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Proper Sizing of a Steam Humidier
The Air-Conditioning, Heating and Refrigeration Institute (AHRI) has set guidelines for determining humidication
capacity requirements. The recommendation is based on the cubic footage (volume) and type of home
construction – assuming typical conditions. It is important to realize many homes will have humidication
requirements that differ from the guidelines depending on how the circumstances differ from standard conditions.
Factors that impact the amount of humidity needed:
• Geographic Area
• Elevation
• Ventilation type
• Number of people living in the home
• Ceiling height (i.e. cubic volume)
• Window type (i.e. structure type)
• Insulation type (i.e. structure type)
• Equipment type
Converting square footage to cubic volume requires multiplying the square footage by the ceiling height (i.e. 2000
square foot space with 10 foot ceilings is 20,000 cubic feet). In general, the higher the ceilings, the smaller the
square footage space each steam humidier will cover since it must humidify the additional air volume.
AHRI denes structure type as follows:
• Tight construction: Well insulated with vapor retarders, tight storm doors, windows with weather stripping,
dampered replace, and using ½ air change per hour of air inltration.
• Average construction: Insulated with vapor retarders, loose storm doors and windows, dampered replace
with 1 air change per hour of air ltration.
• Loose construction: Generally built before 1930 with little or no insulation, no storm doors, no insulated
windows, no weather stripping, no vapor retarders, undampered replace, and with 1-1/2 air changes per hour
of air inltration.
It is vital to take all of these factors into account when sizing a steam humidier for a particular home. Undersizing
the humidier will not only reduce the potential to meet the a desired humidity set point, it may also lead to
extensive system fan run time or higher operating costs as the system tries to deliver to the control’s setting. While
oversizing the humidier may lead to higher amp draw, the system run time will be less, which in some situations
may be less expensive at the bottom line.
Steam Humidier System 69-2285—09 5
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