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How do you size a humidifier? Once RH is determined for the conditioned space, can find grains of moisture from psych chart...then what? How about a dehumidifier? The opposite? Thanks!

Find the grains difference between desired indoor and outdoor conditions, convert to pounds per hour, then size equipment accordingly. Yes, dehumidifier sizing is the reverse process.

Once you have the grains difference, multiply by the air infiltration rate and the volume of the space to get total grains per hour, then convert that to gallons per day. I use a humidifier size calculator for that step and it does the load math for you, giving a specific unit size. A dehumidifier is the opposite, but you still size by moisture removal rate in pints per day.