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Unmet Hours from Outdoor Air Units in EnergyPlus

asked 2015-03-27 05:37:34 -0500

ad3_pras gravatar image

We are undergoing energy model calculation in e+ to meet LEED's mandatory requirement. Our new construction will be a medical office and lab conditioned with VRF system complemented by independent Outdoor Air Units (OAU) to meet IAQ requirement. This OAU consists of supply and exhaust fans only (no coils, mixers, or exchangers). The result of this combination is a huge amount of unmet hours (up to 3700 hours a year). Therefore in e+, it is also possible to introduce OA without adding independent OAU by using mixer embedded inside VRF's Terminal Units. When I implemented this (both cooling and OA provision only through VRF Terminal Units), with the exact technical specifications and the same amount of outdoor air provisions, there was no unmet hours problem occurred (only around 30 hours a year). How this could be happened? Logically, it would behave similar with the OAU equipped. Or maybe I missed something?

For additional information, our project location is in tropical humid climate, Surabaya-Indonesia


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Is your heating and cooling autosized? That would explain it...

Julien Marrec gravatar imageJulien Marrec ( 2015-03-27 07:38:51 -0500 )edit

Yes, most of them were autosized. The two and only defined input were just the rated COP of each condensor and the volume flow rate (debit) of outdoor air required in each zone. Are they still far from enough? We are still collecting other technical information from our MEP team and it seems that this is not an easy task since this is new to us hence we are not used to in terms of documenting details needed by LEED.

Best regards,

ad3_pras gravatar imagead3_pras ( 2015-03-30 06:37:50 -0500 )edit

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answered 2015-03-30 07:24:34 -0500

Your problem is that the zone equipment is autosized, and during the sizing it just doesn't "see" your added load from dumping unconditioned air into your zones. It assumes the central air handling unit is providing "neutral" temperature air. In other words, your zone equipment is sized to meet only the thermal load (internal gains and envelope losses).

So you end up with a system that is undersized to meet thermal loads + ventilation loads, and therefore you get many unmet hours.

If you want the sizing to work correctly for your case with a Dedicated Outdoor Air System (DOAS, or OAU as you call it), I think you'll have to change your infiltration object to have an increased value for design days only that equates to your typical infiltration + the Outdoor Air CFM from the DOAS unit. This will be painful if you have a lot of zones though.

Source: EnergyPlus Support Group thread: Why autosize doesn't consider System Outdoor Loads

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There is development work in progress to include the impact of a non-neutral DOAS when autosizing zone equipment.

MJWitte gravatar imageMJWitte ( 2015-03-30 18:08:13 -0500 )edit

Thank you for the advice. I had tried to include infiltration rate equal to oda's cfm. The result shows a slight reduction in unmet hours (3700 become 2200 hrs). In the other hand it increases total energy consumption from 1700 to 2000 GJ. I'm afraid this wouldnt be realistic since in a case where I include oda inside VRFs TU

ad3_pras gravatar imagead3_pras ( 2015-04-01 20:54:21 -0500 )edit

.. it only produces around 1800 GJ without any issue on unmet hours (30 hours a year). It is set with the exact same autosized settings. When I allowed VRFs TU to provide autosized oda, instead of just relying to OAU, unmet hours is decreased further to just 700 hours. Yes this shows us that OAU is neglected from HVAC sizing. Our model has 109 conditioned zones and 26 mech vented zones.

ad3_pras gravatar imagead3_pras ( 2015-04-01 21:10:17 -0500 )edit

Currently there is a new published version of eplus (v8.3). Does it include ventilation loads from OAU already? How long we should wait for the mentioned version?

Best regards,

ad3_pras gravatar imagead3_pras ( 2015-04-01 21:15:33 -0500 )edit

@MJWitte, any update on the inclusion of a non-neutral DOAS during sizing?

pflaumingo gravatar imagepflaumingo ( 2015-05-11 12:53:40 -0500 )edit

answered 2015-11-10 10:10:54 -0500

updated 2015-11-10 10:11:42 -0500

EnergyPlus v8.4.0 (Sept 2015) has a new feature to account for DOAS effects during sizing. Several new fields have been added to the Sizing:Zone object. From example file 5ZoneFanCoilDOASCool.idf

    SPACE1-1,                !- Zone or ZoneList Name <snip>
    SZ DSZAD SPACE1-1,       !- Design Specification Zone Air Distribution Object Name
    Yes,                     !- Account for Dedicated Outdoor Air System
    ColdSupplyAir,           !- Dedicated Outdoor Air System Control Strategy
    12.8,                    !- Dedicated Outdoor Air Low Setpoint for Design
    15.6;                    !- Dedicated Outdoor Air High Setpoint for Design
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That's great news!

Julien Marrec gravatar imageJulien Marrec ( 2015-11-12 04:03:52 -0500 )edit

Nice, and what exactly is Setpoint for Design? Is it as DesignDay for Sizing the HVAC? When I tried different number, the results are changed too. Then for unconditioned outdoor air (NeutralSupplyAir), is it okay to set the low setpoint same as winter designdays and high setpoint same as summer designdays?

ad3_pras gravatar imagead3_pras ( 2015-11-25 03:36:13 -0500 )edit

Is it possible to model DOAS system without Return Path? Frankly, we are trying to introduce outdoor air from outside via ducting network with a box-in-line fan which delivers fresh air into several diffusers/grilles in different rooms. Fresh air is then blown into destined rooms without any return network, the air is assumed gone via exfiltration. Should I go with DOAS system or Unitary Outdoor Air Unit? Any help would be appreciated.

ad3_pras gravatar imagead3_pras ( 2015-11-25 03:53:02 -0500 )edit

ad3_pras EnergyPlus requires a return path for air loops. You will still dump all the air at the OA mixing box, so it's just a couple of extra components to keep the simulation happy. If you want the DOAS flow to impact the zone with reduced infiltration, then (as of v8.4.0) you can force the DOAS return flow to be zero (see ZoneHVAC:EquipmentConnections) and use new options in ZoneAirMassFlowConservation to couple the DOAS flow to infiltration.

MJWitte gravatar imageMJWitte ( 2015-12-02 16:02:11 -0500 )edit

Julien Marrec the DOAS Setpoint for Design fields are the high/low DOAS supply temperatures which will be used to adjust the zone load on the design days. The meaning of these setpoints varies depending on the option specified for Dedicated Outdoor Air System Control Strategy.

MJWitte gravatar imageMJWitte ( 2015-12-02 16:15:23 -0500 )edit

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Asked: 2015-03-27 05:37:34 -0500

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Last updated: Nov 10 '15