Drawing No. EH–TH–021 // Thermal Engineering & HVAC
Humidification Calculator
Reviewed August 2026
Find the water addition rate and steam energy needed to raise supply air from an entering to a target humidity ratio.
What problem does this solve?
Cold outdoor air holds very little moisture, so heating it for winter comfort often drives indoor relative humidity uncomfortably low unless moisture is deliberately added back. This calculator sizes that addition — how much water needs to be added per hour, and how much energy a steam humidifier needs to supply it — for a given airflow and target humidity ratio change.
Inputs
Results
Background
Water needed = dry-air mass flow × Δw. Dry-air mass flow [kg/s] ≈ 1.2 × airflow [L/s] / 1000, using standard air density. This gives the mass of water vapor that must be added per second to shift humidity ratio by Δw.
Steam-injection humidifiers boil makeup water to steam and inject it into the airstream. Energy required ≈ water flow × (hsteam − hmakeup water), approximated here using the latent heat of vaporization at atmospheric pressure (≈2257 kJ/kg) plus the sensible heat to raise makeup water to boiling — a simplified but representative estimate of humidifier energy input.
Steam injected into an airstream at close to its own condensing temperature transfers mostly latent heat to the air's moisture content with comparatively little sensible temperature change to the bulk airstream — unlike evaporative (adiabatic) humidification, which cools the air by drawing latent heat directly from the airstream's own sensible heat.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
Evaporative (adiabatic) humidifiers spray water or pass air over a wetted media, and the water evaporates using latent heat drawn from the air itself — this cools the air's dry-bulb temperature as it humidifies (following a constant wet-bulb process). Steam humidifiers add pre-vaporized moisture with an external heat source, adding humidity with comparatively little cooling effect. This calculator models the steam-injection case.
Cold air holds very little water vapor at saturation. When that air is heated to room temperature without adding moisture, its relative humidity drops sharply (since relative humidity depends on temperature) even though its absolute moisture content hasn't changed — this is why heated buildings in cold climates often need active humidification to maintain comfortable indoor RH.
It's a representative approximation using standard latent heat of vaporization. Real humidifier energy consumption also depends on standby losses, makeup water temperature variation, and the specific humidifier technology (electrode, resistance, or steam-to-steam heat exchanger types), which this simplified estimate does not fully capture.