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The supply airflow rate passing through the humidifier section.
Use the design supply airflow rate at the point in the ductwork where the humidifier is located, not the total system outside-air or return-air CFM.
The target increase in the airstream's moisture content, in grains of moisture per pound of dry air.
This is the difference between the desired leaving humidity ratio and the entering humidity ratio, expressed in grains per pound (7,000 grains = 1 lb).
Required humidifier moisture capacity
38.6 lb/hr
This uses a standard simplified psychrometric approximation (grains per pound of dry air, which metric practice writes as grams per kilogram, with the 4.5 factor for standard air) common in manufacturer humidifier sizing guides — for a final humidifier selection, use the manufacturer's full psychrometric sizing software, which accounts for actual entering air conditions and altitude.
They open the calculator with your figures already in it
Steam Humidifier Moisture Capacity Calculator: 38.57 lb/hr — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Moisture addition rate (lb/hr) = 4.5 × Airflow (CFM) × Humidity Ratio Change (grains of moisture per lb of dry air) ÷ 7,000, a standard simplified humidifier sizing formula derived from ASHRAE psychrometric relationships (7,000 grains = 1 lb) and widely used in commercial humidifier manufacturer sizing guides.
Inputs used
- Airflow
- 2000 CFM
- Required Humidity Ratio Increase (grains/lb of dry air)
- 30
Confidence note: This uses a standard simplified psychrometric approximation (grains per pound of dry air, which metric practice writes as grams per kilogram, with the 4.5 factor for standard air) common in manufacturer humidifier sizing guides — for a final humidifier selection, use the manufacturer's full psychrometric sizing software, which accounts for actual entering air conditions and altitude.
What this calculation does not cover
- Nothing here is the absorption distance, and that is what most often goes wrong on site. Steam leaving a dispersion manifold needs a clear run of duct before it meets anything — a coil, a filter, a turning vane, a branch takeoff or the duct wall — and that distance depends on the leaving air's relative humidity, the velocity, the duct dimensions and the manifold type. Too short a run and the steam condenses on whatever it strikes: wet filters, corroding duct, and mold in a system installed to improve the air.
- Pounds per hour is not the service. Steam takes roughly 1,000 Btu to make each pound, so the 38.6 lb/hr (17.5 kg/h) on these defaults is about 38,000 Btu/h (11 kW) of gas input, or the same again in electricity on an electrode or resistive unit — a dedicated feed, and often a three-phase one. Add the water supply, whatever treatment the cylinder needs at your water hardness, and a drain rated for hot water.
- The humidity ratio increase is a figure you supply, and taking it off a mild day undersizes everything downstream. The design case is the coldest outdoor dew point the site sees, because that is when ventilation air arrives driest — and a humidifier that comes up short does not catch up the way a heating coil does, it simply holds the space under setpoint for the whole cold spell.
Add the equipment this sizes
This result is a specification — 38.6 lb/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Moisture addition rate (lb/hr) = 4.5 × Airflow (CFM) × Humidity Ratio Change (grains of moisture per lb of dry air) ÷ 7,000, a standard simplified humidifier sizing formula derived from ASHRAE psychrometric relationships (7,000 grains = 1 lb) and widely used in commercial humidifier manufacturer sizing guides.
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