Make-Up Air Unit Moisture Loads Tool

This tool demonstrates the effects of moisture capacitance on the latent sizing calculations for a makeup air unit (MAU). For peak humidity periods, the latent load in the space can be much larger than the dehumidification capacity but reasonable space conditions can still be maintained. The reason is moisture capacitance. The moisture capacitance is estimated by taking the space volume and multiplying by the moisture capacitance factor (typical values range from 1 to 20). By selecting different inputs below, the effects of moisture capacitance on space humidity levels can be demonstrated. Slides from a recent ASHRAE Seminar on this topic are available here. Shirey et al (2001) prepared a Reference Manual for the EPA's IHAT simulation tool. Section 10 and 14 of that manual provide background and guidance on selecting Moisture Capacitance Factors for various building applications.

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