Firstly, let me point out that this is probably old ground and that many before me have made a similar post. However, I thought as new people join all the time it wouldn't hurt to make the post any way.
I have been a bit slack with cleaning my Resmend H2i humidifier. As a result there was a bit of calcification build up on the metal plate and on some of the nooks and crannies of the humidifier.
I took to it with a scrubbing brush, hot soap and water. The result was not what you may say 'impressive'.
My wife suggested I try soaking the humidifier in vinegar. Well it came up brand new after just a few hours soaking. The calcification just wiped off. I then soaked the unit in hot soap and water for another couple of hours to remove any remaining vingear scent. Truly worked wonders.