[Bldg-rate] ASHRAE 90.1 - VAV Fan Part-Load Power Limitation

David S Eldridge DSE at grummanbutkus.com
Tue Aug 26 09:14:22 PDT 2008

The baseline building will still be constructed in the same method using Appendix G regardless of your choice of proposed AHU.  The baseline system will have a single fan/motor AHU, and it will have VAV characteristics if required based on the system selection table and size of the system.

The portion of the Standard 90.1 you are referring to in Section 6 is for prescriptive compliance.  If your project is using the prescriptive section for code submittal, then you just have to meet or exceed those requirements without a quantification of "how much better".

Hope this helps!


From: bldg-rate-bounces at lists.onebuilding.org [mailto:bldg-rate-bounces at lists.onebuilding.org] On Behalf Of Dan Russell
Sent: Tuesday, August 26, 2008 10:04 AM
To: bldg-rate at lists.onebuilding.org
Subject: [Bldg-rate] ASHRAE 90.1 - VAV Fan Part-Load Power Limitation

My question regards Section Part-Load Fan Power Limitation from ASHRAE Standard 90.1-2004, which states: Part-Load Fan Power Limitation. Individual
VAV fans with motors 15 hp and larger shall meet one of
the following:
(a) The fan shall be driven by a mechanical or electrical variable-
speed drive.
(b) The fan shall be a vane-axial fan with variable-pitch
(c) The fan shall have other controls and devices that will
result in fan motor demand of no more than 30% of
design wattage at 50% of design air volume when static
pressure setpoint equals one-third of the total design static
pressure, based on manufacturer's certified fan data.

I have a project that is using Fanwall technology in the air handlers that serve the VAV system.  The air handler has 3 rows of 8 fans each of them only 7.5 hp each (at total of 24 - 7.5 hp fans for a total of 180 hp).  When the Standard here says "Individual VAV fans..." is it saying that for this type of air handler where no individual fan exceeds 15 hp that part-load control is not required?  Certainly, if this is true, using this technology would be great for demonstrating energy points (aka LEED), but I'm having a hard time justifying this in my mind.

Does anyone know if this Section applies to "fanwall" technology or if it is simply intended to make distinction between supply, return, relief, and exhaust fans?


 Dan Russell, EIT

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