Baseline Fan Power Calculation (kW/CFM)

Yes, it is true that LEED reviews are coming back with comments on calculations for baseline fan power. The best approach is to run the calculations as requiried by ASHRAE 90.1 2007 G 3.1.2.9 and submit these backup calculations as supporting documents. Ensure to include both the fan brake horsepower and the total fan power. Use the fan supply volumes for each AHU. The supply CFM should be used to calculate the TOTAL fan power, and that fan power should then be broken up into supply, exhaust, return, etc.

Refer to ASHRAE 90.1 2007 Users’ Manual Page G-28 for more explanation of ASHRAE requirements. Below is an example from the User’s Manual:

The supply air volume for an 80,000 ft² medical office building is 120,000 cfm, as determined by the simulation program using a 20 ºF temperature difference between space temperature and supply air temperature as required by § G3.1.2.8. The baseline building HVAC system is system 5 (packaged VAV with a gas furnace). What is the baseline building peak fan power?
A
Using the equation from Table G3.1.2.9, the brake horsepower for the baseline fan
system is 136.5 hp as calculated below:
bhp = 0.0013 × CFMS + A
bhp = 0.0013 × 120000 + 0
bhp = 156
Lookup Table 10.8 for the next size greater than 156 bhp using the enclosed motor at
1800 rpm, the fan motor efficiency is 95%, therefore, the fan power is 122,501 W, as
calculated below:
Pfan = bhp × 746 / Fan Motor Efficiency
Pfan = 156 × 746 / 0.95
Pfan = 122,501 Watts

Hope this is helpful.

Thank you for the explanation. Pitty there's no ASHRAE forum as good as this LEEDuser.

I had assumed that, although not explicitly stated, exhaust fans should be handled just like supply fans and the respective powers calculated the same way. Seems I was wrong.

2 issues still bother me...

1) If I design a out door air only system that has an exhaust fan half the size of the supply fan and ASHRAE 90.1-2007 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."

...then it forces me to seriously oversize my exhaust fans for the baseline case (as the 90% rule would apply, since I have no recirculation).

2) As the proceedure includes a factor for conditioned spaces, it assumes that there's a cooling or heating coil in my Air Handling Unit, which their may not be. It also thereby excludes pure extraction / exhaust fans. What happens when I have 6 (not all nessesarily the same size) in-room recirculation fancoils per zone and say 20 zones with them, do I do this calculation possibly 6 x 20 times?

I'm diving in the deep end here - pretty good conversation already.

The key for getting the fan power consumption for both the proposed case and the baseline case close to accurate is to make sure that all the air movement taking place in the building (whether OA or re-circulation) is being accounted for in some way. There are several ways to do that. Keeping it simple and following the ASHRAE 90.1-2007, G3.1.2.9 and examples from the ASHRAE user’s manual works best.

What Shilpa has stated below is the method we use most of the time to calculate fan power consumption for our baseline cases and it works with the reviewers.

For the 2 issues stated below by Jean, here are our thoughts:

1) In my world the "goesintos" have to equal the "goesoutofs". If the OA system has exhaust fan just half the size of supply fan, then I would think that excess air would be exhausting through some other fan system in the design (toilet fans, etc.). The design fan power calculations should take account of this. For the baseline case, 90% rule would hold true as stated in AHSRAE 90.1-2007

With some exceptions (schedule differences, heat recovery in Baseline, etc.), it’s OK to consolidate fan types in the Baseline. The requirement is simply that the fans serve the same functions. Consolidating fans by function makes it much more straight forward to apply the ASHRAE rules.

The key to the fan power calcs is that they give TOTAL Fan Power. You basically just need to distribute that power appropriately to account for when it’s in use.

2) For fans which account as part of the process loads, and don't interact with other loads, (for example, garage exhaust fans) we typically perform a simple power consumption calculation, with an assumed usage schedule to estimate the fan’s energy use for a year.

The ASHRAE 90.1 User's Manual backs this approach up.

The 90.1-2004 User's Manual states in Section 6.5.3,
"Fans that ventilate only, such as garage exhaust fans or equipment room ventilation fans that transfer only unconditioned outdoor air, do not qualify as a fan system in this context.  Fan systems must be part of a system with heating or cooling capability."

The user's manual provides a few additional useful clarifications regarding what constitutes a "fan system". Garage exhaust fans, stairwell pressurization fans, etc. should be modeled as miscellaneous load, with both the CFM and BHP per CFM modeled identically (except the motor efficiency in the proposed case can be modeled as improving upon the requirements of Section 10.4). LEED allows you to model other savings via control strategies, as an exceptional calculation measure (ECM).

Regarding the question on fancoils, if we go by the ASHRAE methodology, we would do the calculations for each zone.

As an example, if the baseline system was PZS-AC and the design has 20 thermal zones, then for the baseline case each zone would be served by its own PSZ-AC, and yes, LEED reviewers expect you to size and calculate fan power for each of those PSZ-AC systems. So follow the same the same methodology for the design case.

Also to be noted is that the fan power calculation as described in section G3.1.2.9 is excluding any fan powered VAV boxes which assume a fixed power consumption of .35W/cfm for the baseline case.

Christopher...regarding your following comments:

The 90.1-2004 User's Manual states in Section 6.5.3,
"Fans that ventilate only, such as garage exhaust fans or equipment room ventilation fans that transfer only unconditioned outdoor air, do not qualify as a fan system in this context. Fan systems must be part of a system with heating or cooling capability."

The user's manual provides a few additional useful clarifications regarding what constitutes a "fan system". Garage exhaust fans, stairwell pressurization fans, etc. should be modeled as miscellaneous load, with both the CFM and BHP per CFM modeled identically (except the motor efficiency in the proposed case can be modeled as improving upon the requirements of Section 10.4). LEED allows you to model other savings via control strategies, as an exceptional calculation measure (ECM).

My questions:
1) WC exhaust only fan, from a "conditioned" WC, i.e. it has a radiator in it. Makeup transfer air flows into WC under door slot. "Fan systems must be part of a system with heating or cooling capability." --- does this mean a 'Ventilation' system or is the word system including my radiator? The space is 'conditioned' by the 90.1 definition. NB: There is no Supply Air in the building...it is naturally ventilated.

Jean:

Re: 4/14 comment - we find the real opportunity for improvement is in air distribution design. If we can oversize ducts, coils and filter banks then we can reduce system static pressure, and therefore fan energy.

Re:4/16 comment - that fan energy must be accounted for somewhere in the model. Even with a "naturally ventilated" building, you are still comparing against a mechanically heated base case. I think your design case includes that fan only - it is the "system".

Can we calculate Outdoor air flow and supply airflow as per ASHRAE 62_1-2010, column 6.2.2.1.

I am waiting for your reply.

I am not sure your question is in the right place. LEED 2009 references 62.1-2007. Maybe your question relates to IEQp1? It is hard to tell what your question is in reference to?

As it relates to the subject of this thread, no. It is not that simple. This is part of the outside air calculation. You can download our fan power calculator in the Documentation Toolkit.