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How is the baseline system autosized in CBECC-com?

asked 2019-07-21 11:16:41 -0600

Ranjani gravatar image

updated 2019-07-21 14:18:09 -0600

Hello everyone,

I want to understand what ruleset is used the autosize the baseline system in CBECC-com. I am working with CBECC-Com prototype model (High Rise NR). Does CBECC-Com use 1 cfm/sf and 400 cfm/ton to size the baseline? Does it take into account the envelope loads, type of glass and WWR of the building as well? If the proposed case has 50% WWR, does the baseline also assume 50% WWR? I also tried using the same model in Open studio but the system size was double of what Cbecc-com has. I want to understand why the difference?

Appreciate the help

Thanks Ranjani

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answered 2019-07-22 11:25:02 -0600

I believe CBECC-COM uses EnergyPlus's "autosize" feature to size the baseline model, but I'm not completely sure. The ACM states that the Standard Design boiler is sized to be 25% larger than the peak loads, and chillers are sized to be 15% larger than peak loads. However, I have tried taking the baseline model IDF that CBECC-COM creates, set all available parameters to autosize, set global heating/cooling sizing factors to 1.25 and 1.15, respectively, and the sizing still comes out slightly different than what CBECC produces. So I'm not quite sure exactly how CBECC sizes the baseline model.

Whichever way it sizes, it definitely accounts for all internal/external loads, and does not use rules of thumb (i.e. 1 cfm/sf and 400 cfm/ton). The sizing accounts for all envelope components, such as wall u-factors, glass type and WWR. Regarding WWR, the prescriptive maximum is 40%, so if the proposed building has 50% WWR, the baseline model will only have 40%.

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ericmartinpe gravatar image ericmartinpe  ( 2019-07-24 11:02:48 -0600 )edit

@ericmartinpe Thanks for the reply. Also, is the TDV measure in OS/E+ same as CEBCC-Com? I know that OS/E+ s not a T24 Compliant software. I want to understand what the difference is?

Ranjani gravatar image Ranjani  ( 2019-07-24 13:34:13 -0600 )edit

I'm not familiar with the TDV measure in OS so I don't know how that works.

CBECC-COM generates EnergyPlus IDF files for the Standard and Proposed building models based on the inputs in CBECC-COM, runs both simulations in E+, and applies the hourly TDV factors for the output report.

You could take the IDFs that CBECC-COM creates and apply the hourly TDV factors as FuelFactors in E+, and you will get the same TDV results that CBECC-COM reports (except for DHW, Solar PV, and Battery, which are simulated using CSE).

ericmartinpe gravatar image ericmartinpe  ( 2019-07-24 13:48:47 -0600 )edit

I received the following response from CBECC-COM support (Roger Hedrick, Principal Engineer):

ericmartinpe gravatar image ericmartinpe  ( 2019-07-26 18:03:41 -0600 )edit

The process used to set the sizing values for the annual baseline models is a multistep process. The sizing baseline run is simulated using EnergyPlus autosizing and sizing factors set to 1.0. The results of this run are read by the ruleset, which then determines the appropriate number of pieces of central plant equipment, calculates capacity adjustments for pump and fan heat, applies the 1.25 and 1.15 sizing factors, and makes any other needed adjustments to capacity or flowrate. The calculated capacity and flowrate values are then passed to EnergyPlus for simulation.

ericmartinpe gravatar image ericmartinpe  ( 2019-07-26 18:03:51 -0600 )edit

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Asked: 2019-07-21 11:16:41 -0600

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Last updated: Jul 22 '19