[Bldg-sim] ASHRAE 90.1 unmet load hours

[Bishop, Bill]

Jim,

 

I concur with your understanding of how to define unmet load hours. Your
hotel poses challenges for meeting the unmet load hours requirement, as
you described.

Unfortunately this can be a difficult problem to solve, especially in
the proposed design where the actual HVAC system design might not be
able to handle the diversity of loads.

There have been a lot of good suggestions in the forums about how to
reduce unmet hours. I have a few general suggestions (by no means an
exhaustive list).

 

*         For zones that are not controlled by a thermostat, or are only
loosely temperature-controlled by being next to a tightly-controlled
space, change to "unconditioned" or relax the thermostat setpoints as
Chris suggested.

*         Use a lower heating setpoint for spaces that are heating-only
and not kept "comfortable", like stairwells, mechanical/electrical
spaces etc.

*         Tweak the systems, especially at the zonal level - flowrates,
capacities, supply setpoints etc., while taking care not to deviate
substantially from the actual design.

*         Back off from aggressive nighttime setbacks and/or model a
warmup period in the MIN-AIR-SCH if the unmet hours are when the system
fans first come on.

*         Adjust space load amounts and schedules (occupancy, misc equip
and lighting).

*         Use air walls where appropriate to even out loads/temperatures
between zones.

 

These suggestions assume that your systems are operating in the model as
you intend, and that we're not talking about thousands of unmet hours.
Most of these suggestions need to be applied to both the baseline and
proposed models and it is easy to lose track of changes if you're not
careful.

Some people advocate increasing the throttling range in problem zones
(in both baseline and proposed models), which always works because you
are redefining what "unmet hours" are. My opinion is that this should be
done only as a last resort since you are not actually improving the
modeled system performance.

 

Regards,

Bill

 

  <mailto:wbishop at pathfinder-ea.com> 

 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Chris Yates
Sent: Friday, December 02, 2011 9:40 AM
To: Jim Dirkes
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE 90.1 unmet load hours

 

Hi james
Is it possible to relax your setpoints in proposed and baseline whilst
still meeting A55 requirements? 
Would this help?
Chris

On 2 Dec 2011 14:32, "Jim Dirkes" <jim at buildingperformanceteam.com>
wrote:

Dear Forum,
Feedback requested, please!  I've looked at some past posts on this
general topic and think I am asking about aspects not previously
discussed....

>From previous posts and my own understanding:

*         If one or more zones is not meeting its setpoint in a given
hour, you have 1 unmet load hour.  If 500 zones all fail to meet their
setpoint in one hour, it's till one unmet load hour.

*         Add all such hours in a year to obtain your total unmet load
hours, which cannot exceed 8,760.

I am now modeling a hotel with:

*         "Massive" wall elements in the lower floors with moderate
amounts of glass, "lightweight" construction in the upper floors with a
lot of glass

*         Occupancy schedules that are all over the map for Office,
Assembly / Meeting Rooms, Kitchen & Restaurant, and Guest Rooms

*         Individual HVAC systems for almost every occupancy type.

*         Presentation to all points of the compass, especially the
Guest Tower, which is a 16-sided (roughly cylindrical) tower

This model is making my life very difficult trying to get unmet load
hours below 300!  As I think about why that is happening, it strikes me
that the combination of mass, schedules and orientations are spreading
the unmet loads around to different hours than is normal for many
buildings.  Can any of you share your thoughts about that perspective?

Thanks!

ASHRAE 90.1-2007

unmet load hour: an hour in which one or more zones is outside of the
thermostat setpoint range.

 

 

 

 

The real measure of a person's wealth is how much they're worth after
losing all their money.

 


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