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Latent cooling with no dehumidification

asked 2018-01-04 19:05:04 -0500

MiguelPereira gravatar image

updated 2018-01-10 16:04:57 -0500


I am using HVACTemplate:IdealLoadsAirSystem in EnergyPlus to estimate the heating and cooling loads to keep all zones within a building between 20 and 25 degrees Celsius.

For the time being, I have both the humidification and dehumidification control types set to None. However, looking at the output variables, I notice that DistrictHeating and DistrictCooling do not equal the Sensible Heating and Cooling being reported.

Sensible heating is reported at 973 kWh while DistrictHeating is at 977 kWh - this might seem negligible, but I am particularly concerned about sensible cooling being reported at 2745 kWh while DistrictCooling is at 3122 kWh - almost a 15% difference!

This leaves me with considerable amounts of Latent Cooling and some amounts of Latent Heating being reported - which in my understanding should not happen since humidification and dehumidification controls are both turned off.

Turning dehumidification ON does increase latent cooling loads considerably (sensible cooling loads remains exactly the same), but turning it off does not set latent cooling to zero.

So my question is, what are some possible sources (besides dehumidification) for the existence of latent cooling in the results?

Many thanks in advance.

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Do you introduce outside air into the zones? What is your climate zone?

Avi gravatar imageAvi ( 2018-01-05 00:05:31 -0500 )edit

The values I referenced are for OutdoorAir set to None.

With OutdoorAir set to 0.00944 m3/s Flow per person, there's even more latent cooling and latent heating, again with humidification and dehumidification set to None.

The I/O reference document states that the IdealLoads unit will use Humidistat instead of the selected humidification control ("None", in my case) whenever the unit is in Deadband mode - could this be the source of the issue?

My climate zone is temperate, specifically Mediterranean.

MiguelPereira gravatar imageMiguelPereira ( 2018-01-05 14:31:19 -0500 )edit

@MiguelPereira How about infiltration?

Avi gravatar imageAvi ( 2018-01-06 02:08:06 -0500 )edit

@Avi I am not modelling infiltration, but I am modelling natural ventilation from open windows, which the Sensible Heat Gain Summary report seems to file under the Infiltration columns.

With (de)humidification set to None and ventilation turned on, 88% of district cooling is sensible cooling. Without ventilation, the number drops to 81% (these two should be 100%, I assume).

Using Humidistat with 30%-50% humidity setpoints, 76% of district cooling is sensible with ventilation, 73% without ventilation.

MiguelPereira gravatar imageMiguelPereira ( 2018-01-06 18:36:18 -0500 )edit

@MiguelPereira I might be wrong but I think that once you are cooling below dew point temperature you must have some latent heat removed as you condense water. Try graphing Outside Temperature against Dew point.

Avi gravatar imageAvi ( 2018-01-06 23:48:38 -0500 )edit

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answered 2018-01-09 15:52:38 -0500

The following output variables should add up to the district heating and cooling:

Output:Variable,*,Zone Ideal Loads Supply Air Sensible Heating Energy,hourly; !- HVAC Sum [J]
Output:Variable,*,Zone Ideal Loads Supply Air Latent Heating Energy,hourly; !- HVAC Sum [J]
Output:Variable,*,Zone Ideal Loads Supply Air Sensible Cooling Energy,hourly; !- HVAC Sum [J] 
Output:Variable,*,Zone Ideal Loads Supply Air Latent Cooling Energy,hourly; !- HVAC Sum [J]

If there is no outdoor air in the ideal loads system, then these should also equal what it delivered to the zone:

Output:Variable,*,Zone Ideal Loads Zone Sensible Heating Energy,hourly; !- HVAC Sum [J]
Output:Variable,*,Zone Ideal Loads Zone Latent Heating Energy,hourly; !- HVAC Sum [J]
Output:Variable,*,Zone Ideal Loads Zone Sensible Cooling Energy,hourly; !- HVAC Sum [J]
Output:Variable,*,Zone Ideal Loads Zone Latent Cooling Energy,hourly; !- HVAC Sum [J]

Latent cooling can occur even when the Ideal Loads Dehumidification Control Type = None, because "For all options, the supply air humidity ratio will never be allowed to exceed saturation at the supply dry bulb temperature." (from I/O Ref for Dehumidification Control Type)

If this does not explain what is happening, then please reply here with a link to the idf file or send it to the EnergyPlus Helpdesk.

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@MJWitte Those variables do add up to District Heating/Cooling. I am trying to figure out if that line in the Reference does account for the outputs I have. What variables should I look at to find if the system is preventing the supply air from exceeding saturation? I can't find a SupplyAirHumidityRatio variable.

I attached an image to my original post showing timesteps where the system uses Sensible Cooling to keep the zone temperature at 25C, with Latent Cooling being used at the start and end of the data frame. It should show a pattern that justifies the latent cooling, but I can't see it.

MiguelPereira gravatar imageMiguelPereira ( 2018-01-10 16:18:25 -0500 )edit

Look at the System Node Temperature, System Node Humidity Ratio, and System Node Relative Humidity for the supply node leaving the ideal loads system. Also, look at the return node (or exhaust node) that is feeding the ideal loads system.

MJWitte gravatar imageMJWitte ( 2018-01-12 17:54:36 -0500 )edit

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Asked: 2018-01-04 19:05:04 -0500

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Last updated: Jan 10