Re: [EnergyPlus_Support] Zone Level Airflows

On Mon, Jul 8, 2013 at 11:59 AM, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

I think the LEED reviewers need to take a course in HVAC physics. You cannot oversize by 25% without also increasing the fan size by 25% or the delta T will rise and they?ll complain about that! An oversized system must be oversized in every component!

You may want to explain this more delicately than I have, but that strikes me as a comment from a reviewer unfamiliar with normal HVAC sizing and modeling processes.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Marcus
Sent: Monday, July 08, 2013 5:15 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx

Subject: Re: [EnergyPlus_Support] Zone Level Airflows

We are also confused by this issue!

Review comment for LEED:

The narrative response indicates that the Baseline Case fan air flow rates have been sized based on a

20 degrees F supply-air-to-room-air temperature difference. However, the simulation reports provided indicate that the Baseline Case

fan air flow rates have been oversized by 25%. For future submittals, revise the Baseline Case systems to remove the oversizing for the

fan air flow rates and provide updated simulation reports reflecting any changes.

How else shall we meet the 25% oversizing in G3.1.2.2? I confirmed that I have a 11C dT for both calculated and user conditions, so I don't have the issue Mathew described, they all match. But since the oversized cooling load is 25% higher than calculated, therefore the air flow rate is 25% higher. How shall we remove 25% oversizing for air flow without removing the overall 25% zone oversize?

Cheers,
Marcus
--
Marcus Jones, M.Sc., LEED^®AP BD+C
Freelance energy consultant
Vienna, Austria

On Fri, Jun 28, 2013 at 9:22 PM, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

p.s., Just in case it escaped your notice, the ?SystemSummary? report provides a very nice summary, zone by zone, of unmet load hours for each month.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Jim Dirkes
Sent: Friday, June 28, 2013 2:38 PM

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: [EnergyPlus_Support] Zone Level Airflows

In addition to Jean?s comments?..
I have not looked into it in detail, but every so often an auto-sized zone will not meet all of the loads (i.e., there will be lot of unmet load hours).

ASHRAE 90.1 anticipates this and allows the modeler to increase the sizing factor as needed to get the unmet loads under control.
If this situation occurs, I check my inputs for the problem zone(s) and if they look OK, I try raising the zone sizing factor. That normally helps.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of jeannieboef@xxxxxxxxx
Sent: Friday, June 28, 2013 2:28 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Cc: <EnergyPlus_Support@xxxxxxxxxxxxxxx>
Subject: Re: [EnergyPlus_Support] Zone Level Airflows

Sizing:zone object you specify the air temperature and humidity with
which the air should enter the zone (it doesn't matter where the air
comes from or what amount of it is ODA). This air's energy (enthalpy
and moisture) is added to the zones energy and moisture balance
equations. In effect the volume airflow to meet the load at specified
supply temp is calculated...sizing factor is included.

This is how the load is deturmined for the zone. The equipment is then
sized (sizing:system object) to meet this load (air flow at specified
temp). It may be specified that the equipment must supply a minimum
ODA amount (and the equipment looks up this value from the sizing:zone
object). If the value of the minimum ODA flowrate exceeds the total
needed for cooling, the ODA rate will be used (100% ODA) and
conditioned to the setpoints. The setpoint manager assigned air
setpoint temps should be setup to deliver at least the air at the same
setpoints as the design air setpoints that you put in the sizing:zone
object.
>

--
Marcus Jones, M.Sc., LEED^®AP BD+C
Freelance energy consultant
Vienna, Austria