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

Bill Talbert btalbert at aeieng.com
Tue Aug 26 09:06:37 PDT 2008

If you are pursuing LEED points, A90.1 Appendix G outlines the fan power requirements and provides VAV fan part load performance requirements. If your baseline system is VAV, then it would need to utilize the G3.1.3.15 part load curve and the fan power limitations per Addendum ac or the fan power limits outlined in G3.1.2.9. Per Addendum ac, fan power is based on total system CFM for systems over 5 hp, not individual fan power. Section largely applies for the ECB compliance path and the prescriptive compliance path, not performance comparisons like LEED.

You could utilize a custom part load curve that reflects individual fans staging off in the fan wall system to take credit for improved part load conditions. My experience with fan wall is that system static pressure is reduced due to less system effect, sound attenuation reqs, etc., so perhaps your proposed fan power is lower than what is allowed for the baseline.

Bill Talbert, PE, LEED® AP
Mechanical Systems
Phone: (608) 441-6677
E-mail: btalbert at aeieng.com

Affiliated Engineers Inc.
5802 Research Park Blvd.
Madison, WI. 53719
Tel. (608) 238-2616
Fax. (608) 238-2614

>>> "Dan Russell" <danr at engineeringinc.com> 8/26/2008 10:04 AM >>>

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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