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Its also the fact that heat of the water helps melt the fat that collects in drains from body shed, soap, (dishes if its the kitchen).
It works even better if you get it hotter by pouring boiling water down, after the sink of hot water.
Then the fill sink with hot water again - and do one more flush to force out the melted fat and grease.
(or do some plunging at this point [now that everything is heated up and soft], with hot water). It's AMAZING how much crap it dislodges).
If ya wanna finish the process off to make it last longer - pour about a litre of gel bleach into the sink, add about another liter of hot water, and then pull tjr plug - and let it flush the drain.... DO NOT USE SINK AFTERWARDS - Let it sit overnight .
This allows the bleach time to attack bacteria and kill it .
Do one more sink flush or plunger of hot water in the morning .
With a little affort, and some time, its amazing how much you can clean a drain without those awful corrosive chemicals.