Humidity control question

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So I have an HVAC system setup with a preheat coil, a cooling coil and a heating coil. The humidistat is set to 50%, and it's a 100% OA system.

I assumed you set the LAT for the cooling coil with the cold deck min leaving temp and then the LAT for the heating coil with the Zone entering min supply temp.

I'm getting curious results though - I get no heating loads in the summer months - which I should get for humidity control. I setup some hourly reports to track the temperature of the air leaving the heating & cooling coils and the dehumidification reheat heat as well as the moisture added or removed from the air for dehumidification. I notice that while the cooling coil & heating coil temps seem to be ok and there is moisture removal reported, there seems to be no energy reported for reheat for humidity control.

What am I missing?

Vikram Sami, LEED AP BD+C

Sami, Vikram's picture
Joined: 2011-09-30
Reputation: -1

I don?t necessarily see a problem from the info presented, but the complete/correct answer requires more knowledge about your design conditions and system configuration. From what was explained, it sounds like your 50% RH set point can be maintained with whatever you specified for the cooling LAT.

I would only expect summer reheat if you had to cool below your desired LAT to achieve sufficient dehumidification (or if you had a multi-zone VAV with reheat and had interior zones being overcooled in the summer, but that does not sound like what you?re modeling). If you?re trying to supply at 55 F but have to cool to a lower temperature to dehumidify, there should be reheat. Otherwise, coming off the coil at or near 55 F & 90%+ RH is often sufficient for maintaining 50% RH and a dry bulb temperature in the low to mid 70?s. Note that the reheat does nothing for moisture removal, it simply heats the airstream back up to avoid over cooling the zone.

Plot your design conditions on a psychometric chart and see if they make sense ? I?d be happy to help in this regard. Just send me your supply air, exterior design conditions, and interior design conditions.

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Dakota Kelley's picture
Joined: 2011-09-30
Reputation: 1