Ok equesters,
I imagine my problem has something to do with my understanding of thermal blocks, so I wanted to get some feedback from other modelers.
Currently, I'm performing a LEED analysis of a 166,000 f^2 school building. I have created a space for every terminal box to accurately represent that system. Unfortunately, this works out to one or two hundred spaces (I haven't actually counted) and a very large model that takes a lot of time to simulate.
Is it possible for me to create a model that represents each HVAC zone? I have created a quick set of shells and air-side systems and a preliminary simulation reports almost double the energy usage as the detailed model.
If I represent the entire HVAC zone as one space (one eQuest zone), what is the best approach to represent the terminal units? It seems to me that using the PIU option would be like having one vav box serving an enormous room. While that's not the reality of the situation, is that good enough for simulation?
I suppose I will report back after I tinker with this some more.
Regards,
Matt Clough
Your simplified and detailed model should agree quite closely. There
are at least two options I know of: 1. simplify zoning by combining
blocks into bigger zones, or 2. use multipliers to have the same size
zone counted many times.
Option 1: I've done something similar to your school with a large
office building (200,000 ft^2) had about 300 zones. In this case the
simplified model, with only 15 zones, (I used the default perimeter
and core zoning built into eQuest) matches the detailed model really
closely (3%)- in the relatively few comparative models I've done the
simplified model is always with 10% of the detailed model if I use the
same systems, and match window %, shell areas etc. fairly closely.
Thinking about this in more detail my models have always been fairly
open plan offices but if your zone temperatures are all controlled to
the same then they should match pretty well with a simplified model
given roughly the same shell/loads.
Option 2: I work Green Star here in NZ so my LEED knowledge is only
book and chatter but I've heard you can combine spaces (per ASHRAE
90.1) with similar orientation/ size etc. Nick Canton (see the
archives - pretty sure it was him) wrote a bit about combining lots of
hotel rooms into just a few blocks with lots of multipliers. 100's of
zones down to 30 or so.
Jason
Hi Matt,
First a suggestion to avoid steering the discussion into confusion: when I say "zone" or "space" I'm consciously speaking in eQuest/DOE2 terms.
For each terminal unit, I most commonly define a single space whose boundaries represent the net interior area the terminal unit serves. Each zone may represent multiple rooms. It's conceivable for large models this approach can become cumbersome, but for clarity this is not defining a space for each room. For any size project I would start with this typical approach and consider simplification strategies from there as necessary.
There's a natural emphasis we modelers place on accuracy, but keep in mind LEED & 90.1 recognize and permit you, the modeler, a lot of leeway to build simplification strategies of your choosing into the model. For a large school, perhaps you can delete all but one of a block of similar classrooms and assign a multiplier to the remaining space/zone. 90.1 and the 90.1 User's Manual include further simplification guidance (as well as the mailing list archives). All that said, bigger projects ultimately result in bigger models with lots of inputs - additional time investment is to be expected.
A lot of things might have gone wrong with your 'quick comparison...' Hard to do more than speculate, but if you didn't carefully consider system quantities to ensure a level playing field along the way that would be a place to start.
Hope some of this helps!
~Nick
[cid:489575314 at 22072009-0ABB]
NICK CATON, P.E.
Matt,
There are a few things that you can do in the wizard to help the model
simulate quicker. I would start with having one HVAC system serving the
entire building. This will reduce the amount of plenums from one per
zone to one per shell. Once you are in the detailed edit mode, you can
create more systems. You will also want to make sure that you have the
Code Analysis set to none. I have not found eQUEST helpful with creating
the baseline model so there is no point in having the code specified. I
would also try to reduce the amount of shells that are created in the
model. The more shells usually results in a slower model.
Each modeler will have different opinions but I like to have the model
set-up as a space per terminal box because I think it makes the inputs
for occupancy, lighting and misc loads easier to enter. It also makes it
easier to adjust your model if the design changes and some rooms go on a
different HVAC system. You will have to decide what is the best option
for you.
Good luck,
Otto