Flushing central heating

I'm about to change a couple of radiators, and as water bled from the system is grey and cloudy I thought a flush would be in order. A few years ago I removed the rads and flushed them through with a hose, but in spite of using Fernox MB1 the system still isn't clean. I'm inclined to blame the three ancient cast iron radiators, one of which is about to go. Having read the threads about commercial power flush operations I can't justify the expense, and anyway things can't be too bad as none of the rads have cold spots. I can't be bothered to take the rads off again, but I intend to try Sentinel X400, filling up later using Sentinel X100. The house is a bungalow so there isn't much head for flushing and I wondered if I could use mains pressure. If I tee across from the mains supplying the CH header tank into the boiler supply pipe (would a non-return valve be sensible/ mandatory here?) via a cut-off tap of course, and insert a cut-off tap between my tee and the header tank, would this allow a stronger flush? Can anyone see any flaws in this plan? The system has 8 rads, Gloworm Space Saver, 15mm pipe.
Rob
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On Thu, 11 Dec 2003 11:57:29 -0000, "Rob Bradley"

You can rent the powerflushing equipment. This is quite a bit less expensive than paying the rip off merchants.

You could do this. You would also need to block off the vent pipe over the tank or the water will come out of there. A push fit plastic end cap would do the trick or a brass compression fitting.
Theoretically you should have a non return valve to the mains, but for a one off exercise that you will be supervising the whole time, I see no reason to bother. You could use a sealed system filling loop as the means of hooking this up. That way you would have all of the valves inclusing a non return.
To make this effective, you could also do with a valve on at least one side of the boiler. This way you can direct the flow around the system and through one radiator at a time by operating the radiator valves. In that way you can flush them individually as well as the pipe work.
If you do put valves around the boiler, then lever ball valves are good, and you can the handles off to avoid inadvertently closing them.

.andy
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