defining the windows for natural ventilation

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Hi all,

I just understood that e-Quest dosn't calculate natural ventilation without making some definitions.

I found that in the detailed mode, in the HVAC window, I can define the outdoor air flow, but how do I define what is the area of the window that opens? It could be only half of the window area or all of it for example.

So what are the parameters that I need to fill out in order to be exact in the natural ventilation definition? And where could I find it?

Thank you

Noa Hakak,

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Hi Noa,

My understanding is that eQuest does not calculate natural ventilation at all. The program is not designed to support calculations of the movement of air between zones. The outdoor air flow in the HVAC window is outdoor air introduced by the mechanical system and not through windows. I think you would be better off looking to another tool (maybe EnergyPlus/OpenStudio?) that explicitly supports these calculations.

All the best,
Dan

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Daniel Knapp, PhD, P Phys, LEED? AP O+M

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Noa:

You can select natural ventilation in eQuest under the Air-Side HVAC System
Parameters, Outdoor Air, Natural Ventilation. I would use the Sherman-Grimsrud
method. From eQuest help - "Sherman-Grimsrud" method in which the air change
rate depends on wind speed, inside-outside temperature difference, and wind
shielding of the building. In this case, the keywords MAX-VENT-RATE,
HOR-VENT-FRAC, and FRAC-VENT-AREA are applicable.

Therefore I would calculate the vent area (window opening) and set up a
fractional schedule to account for the percent the windows are opened throughout
the occupied schedule.

Paul Diglio, CEM, CBCP

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Are you sure about that?
How about natural lighting? Does it calculate it?

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So what then is infiltration in eQuest? THAT is separate of the outside air introduced into the HVAC.
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There is the ability to perform rudimentary daylighting. It does ask for minimum footcandles in the data. So it must have SOME idea about natural illumination when the lighting controls step down.
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John Aulbach, PE

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It looks like eQuest can simulate infiltration and can simulate the movement of air through open windows into an exterior space using the Sherman-Grimsrud method as pointed out by Paul Diglio. Maybe this is all you need for your model in which case you are set. I was more concerned about the fact that eQuest does not calculate the movement of air from zone to zone and is not a good simulation tool for calculating natural ventilation throughout a building. It looks like it may well be a good tool for calculating natural ventilation in a given room with open windows though.

Cheers,
Dan

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Daniel Knapp, PhD, P Phys, LEED? AP O+M

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eQuest does calculate light through exterior windows into a zone and reduce
electrical light usage. It uses a split flux method that when compared to
Radiance can over/under estimate the effect but it is a great first order
tool (my opinion). Be aware that it only has light into the first zone with
exterior windows - other zones will only "feel" the light as conductive
heat transfer through the matched zone boundaries. There is a sunspace
module in DOE2 that might work if you need to bring daylight more into
central zones but I never sorted that out.

eQuest is much faster for daylighting studies than E+ and OpenStudio right
now. I've switched and miss the ease of eQuest on daylight harvesting. Once
my ruby is up to scratch I'll automate it similar to eQuest (or maybe
someone else will) but until that point assigning sensor locations and
setting up daylight harvesting in E+/OS is much more time consuming. E+ can
model using similar methods to eQuest and can use interior windows to allow
light to pass from an exterior zone to the next zone to the interior.

Jason Quinn

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eQUEST, or more specifically the DOE-2 engine behind it, has had the ability to simulate
natural ventilation through windows for RESIDENTIAL systems since the late 1980's. I
believe this capability was also added to all the other system types sometime in the late
1990's, but I'm not certain and someone who knows DOE-2.2 well might want to describe the
simulation there.

In DOE-2, natural ventilation is modeled with the same Sherman-Grimsrud model as used for
infiltration, except that it is done during SYSTEMS so that it can be controlled based on
the actual zone conditions of temperature and enthalpy. The inputs are similar as for
infiltration, except that the EFFECTIVE-LEAKAGE-FRACTION now refers to the area of window
opening, rather than of cracks and leaks. I once asked Max Sherman, co-author of the S-G
Model, whether he thought his model was suited for modeling natural ventilation, and got
the surprising response that he felt it was MORE suited than for infiltration, since the
openings were so much larger and regular than are cracks and leaks.

The main limitation of DOE-2 natural ventilation is that it's a one-zone model, so it
can't handle air flow from zone to zone. That doesn't mean it fails completely in a
multi-zone case, but just that you will get natural ventilation only in the perimeter
zones with windows as if those zones were independent. To model multi-zone air flow, one
needs a network model like CONTAM or COMIS. I was involved in linking EnergyPlus to COMIS
15 years ago, but EnergyPlus is now linked to Airnet, the solver behind CONTAM.

I think it would be interesting to compare DOE-2 to EnergyPlus in their modeling of
natural ventilation. Many of the inputs needed for multi-zone air flow modeling are
difficult to determine, e.g., how much are the interior doors opened? Also, don't
buildings designed for natural ventilation tend to be large open-plan, i.e., more like
single-zone? I'm not saying that multi-zone air flow modeling is unnecessary, but I'd
like to know whether the simple single-zone model is conservative or overly optimistic.
Then, I can decide how much effort to put into the multi-zone modeling.

Another observation about natural ventilation modeling in DOE-2 is that because it's so
quick and implemented in SYSTEMS, the program actually checks to see if natural
ventilation can maintain indoor temperatures below the setpoint and if not, it closes the
windows and goes on to mechanical cooling.

Joe

Joe Huang

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