I us only distilled water in my humidifier and while it's cheap, I don't like to waste it. I also like to empty the water each day and use fresh every night.
So, here is my solution. I bought a small 6 oz measuring cup from the baked goods isle in the grocery store and measure the amount I need. Pouring from the gallon jug to the measuring cup and then to the humidifier is easier than straight to the humidifier. And the formula could not be easier. It's a 1 to 1 relationship. Humidifier set to 3, then 3 oz. humidifier set to 5, then 5 oz. This will be good for 8 hours on the hose.
Works for me.
I am an accountant so any advice given here is not medical. If I give any financial advice, you can take it to the bank. However, you will have a hard time cashing it in.
I use leftover water for my plants.
That correlation may hold under present conditions; but temp and ambient humidity will impact how much water will be used.
I'd rather not run a tank dry.
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