Draining down combi system

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I own a terraced house, the boiler is upstairs, the one Rad with a drain fitting is a step up from the others downstairs so im a bit concerned that the 3 others downstairs won't empty. Can I turn off gas to boiler, leave the cold water flow to the boiler on and attach a hose to the Rad drain fitting, wouldnt that literally feed clean water through the system and then I can run it until clear , a lot of advice says to turn off all water supply to boiler and drain system I'm assuming there is a reason for this, however rather than emptying system wondered if I could do it this way and get water pressure through system and then bleed radiators. Thanks in advance, happy Christmas.
 
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OK so you're just wishing to perform a gravity/mains flush of the system/rads? Do you have a filter on the system?

Maybe want to check and see if there is another drain out through the wall somewhere? if the only drain you have is a step up from the other rads then obviously they won't drain completely.

Best you could do is fill with some cleaner, run and then drain and fill, run system again, drain and fill until it all runs clear. Either that or take off each rad in turn and flush it out.
 
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Apologies for ignorance but if I open the filling loop and open a drain tap with hose attached into the drain, won't it just flush through the water until clear, I've see a youtube video where a professional removed a radiator and attached a drain to one side of the fitting and hose providing ingoing water into the other side, isn't using filling loop just the same principal?
 
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The mains water may well just run through and out of the system ,not necessarily displacing all the existing systems water ,doing as you suggest. The 3 rads lower than the drain off are highly unlikely to empty any of their contents.
 
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