Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page
Ask Your Question
3

How do you model in eQuest a DOAS that supplies air to registers (corridors) as well as other HVAC systems?

asked 2014-09-22 08:51:12 -0500

updated 2015-07-11 20:09:06 -0500

I'm using eQuest and I'm modelling a multifamily building that has a dedicated outdoor air system. It's 100% OA, DX coil for cooling and gas furnace for heating.

The DOAS is supplying fresh air at 55°F in cooling mode and 70°F in heating mode to:

  • Supply registers in the corridors (just a ventilation grille)
  • Indoor units of a VRF/VRV system in the apartments.

I initially created a dummy space and zone to defined it as the control zone of the DOAS system, which is right now a Packaged-Single Zone (could be a poor choice here) (explained in this energy-models.com video)

For my VRV systems, which are Packaged Single Zones (per Daikin's recommendation on modelling VRV units), I've set the OA-FROM-SYSTEM to be my DOAS.

For all my zones receiving fresh air, both corridors and apartments served by VRV, I've defined the outdoor air flow in the ZONE:OUTSIDE-AIR-FLOW attribute.

I was wondering how I should model my corridors. Right now I've put the 8 corridors and two storage zones under the DOAS system (so 8 corridors + 2 storage + Dummy Zone = 11 Zones), but I'm getting two warnings that I'm not sure I can safely ignore or not.

I've defined the DOAS, per mech plans, to have SYSTEM:SUPPLY-FLOW: 4000 cfm SYSTEM:MIN-FLOW-RATIO: 1 SYSTEM:MIN-OUTSIDE-AIR: 1

* CAUTION *********************** DOA RTU System defined as an OA-FROM-SYSTEM has 11 CONDITIONED ZONEs - only one was expected.

* WARNING *********************** DOA RTU System defined as an OA-FROM-SYSTEM has OA flow requirement of 2180. CFM but its calculated/specified OA flow is only 889. CFM. Its MIN-OUTSIDE-AIR has been reset to meet the requirement, however it is strongly recommended you re-run with this MIN-OUTSIDE-AIR specified.

Here are the questions I really have:

  • Regarding the caution, am I supposed to put my corridors somewhere else such as only having one dummy zone under my DOAS?

If yes, where should I put them?

If not, what other system should I maybe use? (the Packaged Single-zone might be a problem here...)

  • Regarding the warning: the 2180 CFM is the sum of the Zone:outside-air-flow I've defined for the systems served by VRV units. I have floor multipliers so it should be 3270 CFM. For example, I've got one zone that receives 190 CFM and has a multiplier of 3. Am I supposed to input 190 CFM or 3*190=570 CFM for this zone? (I think 190 is fine because it does the computation correctly for the corridors... see below)

  • eQuest calculated 889 CFM. That's 850 CFM from corridors (it does take into account multipliers correctly here), plus 39 CFM for my dummy zone. I'm not sure how it computed these 39 CFM. How should I adjust that?

My entire "HVAC System / Zones" section is located here for more information.

edit retag flag offensive close merge delete

Comments

Just as an alternative, you may want to consider this method for modeling DOAS in eQuest: http://energy-models.com/forum/new-eq...

Also, two other points which may or may not be relevent to you: 1) Heat recovery doesn't work when using OA-FROM-SYSTEM, if you have some kind of heat recovery on the OA system. 2) Make sure you've fixed the VRV Heating EIR-F-EWB/ODB AC curve, coefficient d, which doesn't import properly from the Daikin curves document. (Imports as 7.14585, should be 0.0714585). eQuest truncates the exponent because the line is too long.

Benjamin gravatar imageBenjamin ( 2014-11-26 12:32:35 -0500 )edit

2 Answers

Sort by » oldest newest most voted
3

answered 2014-10-10 02:09:10 -0500

yongqingzhao gravatar image

updated 2014-10-10 02:09:48 -0500

I think you can use Unit ventilation system(zero fan energy and modify "Heat Source" to "Not Installed" ) to serve the corridor.And take outdoor air from DOAS by OA-FROM-SYSTEM keyword.

edit flag offensive delete link more
1

answered 2014-11-25 13:34:22 -0500

Modelling DOAS in eQUEST is always a bit tricky. Some things to keep in mind about OA-FROM-SYSTEM:

  • For the borrowing systems, this keyword will set the incoming OA temperature to the return temperature for the DOAS. If the corridors have a set point of 70ºF, the return temperature will be 70ºF, even in cooling mode when the DOAS supply temperature is 55ºF.
  • The OA required by the borrowing systems is NOT automatically added to the OA requirement for the DOAS. The DOAS calculates its OA requirement from the zones listed under it (the corridors). If this OA requirement is lower than the OA required by the borrowing systems, the DOAS OA requirement is set to be equal to the OA required by the borrowing systems, NOT to the sum of the OA for the corridors and the OA for the borrowing systems.

With those points in mind, I will try to answer your questions.

  1. the corridors can safely stay under the DOAS as you have modelled it. You may get high unmet load hours in the corridors. One possible workaround is to relax the zone set points (say to 60ºF in heating and 80ºF in cooling) to allow the corridors to fluctuate in temperature. Another is to model the corridors as unconditioned.
  2. If you want the DOAS to supply 4000 cfm of OA, set the supply flow to 4000 cfm (System Parameters -> Fans -> Flow Parameters). Then set the Minimum OA Sizing Method (System Parameters -> Outdoor Air -> Outside Air and Economizer) to "Set by Critical Zone". Then, for one of the corridor zones, specify the design flow rate (Air Flow tab of the Zone parameters window) and the outdoor air flow rate to be equal to the same number. This should enforce an outdoor air ratio of 1.0. Another approach is to add enough OA to each corridor zone that the sum adds up to 4000 cfm.

You might want to look up the OA-FROM-SYSTEM keyword in the DOE2.2 Dictionary (http://doe2.com/download/DOE-22/DOE22...)

Hope that helps.

edit flag offensive delete link more

Comments

Just to follow up, if you want to ensure that the zones get 55ºF supply temperature, you could set the corridor zone set points to 55ºF in cooling mode. It's a bit of a kludge, but I don't think there's a way to ideally model the system as you have described it.

Daniel gravatar imageDaniel ( 2014-11-25 13:42:00 -0500 )edit

I choose a small corridor as the zone for DOAS so that the OA of borrowing systems is much more than the corridor. However, I found that DOAS would not consider the multiplier of borrowing systems when calculating the OA!!! If I check the OA of borrowing systems itself, it is correct and equal to the number multiplied by floor multiplier. But DOAS does not consider that!

Yan gravatar imageYan ( 2015-12-03 16:25:23 -0500 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

 

Question Tools

1 follower

Stats

Asked: 2014-09-22 08:51:12 -0500

Seen: 2,330 times

Last updated: Nov 25 '14