Noisy central heating pump - cause?

We have a conventional CH system about 30-years old, i.e. gas boiler, water pump, and diverter value for radiators vs coil in hot-water tank. Over the last few days the pump has been making a lot more noise and it sounds to me as if there is air/gas in the pump. Two nights ago the system failed with the boiler reporting insufficient flow. In the morning I removed the large screw from the shaft of the pump but hardly any gas came out, just the usual trickle of water. But it restarted again with much less noise. But today it was noisy again and then failed again.
My guess is that the gas must be coming from corrosion. I put inhibitor in the system the last time it had any attention maybe 6 years ago so perhaps it needs renewing at intervals? I also found that water in the header tank was full of brown floccules. So my current thought is that I should drain down and flush with clean water a couple of times.
Does a central heating cleaner such as Fernox F3 do any good? It seems this should be in the system for at most a day or two, so that needs a few more drain-down and refills, unfortunately.
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Clive Page

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On Monday, 30 January 2017 09:45:26 UTC, Clive Page wrote:

http://wiki.diyfaq.org.uk/index.php/Central_heating_pump_repair
NT
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On 30/01/2017 09:45, Clive Page wrote:

Maybe the pump's just plain knackered! When the pump runs, do the motorised valves control the heating/hot water as you would expect?
Cleaning
Make sure you bail out the header tank, don't drain loads of cr@p through the system. Then scrub out header tank. If the issue looks fungal, then you can get agent from Fernox to tackle this. It's a while ago I did this, but I thought cleaner should be run for a week??? Recommend you put some sort of strainer on the end of your drain hose to see what it catches (net curtain of similar is OK). If you get loads of black crud, consider running another cleaning cycle and also fitting a magnetic filter e. g. Magna-Clean
Refill, bleed and run, refill , bleed and run, then drain, refill and add inhibitor as you do so. Add anti-fungal to header tank if you have bothered. Check that the float valve is working well on header tank - any doubts replace.
This will give the system a chance.
By 30 years old, particularly if the system hasn't been maintained much, you may have solid crud blocking pipes. On a vented system this usually occurs around the tee off for the vent or for the make up water.
Phil
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On 30/01/2017 10:32, thescullster wrote:

Yes, they do, so I think the pump is basically ok, and it's a fairly recent replacement, maybe 6 - 10 years old.

Thanks for the advice. I have bailed out the header tank, and am about to scrub it clean.
But the problem is now obvious: the pipe from the header tank to the CH circuit seems to be very nearly blocked. I tried threading a spiral pipe unblocker but it gets stuck at the second bend, which may or not be the site of the problem. I don't know whether trying to trickle some CH cleaner or even descaling solution down the pipe might work, or maybe connect a hose from it to cold mains and blast it with some mains pressure? A bit tricky to get a tight seal at the header-tank end.
Any ideas would be welcome.
--
Clive Page

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On 30/01/2017 12:51, Clive Page wrote:

Take it apart?
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Cheers,
Roger
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On Monday, 30 January 2017 12:51:47 UTC, Clive Page wrote:

Blow the crap out with a hosepipe connected to mains water pressure. You might have to disconnect some bits. Make sure you don't over pressure anything in the system or blow crap into the system.
The pipe must be blocked virtually solid. Bit worrying as to what it could be. Normally there only water movement in that pipe is due to expansion of water in the system.
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If the system has been left with a continuous leak for some time, you can find the area of the pipe which transitions from cold to hot forms hard water scale when theres a slow trickle of fresh water entering the system there.
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Andrew Gabriel
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