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# LEED Baseline Building: G3.1.2.9

Hi, I have a doubt about System Fan Power with System 8.

G3.1.2.9 says: System fan electrical power for supply, return, exhaust, and relief (excluding power to fanpowered VAV boxes) shall be calculated using the following formulas:....

Moreover, G3.1.2.8 says: If return or relief fans are specified in the proposed design, the baseline building design shall also be modeled with fans serving the same functions and sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger.

Therefore, I understood to apply the G3.1.2.9 method to the supply fan with 100% of design flow rate and to apply it again to the return fan with 90% of design flow (or the supply minus the outdoor air). So I get two different powers to apply to these fans.

My way of thinking appears to be wrong since the power is intended to be the sum of the supply and return fan. This can be noticed also from the 1.4 table.

Can someone make it clear? If the second method is the correct one: how do I redistribute this power between supply and return fans?

Thank you

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The calculations in G3.1.2.9 state to use CFM_s; which is the CFM of the supply. This CFM is auto-sized based on the flow rate needed in the Baseline building. This CFM will depend upon the space load sizing and is based on a 20F delta supply air temperature from setpoint per G3.1.2.8. So, per G3.1.2.9 one calculates the kW allowed per system Supply airflow rate only. This kW is allocated proportionally to the fans in the system (supply, return, exhaust, and relief). example: if the calculated Baseline Fan system is 10 kW of fan power and in the Proposed building the fan power is 50% supply, 30% return, and 20% exhaust; then this 10kW is split to 5kW supply, 3kW return, 2kW exhaust. I've also seen modelers who take this 10kW and split it based on the combined flow rate of the baseline's supply+exhaust+return+relief and apply this kW/CFM_bl_total to all fans in the baseline. The end result is probably highly similar for most cases since the Baseline fan system is still only 10kW. There is no allowance for return, exhaust, or relief flow. There are allowances for things like filtration or energy recovery, but not other fans in the system.

The directions in G3.1.2.8 will impact the system flow rates. So, while there's no power allowance for the return/relief fan, the presence of the return/relief fan should/will impact the calculated supply fan flow rate.

Note: All code references are to 90.1-2007 / LEEDv2009/3. 90.1-2010/LEEDv4 has same/similar requirements, but code section numbers are slightly different.

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Thanks for the answer. It's hard to me to allocate it proportionally to the fans of the proposed since in the proposed I have 2 or 3 AHU for every floor and in the Baseline just one... So which proportion should I take? I write this question just to point out that it is not always possible.

Nevertheless, your explanation is clear: "There is no allowance for return, exhaust, or relief flow"; I think the proportion for supply return fans is up to the modeler.

Thank you.

( 2018-03-20 05:48:32 -0600 )edit
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Agreed - this is where art, science, and judgement all meet. In highly complex cases I have typically just allocated based on the BL CFM by getting the kW/CFM for the system; kW from the supply fan flow allowance divided by all supply ,exhaust, return, & relief fan in that system. This still accounts for all of the kW allowed, just happens to be over a larger amount of airflow. Good luck!

( 2018-03-20 08:39:11 -0600 )edit