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CBECC-com DOAS High-Rise Residential

asked 2019-10-09 18:49:04 -0600

updated 2019-10-09 18:49:30 -0600

Version CBECC-Com 2019.1.0 - Energy Plus v9.0 Project/model is a High Rise Residential which will be designer/permitted in the 2019 Code Cycle. Proposed building will have WSHP, Water-Cooled VRF, or Air-Source VRF; with a decoupled DOAS for ventilation (unconditioned direct OA to space).

The Standard (Baseline) Design is a 4 Pipe FCU with Water Cooled Chiller + Gas Boiler. Ventilation is via a balanced ventilation system (sort of). This is implemented into the model as an exhaust fan system and corresponding infiltration. The exhaust fan power will be adjust to accounting for an Exhaust and a Supply Fan.

Testing systems (and multiple other factors – ex: envelope parameters), indicates that the proposed is losing to the Standard in unexpected/unexplainable ways.

The Proposed building idf has a Zone Inlet Node List with two nodes - one for the TerminalUnit (from DOAS) and the other from the ZoneSystem. This is as expected for a decoupled ventilation system, yes? This should pass raw OA to the zone, and then the zone unit can pick up this load once it passed through a zone exhaust node.

The Standard building has a single Inlet Node from the 4 Pipe Fan Coil; and exhaust node list with 2 nodes - 1 to the 4 Pipe FCU and one to the Exhaust Fan. The Standard Design does have a 'ventilation' ZoneInfiltration object set with Flow/Zone which is equal to the zone ventilation requirements. It (and the Zone exhaust fan) have a schedule set to =1 for all hours (as expected).

The Proposed zone coil sizes at ~10Ton. The Standard zone coil is sizes at ~4Tons.

On a Building Basis. The Standard Design is ~400T. For a 365,000 SF building, this Ton/SF is wildly off. For the Proposed Design, the Load is ~1050T; which is close to the more reasonable 350 SF/T.

It would appear as though that in the Standard Design the Infiltration air load imposed on the zone is not being passed to the coils properly. Is this accurate? Is it possible that the ZoneInfiltration load is not imposed on the Zone 4Pipe FCU and it exhaust before being seen by the zone equipment? Have other had issues with Exhaust ventilation system/Infiltration/zone equipment sizing?

Also, on the Standard Design Fan:ZoneExhaust, the Field: Balanced Exhaust Fraction Schedule Name is set to =0 for all hours. The E+ documentation was confusing on how this either would/wouldn't allow for these infiltration air loads to be unbalanced and impact conditioning/airflow.

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not sure, but seems like the following might be applicable: Isse discussion moved to talking about VAV dampers from an AirLoop; which doesnt exist in the Standard Design but does existing the Proposed Design as the DOAS TerminalUnit.

dradair gravatar image dradair  ( 2019-10-11 08:36:12 -0600 )edit

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answered 2019-10-28 00:38:04 -0600

@dradair, thanks for posting this detailed description. In doing some independent internal follow-up testing of v1.0, I've identified a bug that I'm pretty sure explains this discrepancy. The upcoming v1.1 release will include fixes for this and other issues identified since the v1.0 release.

If you haven't already, please make sure that you submit this issue to the CBECC-Com ticket desk ( along with your model so it can be tested. If the model is very large, it is always appreciated if you submit a model that is paired down to only include as few zones/systems as possible so we can test it more efficiently.

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@DavidReddy Issue has been submitted to CEBECC-com support 10/9/19. Was going to follow up this week to see if there were any updates; design team is hot with us as the results dont align with expectations (ours or theirs).

I have not made a simplified model to test this out; but could try to test/ further confirm the issue (and decrease run-times)

dradair gravatar image dradair  ( 2019-10-28 09:06:26 -0600 )edit

Any recent follow up here? I'm also working on a HR Multifamily/Hotel model. It's the first time I've tried implementing DOAS in CBECC-Com for this building type, and I have to say, my proposed model just seems to be taking a beating relative to the fan energy of the T24 baseline.

NathanMiller gravatar image NathanMiller  ( 2020-01-03 11:32:50 -0600 )edit

@NathanMiller, did you submit your model to the CBECC-Com help desk? Which program version are you using? Does the DOAS supply air directly to the zone, or is it connected to the ZnSys?

DavidReddy gravatar image DavidReddy  ( 2020-01-08 00:04:52 -0600 )edit

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Asked: 2019-10-09 18:49:04 -0600

Seen: 209 times

Last updated: Oct 28 '19