Central heating problem

Recently when I was replacing a radiator I took the opportunity to give the sludged-up system a thorough flush after circulating Sentinel X400 for a few days. I rigged up a system to flush mains water through the boiler supply pipe and did each radiator in turn until the water ran clear. I refilled using Sentinel X100 inhibitor, put the system back into use, and within days the water in the expansion tank was black again, and the boiler much noisier than ever before. A jam jar full is completely opaque, with a brown scum on the top. I had a heating engineer look at it, but he doesn't know why this is happening, unless it's the two old cast iron radiators in the system. He suggested replacing my Glow Worm Space Saver 60B MkII, gravity hot water, with a Ferroli F30 combi, after power flushing the system. I suppose that's the easiest and most profitable solution for him (though he is the brother of a friend so I don't think he was trying it on). The house is a bungalow, and there's not much height difference between the cold cistern and the hot water cylinder, which I think might be the cause of the erratic 3 litres/minute supply to the bath hot tap. Changing to a combi should solve this problem and be more economical, but will the sludge return? One of the cast iron radiators has been repaired with epoxy metal repair where it developed a pinhole after blast cleaning and powder coating. Is the pressure in a combi system likely to be a problem? He says it would be a very straightforward job to replace the boiler, but hasn't given me a price yet. What do you reckon, to power flush, supply and change the boiler and take out the loft tanks and redundant pipework?
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There's not necessarily any need to replace the boiler if it is working.
Descaling chemicals won't fix a totally sludged system. What you need to do, is follow the FAQ on manual flushing. This involves removing each radiator and blasting the insides with water out in the garden.
Also, consider replacing the cast iron rads if they're on the way out, although if they're nice column types, it is probably worth doing what you can to flush them thoroughly.

This is a totally independent problem. Check all the valves are open, but you may have scaled up pipes in the DHW system.
Christian.
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In an earlier contribution to this discussion,

When you flushed, did you physically remove each radiator and take it outside and blast it with a hose? If not, you have probably succeeded in loosening but not removing the sludge - so most of it is still in there. Your cast iron radiator shouldn't be a problem if you use the right inhibitor.
A combi won't cure the sludge problem if the system is not flushed properly. A combi *will* give you a better hot water flow (if it is truly 3 litres/min. Are you sure about that? At that flow rate, the water in the bath would be cold by the time you had filled it!). Make sure the mains is up to it though.
Alternatively, you could keep your existing boiler and install a mains pressure hot water system or heat bank - still providing stored hot water but with a much better flow rate.
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properly.
the
mains is

Yes, it really is 3 litres/min., but it's at 75C so it takes a while to cool down.

mains
water
I'm not familiar with either of these options. More complicated/expensive than a combi?
Thanks for the reply
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Very much so. In your situation a high flow combi is the most cost effective way. It will meet all your needs. No need to overcomplicate matters.
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In an earlier contribution to this discussion,

See http://www.diyfaq.org.uk/plumbing/plumbingpage1.html#thermal for info on heat banks (thermal stores). With one of these, you have a tank of hot water (at low pressure) and a heat exchanger inside - through which mains pressure water passes - and is heated up - on its way to the hot taps.
A mains pressure hot water system has the whole hot water cylinder at mains pressure - and does away with your DHW header tank. When you open a hot tap, water flows out at mains pressure - driven by the incoming mains to the cylinder. Understandably, certain safety devices are needed with such a system, and installation is not a DIY job - whereas it could be for a heat bank.
With either of these you could keep your existing boiler.
Don't be surprised by IMM's reply. Have you ever seen him recommend anything *other* than a combi?
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If it is a large bungalow you should have used 2 cans of X-400, and leave it for 4 weeks as per instructions for heavily sluged systems. Then flush as y ou did. Fernox is more acidic and removes more sludge. The noisier boiler may be displaced sludge caught in the boiler.
Replacing with a high flow Combis is probably a good thing as the Spacesaver is very inefficient. Have a strainer on the return to the new boiler with a full bore isolating valve either side to catch any gunk in the system. The higher pressure "may" pop the repaired rad, so watch out for this. Once the sludge has gone it should not return. Replace inhibitor every 3 years.
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leave it

flush as y

boiler
Spacesaver
boiler with a

system. The

Once the

years.
Thanks IMM, I'll try this. I presume a strainer would only catch biggish bits? I'm guessing most of the sludge is either sat in the rads and boiler, or is in fine suspension.
Rob
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