[Equest-users] FW: Number of systems required by 90.1?

[George Hu]

This brings up the subject of off-time operation of AHU’s.  Having one single zone calling for a large AHU in a (large) building

to run is probably an extreme case. More realistically, probably a small number of zones call for the AHU to run.

With large turn down ratios of the supply fan(s) in the AHU driven by VFD(s), and the ability of DDC control systems to 

shut off VAV boxes in no-demand zones, I would tend to believe the number of AHU’s would not make a big difference

in (fan) energy cost. This is, of course, assuming that the simulation software is capable of modeling all these features

accurately.

 

 

George Hu, PE, LEED AP

President

AWE | Air Water Energy Engineers, Inc.

781.652.8688

www.awe-e.com

 

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: Wednesday, December 10, 2008 1:30 PM
To: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] FW: Number of systems required by 90.1?

 

The difference in energy consumption becomes especially apparent when you have a shutoff VAV system with reheat (like I do with this particular model). If, for example, a ground floor interior zone called for heating after-hours because of heat loss thru the slab, and you only had one system for the whole building that has to operate, the energy consumption is significantly more than if you have a system for each floor.

If LEED gives you the option to use Addenda or not use Addenda, then in my case, I would more than likely NOT choose to use it. As long as this is acceptable, which it appears it is, I can take significant credit for having multiple AHUs in lieu of a single RTU system.

It sort of feels like cheating to do this, but ASHRAE is very specific on how you are to model the Baseline building, and I’m not doing anything “wrong”, per se, by using a single system for the Baseline building. For what it’s worth, there are a lot of buildings that use package penthouse AHUs…

GHT Limited
James Hansen, PE, LEED AP

Senior Associate

1010 N. Glebe Rd, Suite 200

Arlington, VA  22201-4749

703-338-5754 (Cell)

703-243-1200 (Office)

703-276-1376 (Fax)

 <http://www.ghtltd.com/> www.ghtltd.com

 

From: George Hu [mailto:george.hu at awe-e.com] 
Sent: Wednesday, December 10, 2008 1:20 PM
To: 'Bill Talbert'; James Hansen; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] FW: Number of systems required by 90.1?

 

It seems odd to me that 90.1-2007 requires the baseline building to have one system per floor (system types 5, 6, 7 & 8).

These are all VAV systems, where you have individual VAV boxes responding to different thermal zones. Why would different

number of AHU’s in this case make a difference in heating and cooling energy consumption?

In terms of fan energy, however, different number of AHU’s may make a difference. This is because 90.1 regulates

the fan brake horsepower, which is depending on the size of the fan (Table G3.1.2.9, <20,000 cfm or >20,000 cfm).

For the same sized  building, assuming the total supply airflow being roughly the same independent of the number of 

AHU’s used, each individual fan’s brake house power would be different depending on its fan size, this may lead to 

different total fan energy consumption when you add all individual fan energy up. For this reason, I would argue, it makes

more sense to keep the same number of AHU’s in the baseline case as in the proposed design, for you would then be comparing

“apple to apple”, because the fan sizes in the proposed design would be “comparable” to those in the baseline case.

 

George Hu, PE, LEED AP

President

AWE | Air Water Energy Engineers, Inc.

781.652.8688

www.awe-e.com

 

 

  _____  

The information contained in this communication is confidential, may be privileged, and is intended only for the use of the addressee.  It is the property of GHT Limited.  Unauthorized use, disclosure or copying of this communication or any part thereof is strictly prohibited and may be unlawful.  If you have received this communication in error, please notify me immediately by return e-mail or by e-mail to  <mailto:ght at ghtltd.com> ght at ghtltd.com, and destroy this communication and all copies thereof, including all attachments.  Thank you.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/attachments/20081210/6b4f7d4d/attachment-0002.htm>