Hazel's heater has been pretty well non-existent to say the least for about 3 years now, which has always been a pain in the colder weather & has put me off driving it. As is so often the case, it has been a repair that I have put off because I thought I was going to have to pull the heater box out & that would have been a faff.
Anyway, the time came to investigate this week & it was decided to have a ride round (as you do) to warm things up. When the hoses were inspected it was seen that the first part of the 'in' pipe was very hot (bottom arrow) & the other was cool (top arrow):
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It seemed to imply that there was a blockage in this hose but after taking it off, the hose was clear. Presumably then, the heater matrix was blocked & the cool water in the hose would be merely because there was no onward flow & that the water hadn't yet warmed up purely by heat transfer from the engine outlet pipe.
The next stage was to flush the box through with a normal hose pipe. This was done in the reverse direction & the water quickly became very clear & the flow was apparently unrestricted. The same happened for the normal direction of flow. This was puzzling as there's not a fat lot else that could be wrong.
However, we put everything back together & then had another 'test run'/jolly. Surprisingly, the heater then worked perfectly

. When we got back, my friend Mick spotted this on the drive, under where we had been working (coin only for size comparison ...):
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It was a lump of soggy 'grot' that presumably had become lodged in the system & we hadn't spotted it coming out. It looks & feels like an old piece of hose that may have come loose in the past. I did change all the heater hoses a few years ago to try & remedy the heating problem, but it's quite possible that I was already too late!
As I always say (though I hardly ever do it myself), "try the easy things first". If I'd spent the same 2 hours about 3 years ago, I'd have had a toastie interior in all those cold times instead of a freezing one

.