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

James V. Dirkes II P.E. jvd2pe at tds.net
Wed Aug 27 10:24:30 PDT 2008


Dear Dan,
 
It seems that you have two possible scenarios:

1.	

	You need only to comply with 90.1 as a minimum performance level:
In that case, it seems that you comply with the "letter" of 6.5 by having no
motor larger than 10HP.  You also comply with the "spirit" if you turn fans
off as load diminishes.
2.	

	You are using App G for a LEED project:
In this case Section 6.5 does not apply.  You need to model a Baseline case
appropriate for your building type and size and a Proposed case that matches
your actual Fanwall system and its controls.

 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

 

  _____  

From: bldg-rate-bounces at lists.onebuilding.org
[mailto:bldg-rate-bounces at lists.onebuilding.org] On Behalf Of Dan Russell
Sent: Wednesday, August 27, 2008 12:20 PM
To: bldg-rate at lists.onebuilding.org
Subject: Re: [Bldg-rate] ASHRAE 90.1 - VAV Fan Part-Load Power Limitation



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 6.5.3.2.1 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 Standard?

 

All responses welcome, 

 

Thanks,

 

 Dan Russell, EIT 

 

cid:image001.jpg at 01C840A4.E711B250

 

From: bldg-rate-bounces at lists.onebuilding.org
[mailto:bldg-rate-bounces at lists.onebuilding.org] On Behalf Of David S
Eldridge
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 Limitation

 

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!

 

David

 

 

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 6.5.3.2.1 Part-Load Fan Power Limitation from
ASHRAE Standard 90.1-2004, which states:

 

6.5.3.2.1 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

blades.

(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?

 

Thanks,

 

 Dan Russell, EIT 

 

cid:image001.jpg at 01C840A4.E711B250

 

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