# How to model dehumidification load in naturally ventilated zone?

My interest is in how much dehumidification load I would need, if I want to limit the humidity ratio in a naturally ventilated zone to e.g. 12 g/kg. So my idea was to model a naturally ventilated zone in DesignBuilder and add a FCU (fan coil unit) for circulation ventilation (no mechanical ventilation from outside) including dehumidification. But with the FCU available I can not set a "setpoint manager" for upper humidity ratio limit. Is there a solution to that?

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It may be simpler and quicker to model the zone using Ideal Air Loads. You can set a humidistat on Ideal Air Loads.

( 2017-05-29 01:58:53 -0500 )edit

thank you for quick answert. as far a I understood the humidistat in IdealAirLoads can only be of the relative humidity kind?! or is it possible to change that to humidity ratio instead?

( 2017-05-29 02:17:38 -0500 )edit

If you know your upper air temperature in the space and your humidity ratio you can calculate the maximum relative humidity allowable and set a humidity limit that way, you could even alter it seasonally to adjust for the your heating and cooling setpoints.

( 2017-05-30 07:53:21 -0500 )edit

@ Annie: thank you. the idea is good. But the temperature in the zone will change every hour, so that I would have to calculate the relative humidity upper limit by the humidity ratio hourwise .. which is not practicable (in a naturally ventilated zone the temperature would be free-running to some extent, wouldnt it?)

( 2017-05-30 07:58:12 -0500 )edit

You could calculate the RH for the absolute worst condition, highest temperature at 12g/kg, then at lower temperatures the g/kg wouldn't be able to go over that. Not the most elegant solution I will admit, but in general the room temperatures should stay quite stable, even with NV. If you have setpoints of 20C in winter and 26C in summer, Summer RH would be approx 50% RH and winter approx 75% RH. If you set the humidistat to 50% RH then the absolute humidity would never go over 12g/kg

( 2017-05-31 04:00:29 -0500 )edit