[Bldg-sim] System Fan Power in Appendix G

[Zeng, Ming]

My understanding is option 1. The fan energy in that formula is the sum of all the fan horse power. Search the board, I think there is a previous email from Michael Rosenberg talking about the fan energy. Hope this helps.

Ming Zeng, PE, LEED(r) Accredited Professional
Mechanical Engineer

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From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Mike Kaufman
Sent: Tuesday, January 20, 2009 12:16 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] System Fan Power in Appendix G

Hi All,

I have a specific question about using Appendix G3.1.2.9 to calculate baseline system fan power (I have already searched previous posts and could not find the answer I'm looking for).  So, the Appendix G section reads as follows: System fan electrical power for supply, return, exhaust, and relief (excluding power to fan powered VAV boxes) shall be calculated using the following formulas.  I understand the formulas, which essentially use a fan CFM to determine a brake horsepower BHP, which is then used to calculate the fan power (Watts), but my question is this:  Supposing you have an HVAC system with supply, return, and small exhaust fans (in restrooms, for example) - how do you calculate the power to each fan?

Option 1:  Use your supply fan cfm to calculate a power, say it comes to 10 kW, and use this as your total "system" fan power.  This 10 kW, being your whole system power, is then distributed among all of your system fans however you desire (For example, supply fan power = 5 kW, return fan power = 4 kW, exhaust fan power = 1 kW, for a total of 10 kW for the fan system)

OR

Option 2:  Use each supply, return, and exhaust fan cfm to separately calculate an individual kW for each fan (For example, supply fan power = 10 kW using the supply cfm, return fan power = 9 kW using the return cfm, exhaust fan power = 1 kW using the exhaust cfm, for a total of 20 kW for the whole fan system)

Obviously, you will have much more fan energy in a baseline system calculating fan power with Option 2.  Does anyone have a specific example they can share of a fan system calculation that was accepted by USGBC for an EAc1 energy model?

Thanks for your help!
Mike

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Mike Kaufman, Building Performance Engineer
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