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IF you can get pure citric acid (as opposed to lemon juice which tends to be watered down and expensive) that should be more effective as it has a lower pH value, 2 versus 3 for vinegar.
When I went looking for an alternative to the £4.50 for 2 applications kettle defurrer product that my wife had been using as seems to be responsible for rotting the rubber seal on the kettle but barely seemed to touch the chalk, I found brown malt vinegar was available in 5 liter containers wholesale for £3.00. We've been using it twice a week for over a year and its still 1/2 full.
Dunno what it tastes like, but who cares :)
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