[Equest-users] HVAC system - Parametric Runs

Fred Porter FPorter at archenergy.com
Mon Aug 18 07:57:50 PDT 2008

>>> "Patrick Darby" <pdarby at twfp.com> 8/18/2008 7:22 AM >>>

My system involves first a CDQ which is supplied with 100% outdoor sir which heats and/or cools the air and supplies it to two air handlers, which supply VAV boxes. I have modelled this using a CDQ (Variable Air Volume) unit which supplies the two Air Handling Units (Power Induction Units).
First, I believe a CDQ is TRANE's trade name for use of a "wrap-around" desiccant-based "passive dehumidification" wheel aka the "Cromer cycle." The wheel sits DOWNSTREAM of the cooling coil, not upstream as in the DOE-2.2 ERV component.  See http://www.trane.com/Commercial/Uploads/Pdf/1070/clch-prb020-en%200904%20cdq%20eb.pdf Neither this nor the previously more common but less-effective "wrap-around" heat pipes are intrinsically supported by eQuest/DOE-2.2. The eastern utilities that are basing incentives on modeling results need to fund improvements in dehumidification systems modeling in DOE-2. Or maybe we need to chip in. (EnergyPlus supposedly supports both configurations if one can figure out how to input the systems and building and generate useful outputs in less than a month, but then only supplies the OA to rooms, not other AHUs. TRACE at least supports wraparound heat pipes, and would be expected to model the CDQ now or very soon, and integrates its dedicated MAUs into the entire model.)

Second, If I'm mistaken and even if the MAU AHU is an exhaust-makeup wheel in the specified configuration, this usually can't be modeled using the OA-FROM-SYS method in DOE-2 that seems to be indicated here. That is because the OA-SYSTEM itself will think the RA is from its "duct-zone" and this air will not have the space conditions of the building return air. I have used a complicated and brittle work around to partially remedy this, but I really don't recommend it. Would be nice if DOE-2 had a RA-FROM-SYS command and MAU "airflow tracking." But it doesn't, so the modeler needs to be very, very careful.

Some part of the system will need to be neglected in the model, with its effect calculated by a side calc or "exceptional calculation" in LEED speak. Depending on whether the majority of energy savings are determined by dehumidification with raised main SATs, or exhaust energy recovery, the proposed system should be modeled with the OA-FROM-SYS and a very low SAT off that coil, but w/o heat recovery, or without OA-FROM-SYS but with the "ERV" model enabled on the "PIU" systems. But in the first case, an equivalent low SAT (to model  dewpoint from the CDQ) may not be possible with the proposed CHWT. 

>>> "Patrick Darby" <pdarby at twfp.com> 8/18/2008 7:22 AM >>>

I am now creating my reference case using parametric runs and based on the Model National Energy Code (Prescriptive Compliance), and using the article Performance Compliance for Buildings - May 1999. The Zone References can only be AH1 - Single Zone (constant flow) or  AH2 - Multiple Zone (variable flow) according to the Compliance. I was wondering if anyone has suggestions on what reference case I should based each of my three HVAC systems?
Third, As far as the reference case model in Canada, I thought some automated front end to DOE-2.1 was usually used (required?) to generate that, at least for code compliance. For LEED, the 90.1 PRM can still be used to generate a baseline even if it isn't the local code. And if the PRM models demonstrate the now-required 14% savings, LEED deems this to indicate compliance with EAp1, w/o requiring code-specific performance calculations. 

Welcome to our world;

Thank you,

Patrick Darby, BScE
Mechanical Engineering Services 

The Walter Fedy Partnership 
Phone: 519-576-2150 x271
Fax: 519-576-5499
Web: www.twfp.com

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