return air node enthalpy not matching plenum enthalpy

openstudio
output-variable
plenum
return-air
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Hi I'm modeling a VAV air handling unit with zonal hot water reheat coil with 4 zones and plenum return.

I noticed the return air of the AHU has very low enthalpy (78F drybulb, 21.7 enthalpy and 13% RH). When I track it to the zone level, all room and plenum conditions looks normal (78F drybulb, 45-55% RH).

image description

78F dbt and 47.4% RH is about 29.1 for enthalpy. I don't understand why the condition can change so drastically between the plenum and the return air node (21.7 vs 29.1, 13.4% RH vs 47.4% RH). And it's typical for all room under the system.

Any inputs appreciated. I'm using openstudio v2.4.1.