depends what you mean by flushing exactly. if you just want to clean it through and replenish with descaler/corrosion inhibitor then isolate the electricity and gas supply to the boiler and drain down from the drain cock (should be found on the pipework on the LOWEST radiator). once the pressure has been released and the system partially drained then open all rad bleed valves (upstairs first) and wait - can take up to a couple of hours or even longer to completely drain down. if pipes are run from above to the radiators you may have to remove each one and drain manually.
if rads are particularly mucky then it does no harm to take them outside and flush through with a hosepipe attached to the main.
replace, close all bleed valves and drain cock and refill the system at the boiler. switch back on and let it run for a short time just to get things pumping and stirred up.
then repeat the whole process! this will flush through the pipes. drain down again and refill. and add descaler (if required) and corrosion inhibitor - easiest way is to remove the bleed valve or blank on top of towel rail in bathroom (if you have one) and pour in with a funnel - BEFORE the system is pressurised! otherwise you will get very wet when you remove the blank.
if no towel rail then you can get kits that will add the inhibitor and descaler via bleed valve on a rad. not the best method but certainly the simplest.
if, however, you are talking about a POWER FLUSH - then get somebody in to do it, don't attempt it yourself. you might possibly save £600 but more likely you will cause damage that could run into £1,000's to put right.
frankly though, whichever method you are referring to, get somebody in if you are not entirely confident in your abilities and level of knowledge.
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