a burst of humidity in Energyplus

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First off, you have many options to define latent moisture gain entering a zone in EnergyPlus. If some electricity consumption causes this latent moisture gain, then you should use the ElectricEquipment object in your input file. There are other objects to use if latent moisture gain is caused by using gas, or as @rraustad pointed out in their answer the OtherEquipment object that will add moisture without consuming any energy.

No matter what option you use to define the latent moisture gain, you cannot directly define moisture addition to a zone in terms of water mass rate like $g_{water}/hr$. You will instead need to define multiple inputs that will together set this water mass rate gain:

• Energy use of the equipment (total design level in $W$, power per area in $\frac{W}{m^2}$, or power per person in $\frac{W}{person}$ )
• Fraction of the equipment's energy use that becomes latent gain. As @rraustad says, this should be set to 1 so that all of the energy use becomes latent gain to the zone.

The basic calculation for the "power" of latent gain into a zone is:

$$Power_{latent} = \dot{m}_{water}*h_{fg}$$

Where $h_{fg}$ is the heat of vaporization of water (2256 $\frac{kJ}{kg}$ at 100 $^oC$). So, you should multiply your desired $g_{water}/hr$ by $h_{fg}$ to calculate the equivalent $W$ of latent gain to enter as an EnergyPlus input for Design Level.