How do you like to handle limited local humidifiers?

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Hi all,

So, here's a scenario. Not the first time I've had to deal with this but I'd like to know if there's a better/alternative approach:

* A VAVS system serves hundreds of zones. Limited humidity control in this system accomplishes maintaining a minimum of 20% RH in the return airstream.

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* For a selection (minority) of zones under the large VAVS system, humidity is further controlled to a tighter band of 40-60% by localized electric humidification units (representative example:
* Because the VAVS runs 24/7, supplies mostly OA, and the spaces of concern have relatively light occupancy profiles being the only substantive contributor to moisture in the space, the RH measured in the field for these select spaces is determined primarily as a function of the air supplied by the VAVS system.

How should one go about accurately simulating the energies associated with localized humidification equipment?

I cannot simply add the more stringent humidity range of 40-60 to the central unit - this results in way more humidification and active dehumidification than is actually happening./

Presently, I'm making a copy of the VAVS, assigning only those critical spaces to the copy, and applying the following system level humidification inputs to the (smaller) copied system only:
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The main perceived problem with this approach is that the separate SYSTEM will not during the simulation observe the actual RH conditions caused by the larger (actual) VAVS system, which is determined as a combination of effects due to OA conditions, RA conditions, and active humidification at the central unit.

Secondary problems (seemingly easier to address) include:

* Ensuring the (smaller) copied system is not over/undersized in airflow/coil capacities
* Introducing the effects of the localized active humidification/dehumidification on the RA stream of the main unit requires transposing "free" latent loads to other zones

... Is there a better way to skin this cat? Can localized humidification equipment be more explicitly included in our doe2/eQuest simulations? Also interested to learn of any 3rd party tools or spreadsheet based calculations others are used to performing for such situations.

Thanks in advance!


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Nick Caton, P.E., BEMP
Senior Energy Engineer
Regional Energy Engineering Manager
Energy and Sustainability Services
Schneider Electric

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Nicholas.Caton at's picture
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Hi Nick,
You might need to break out your trusty psychrometric chart for this one...

First, I would do a few runs changing the MIN-HUMIDITY at the VAVS system(s), to put an upper bound on total humidification energy. Varying the humidity settings across a few runs also serves as a reminder for where the humidity energy shows up in the output reports.
Then I would use the system structure you describe below (creating a smaller VAVS system to serve the critical zones), except I would set the system for 40% MIN-HUMIDITY instead of the 30% you used. Seems like you are adding the equivalent amount of latent heat that way relative to what the actual systems are doing. The smaller VAVS system will humidify the zones (including the ventilation load) to 40% RH and the larger VAVS system will maintain 20% RH.

I was also going to suggest trying the DOAS system in DOE-2.3 to provide 20% RH to all zones served by the VAVS. This may not be a good solution to your problem, however. DOE-2.3 does not support HOT-WATER humidifiers (or, from what I can tell, Steam Loops). Only ELECTRIC humidifiers can be used, but you could still use ELECTRIC to determine the BTUs of humidification energy. Other issues are that the new DOAS system type is always 100% OA, and they measure humidity conditions in the supply flow instead of the return air. (They also use the keywords MAX-DEWPOINT and MIN-DEWPOINT instead of MAX/MIN-HUMIDITY.) Still, you may want to explore the option.


William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
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