How humidity is controled in HVACTemplate:System:VAV

asked 2017-10-06 01:17:55 -0600

2085 ●1 ●4 ●14

I modeled VAV for air side with water cooled chiller for water side using HVACTemplate.
In HVACTemplate:System:VAV, I set Dehumidification Control type:CoolReheat,Dehumidification Setpoint:50%.
However, when I look at result, humidity seems strangely low (sometimes it reaches to 20%) in summer compared to the climate data.
(below image shows annual humidity data left:Outdoor, right:conditioned area)
Can anyone tell me what is causing such a low humidity control?
image description

answered 2017-10-09 01:26:03 -0600

Chandan Sharma

2432 ●1 ●2 ●9

updated 2017-10-09 01:28:21 -0600

Running simulation for one day of summer and winter each would show whether CoolReheat control is actually working or not. If CoolReheat is found to be working then during summer months low zone RH could be due to night cycle manager. Turn night cycle off by making the fan to be always on. For winter months, see attached Psychrometric chart (Ref) for cooling and dehumidification process. image description If point 1 is further right and point 3 is further left, RH could be same or higher as a result of cooling and dehumidification. Plotting temperature and humidity ratio instead of RH at various air loop nodes from expidf file will show why zone is not maintaining RH setpoint. It could be that even if the outside RH is high, the outside humidity ratio is low during winter and zone moisture load is not enough to hit the RH setpoint. If this is the case, then adding humidifier may help improve the results. Once the air gets saturated though, RH setpoint would still not be met during winter. Running model at different weather locations would also help understand the results.