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how to autosize for dehumidification load in EnergyPlus

asked 2015-03-25 17:42:21 -0500

mleach's avatar

updated 2015-11-12 20:00:59 -0500

I have a built up PSZ air handler (I'm not using a Unitary system; I have a standard built up air handler with a chilled water coil, a max humidity setpoint manager on the cooling coil air outlet node, a humidistat in the zone, and the controller water coil is configured to meet both sensible and latent load) configured to provide max humidity control. In a proposed case model where the cooling coil is hardsized everything works fine. When I autosize for a baseline case, the cooling coil can't keep up with the dehumidification load. I tried to change the type of load field in Sizing:System to Latent or Total (instead of Sensible) but that didn't have any effect. Is there any way to get the autosizing routine to size the system to meet both the sensible and latent load?

UPDATE: After doing a bit more digging, it looks like the limiting factor is the fan flow rate, not (necessarily, at least; could be both...) the cooling coil. When I hard sized the cooling coil, the system performance didn't change. So I'm currently thinking that the issue has to do with Sizing:Zone, which I'm guessing does not account for the humidistat in the space when calculating cooling design flow. This is by no means a solution, just some extra info to consider.

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answered 2015-07-22 23:06:04 -0500

updated 2015-07-23 07:25:03 -0500

At this time there is no method for sizing coils based on latent loads. There are plans to add this capability in the near future, however, for now the ultimate sizing methods are based on sensible and ventilation loads.

The code does allow using a sizing method based on total loads, but this method simply calculates the load based on Mdot * DeltaH instead of the sensible calculation of Mdot * Cp * DeltaT to identify the time of the peak zone load. Note that choosing the time that the peak zone load occurs (based on total load) is not the same as identifying when the maximum latent load occurred nor that the coil is sized based on the maximum latent load using the corresponding coil sensible heat ratio.

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Just out of curiosity, why are latent loads omitted in sizing the coils? In reviewing the DOE2.1A Engineering Manual, both sensible and latent loads are used to size the coil. Not that DOE2.1 does a particulairly good job of sizing cooling capacities but it would appear at least an attempt is made to include the latent load effects.

Chris Jones's avatar Chris Jones  ( 2015-07-23 07:30:25 -0500 )edit

DOE2 does pretty much the same thing as EnergyPlus, using total (sensible + latent) load to size the coil. What we want to add to EnergyPlus is the ability to size the coil based on the latent load, which for some constructions and geography, can be very different than sizing on sensible and/or total load. EnergyPlus development has taken a stab at sizing on latent loads, however, the concept was never finalized (I still have hope).

rraustad's avatar rraustad  ( 2015-07-23 10:19:37 -0500 )edit

Was this ever developed?

Luis Lara's avatar Luis Lara  ( 2019-02-25 16:42:04 -0500 )edit

Not at this time.

rraustad's avatar rraustad  ( 2019-02-25 18:49:29 -0500 )edit

I am currently wrestling with this problem, ie autosizing is not doing a great job for my high-latent zones. Is there any new information or progress since this 2015 post?

Jim Dirkes's avatar Jim Dirkes  ( 2019-10-16 12:17:28 -0500 )edit

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Asked: 2015-03-25 17:42:21 -0500

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Last updated: Jul 23 '15