DOE2 peak loads and equipment sizing

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Dear Bldg-Sim users,

In DOE2 , have any users come across major differences between building
peak cooling loads (in the LS-D reports) and the maximum cooling loads
listed in the SS-D reports?

I'm troubleshooting equipment sizing in a DOE2 model and I'm trying nail
down some reasonable explanations for the disparities in the loads.

I would appreciate any words of wisdom.

SP Henry

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Whoa!

Hugh differences by design!

LOADS reports are just that...space heating and cooling loads plus assigned electric loads, which includes occupants, schedules, etc. All at constant thermostat temps for each SPACE.

SYSTEM loads are the loads on the air-side systems, which are system dependent, and include ventilation loads, exhaust fans, thermostats, economizers, etc.

PLANT adds yet another layer of equipment, etc.

Therefore, values from SYSTEMS or PLANT are best used for sizing depending on system type.

Jeff S. Haberl, Ph.D., P.E., FASHRAE

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Thanks Jeff, that?s I good point. I appreciate your summary of the differences between those report types.

In my particular case the ?major? differences I?ve encountered between LOAD and SYSTEM were much larger than what?s reasonable for ventilation/exhaust/economizer.

Any thoughts?

SP Henry-?the tired troubleshooter?

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SP Henry,

Ah the "DOE-2 blues" on a dreary day in March...

If you are looking at peak loads, I suggest looking at when (month, date, and time) the load and system peaks occur in the model.
How different are they from each other?
Are they intuitive?
Do they coincide with a "step" change in the input schedules?
What is the weather file doing then? There are some bad ones out there.
etc...

Good luck,
Paul Riemer

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I posted an ASHRAE 62.1 question to this list a few days ago. I have
made a lot of progress since then, but there are still a few nagging
questions. I know that ASHRAE 62.1 is not a primary focus of this list,
but I hope that some of you LEED gurus out there will help me out.

Basically I am looking to find out if population diversity can be used,
across the board, to reduce the system outside air requirements in 62.1
and if you can do that, how do you document room-by-room compliance as
required by LEED Existing Building requirements? My thoughts right now
are to use population diversity to reduce the entire system OA
requirement, but to check that every room with only one person gets the
required OA allotment.

This is perhaps more strict than necessary.

All ideas are welcome, but if somebody is an ASHRAE 62.2 guru, I 'd love
to correspond off-list on this.

Cheers!

Robert P. Wichert, P.Eng.

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Robert,

I haven't seen any replies to your Std. 62.1 question, so I'll assume you're
still wondering.

The standard calculation of the required outdoor air (OA) amount already
includes population diversity, which typically reduces the total OA
required. So, to your question: yes, diversity can be used to reduce
system OA requirements (and is a normal part of the Std. 62.1 calculation).
Diversity is calculated with equation 6-7, and it's factored into the total
uncorrected OA intake amount in equation 6-6 (referencing 62.1-2007).

If you're correctly using the standard for a system that serves multiple
zones and uses some recirculated air, you will have calculated which zone is
the critical zone: the that zone needs the greatest fraction of primary air
at the ventilation design condition. (Note that the ventilation design
condition is not the cooling or heating design condition, necessarily). The
critical zone drives the total OA requirement. All other zones
automatically get the required OA allotment or more.

To know which zone is the critical zone, look at Appendix A. There's a
section called "Selecting Zones for Calculation".

To your comment "every room with only one person gets the required OA
allotment", note that every zone has a population AND an area component to
the required OA.

There are ways to meet the standard and reduce the total OA required. These
generally involve changing the critical zone until it becomes non-critical
(the next most ventilation intensive zone becomes the critical zone). This
can be done with a transfer air fan (bringing air from an over-ventilated
zone to reduce the required primary air in the critical zone), or by
increasing the minimum VAV flow setting on the critical zone (this zone may
use more reheat energy, but this can be offset by the cooling energy saved
by reducing the total OA, for example). It depends on your particular case.

If that doesn't answer your questions, feel free to email me off-list.

David Yuill, P.E.

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Thanks David,

I have learned a lot since I first posted on this topic.

Peter Moberg was nice enough to provide great information and a
spreadsheet from 62.1: 2004. I also bought the User's Guide for 62.1:
2007. In it there was a spreadsheet that, although it was difficult for
me to decipher originally, gave a great tool for solving this dilemma.

I had already developed my own spreadsheet, which was much more
rudimentary than the ASHRAE spreadsheet. Although my spreadsheet wasn't
up to snuff, it helped me to understand the issue and to enter the data.

I am still not done with the analysis, but I do believe the I am well on
my way to understanding the issue and providing a solution. You are
quite correct, once the critical zone is identified, changes can be made
to effect a solution

Thanks for helping me to understand this issue,

Robert P. Wichert, P.Eng.

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