I’ll echo everyone else and say vinegar for sure. A gallon of vinegar is a few bucks at worst.
Anymore, though, I buy distilled water for my coffee maker. It eliminates the possibility of scale. I use 750 mL when I make my coffee and a gallon of distilled isn’t even a dollar. It’s still a good idea to run some vinegar and water through it every once in a while because vinegar kills mold.
Here are some other vinegar uses for those who don’t have much use for vinegar:
Vinegar TipsPut a small amount (like 15-30 mL, depending on size) of vinegar in when you wash your laundry. It’ll help keep mold in the washing machine down and eliminate odors. It won’t make your clothes smell like vinegar either. If you need buttermilk in a pinch, but don’t have any, mix 15 mL vinegar with 225 mL of regular milk and let sit for 10 minutes, and you’ll have something very close to buttermilk. I use this in my cornbread and waffles. If you’re removing wallpaper, spray some vinegar mixed with water on the wallpaper. It’ll soak through and break down the adhesive. The wallpaper will detach much more easily and in larger pieces. Very useful stuff to have around.