jw,
I am switching to no kettle this season....
Last season, i used a cast iron kettle, either directly on the stove-top, or on the trivets. If you put it on the stove-top, you have to be extremely careful!! more than a couple times, with the kettle fairly full, and the stove-top up to 600+ you can have violent boil overs, which will wake you up in the middle of the night. First couple of times, it was scary, I was awoken thinking that the glass had exploded. Water was literally all over the stove, which quickly evaporated, and at times three feet from the stove. I thought, no big deal, but this past summer, there was the beginnings of rust, mainly where the water had spilled. A couple of the boil overs, had the nasty rusty water sprayed all over the place. Luckily the tile floor was easy the clean.
I then switched to putting the cast kettle on the opposite side of the stove (from where I had put it on the stove-top) on top of the trivet. Did not have boil overs, refilled less often, but still have beginnings of rust under where the kettle sat on the trivet top.
I think the amount of humidity added by the kettle is minuscule to what our home needed in the dead of the winter season, and caused more damage and aggravation than what it was worth.
Secondly, the water in a cast iron kettle gets real rusty and nasty, really fast. So I would HIGHLY recommend an enamel or stainless container if you decided to proceed.
Thirdly, I would suggested distilled or rain water, as you could see mineral/salt buildup (water softener) around the area where the kettle resided on the stove top OR trivet (even without boilovers), and major salt buildup inside the cast iron kettle.
Unless you like to paint your cast iron stove, I would strongly suggest avoiding putting any type of kettle on a cast iron/steel stove.
The insides of the side panels are badly rusted after one season, as are the front and front legs, and the steel stove top is slightly pitted and starting to rust where the kettle sat directly on the stove, and the trivets showing signs of rust.
I will be brushing painting next summer.....