; after a long life, or with poor coolant maintenance, the relatively small diameter pipes are vulnerable to getting blocked by circulating debris/sludge.  The ones farthest from the in/out pipes are probably the first ones to suffer.  You can see a bit more muck at that end of the unit, even after it was flushed out very thoroughly whilst still intact.

And there's the real problem with flushing.  Because there are 12 pathways all in parallel, just because you see a good stream of water going in from your hosepipe, and coming out (whichever way you flush it), doesn't mean that all these pipes are flowing.  I can only imagine that you need a really high flow rate, for a long time; or else some sort of  de-sludging chemical (I've heard biological washing powder recommended but never tried it myself) in order to stand any chance of fully clearing out every little pipe.

As it happens, this matrix developed a leak, rather than just working poorly, at a seam between two parts of the plastic end fitting parallel to where I've applied my hacksaw (under the foam strip).

I'd think that the main radiator at the front of the car has a pretty similar construction, and probably gets partially clogged up in much the same way, with the same difficulty facing an effective flush out.

Hope this is of interest and use to some of you lovely OOFers  

Pete