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

Dan Russell danr at engineeringinc.com
Wed Aug 27 09:20:18 PDT 2008

David - a couple follow-up questions regarding the two separate
circumstances you bring to light:


1)      Using Appendix G as the governing authority for performing
energy simulations

a.       I now see that Section G3.1.3.15 states "VAV system supply fans
shall have variable speed drives..." with no mention of this requirement
applying only to individual fans greater than 15 hp.  So, for energy
simulation purposes it seems the baseline is required to use VFD's.

b.      The wording specifically identifies "supply fans" but would this
apply to return or relief fans as well?  Or, can the baseline return or
relief fans be modeled without VFD's using constant volume w/ motorized
damper on a bypass loop modulated to maintain building pressure?

2)      Using section 6 for prescriptive code compliance.

a.       Is it your interpretation of Section that fanwall air
handlers using banks of individual fans rated at less than 15 hp each
would be exempt from the VFD requirement?  (i.e. Fanwall controlled
simply by staging individual fans on/off to maintain system flow &
pressure requirements would meet the intent of this prescriptive
section?) OR is this an unintended loop-hole in the wording of the


All responses welcome, 




 Dan Russell, EIT 




From: bldg-rate-bounces at lists.onebuilding.org
[mailto:bldg-rate-bounces at lists.onebuilding.org] On Behalf Of David S
Sent: Tuesday, August 26, 2008 10:14 AM
To: bldg-rate at lists.onebuilding.org
Subject: Re: [Bldg-rate] ASHRAE 90.1 - VAV Fan Part-Load Power


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
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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