At work we rejuvenated a drip maker by running a pot of straight vinegar through it followed by a couple of pots of water (to get that awful vinegar smell and taste out). Vinegar will also stink-up the place as it boils. We have iron scale around here and the vinegar dissolves it pretty readily. It oughtta work similarly for calcium, I should imagine. I'll bet somebdoy sells something that works better, but we're cheap.

Next time I'm gonna try some 15% H2O2 (what you get for antiseptic use is like 3%). That stuff oxidizes all kinds of metallic and organic compounds but has no odor of its own, though as it oxidizes organics at high concentration it can release some pretty sharp odors. I'm hoping it would be odor-free on metallic scales. Is there a chemist in the house?