Forum discussion

NC-v4 EAp2:Minimum energy performance

Proposed Fan Power higher than the Baseline Fan Power

Hi Marcus, 

We seem to struggle on every energy model when calculating the proposed fan power....it is always so much higher than the baseline. I get pushback from design teams when advising them to use smaller fans.

Have you come across projects that demonstrate fan power savings when compared to the baseline? and if so, what's the secret sauce?

My guess is that it is simply smaller fans, which forces designers to select what is actually needed rather then the CYA design methodology.

Any expert insight is always appreciated.

Best,

dave 

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Wed, 12/15/2021 - 15:28

I think the intent behind the new calculation methodology is explicitly designed to encourage designers to right size systems and not oversize them. So essentially there is now a penalty for oversizing, and the only solution I know of is right sizing. Maybe presenting it to them in these terms instead of saying they need smaller fans might help. I'll see if I can get one of our engineer reviewers to chime in on some more specific strategies.

Thu, 12/16/2021 - 00:26

Thanks Marcus, I appreciate you connecting with your engineer reviewers on this. and I like the terminology "right sizing". Will use this when chatting with design teams. When talking LEED / Sustainability I always present as "best practices" - folks seem more receptive, since nobody wants to shout out on a call that their company does not implement best practices.  thanks again.

Thu, 12/16/2021 - 14:55

Marcus is correct that this is usually a result of over-sizing the design. If supply air temperatures are similar to the Baseline, the space loads would be expected to be lower in the Proposed model than the Baseline and therefore should result in lower airflows when sized to meet the loads. One way to test this would be to run a Proposed model with auto-sizing applied to see the difference. Fan power would simply follow airflow, assuming similar pressure drop, so make sure you're applying all applicable Baseline pressure drop adjustments to keep the fan power comporable. The other thing to keep in mind is that you won't necessarily be paying an energy penalty for over-sized fans as long as they're variable speed and allowed to modulate to meet the loads due to the fan affinity laws.

Thu, 12/16/2021 - 15:19

Thanks Tyler. Very helpful.

Thu, 12/16/2021 - 16:53

David, I have had the same experience with my fans almost always exceeding the fan power allowance.  Sometimes it is over-sizing, but there are also design constraints the PD built into the fan power allowance doesn't account for.  It has gotten better for certain building types, such as labs, in subsequent versions of 90.1.  I have always been advised to increase the duct size or better yet the AHU box size to decrease the PD and fan power for those longer runs, but that isn't always an option.  I run into the same issues with baseline pump W/gpm.  I very rarely have a system with less than 22 W/gpm for CHW.  It is usually more like 35 W/gpm, especially if it is from a central CEP.

Thu, 12/16/2021 - 17:04

thanks for your insight Cory, much appreciated. 

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