Underfloor Modelling and Error Message.

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Hello eQuesters,
?????????????????????????? I had two questions:

(1) Can underfloor modelling be done?in eQUEST.? Does any body know
which 'system type' to consider? I have considered a powered induction
system type.

(2) I got a error message from one of my model as follows. Can some
one help me understand this error?. i have attached my print screens
on the air side for this model.

*WARNING**********************************************************************

Zone: EL1 North Perim Zn (G.N1) has a design cooling

temperature differential of only 4.0F. This

may result in an extremely large design airflow.
***********************************************************************************

Thanks,

Kazim

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A PIU system is fine, especially if you are using fan powered boxes to serve your perimeter. The more important part of trying to approximate an underfloor system is correctly appropriating the loads between the conditioned and unconditioned zones.

As for the error, what is your supply temperature? It seems like the error would be related to a small dT between the room setpoint and the supply air temperature, which would require calling for a large volume of air to satisfy the loads in the conditioned zone. You might also try having different set points for your heating and cooling, there should be at least a 5F dead band per ASHRAE 90.1. 6.4.3.1.2. Hope that helps.

Brendan Hall, M.Sc., LEED AP BD+C

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There's a large chapter in the EDR guidelines that pretty much details an overall "how-to" for UFAD in eQUEST... recommend downloading the PDF linked at the bottom and perusing!

http://www.energydesignresources.com/resources/publications/design-guidelines/design-guidelines-hvac-simulation-guidelines.aspx

IIRC - this is written in a fashion that does a good job of outlining the limitations of UFAD in eQUEST, and provides guidelines for DOE2 system selection by pointing out behaviors that might be desirable to include in the simulation.

~Nick

NICK CATON, P.E.

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In general, I think that the article referenced below is a great start
to UFAD modeling systems in eQuest. However, since the article was
written, much more research has been done (mainly by the Center for the
Built Environment-UC Berkeley) in the area of UFAD modeling. This
article (EDR link) suggests that displacement (TDV) and UFAD systems can
be modeled the same. While some of the dynamics are the same, this is
not the case.

What the EDR article does not expand on is the issue of supply plenum
temperature decay and return air plenum temperature decrease that occurs
with UFAD systems. If you simply use the EDR factors, what you will
find is that return air temperature is higher than what actually occurs,
and plenum supply air is lower than reality as well.

For those of us that have design/installation experience with UFAD
systems, return air temperatures are not in the 78-80 degree range, but
closer to 75-77 degrees (assuming a space temp of 74/75 degrees). Why?
Consider in a multi-story building a section, heat from the return
plenum will be lost both to the floor above due and also the floor below
by means of radiation.

Supply plenum heat gain is a much harder thing to quantify, since you
have the issues of return air gain, as described above, but is much more
complex. Supply air plenum heat gain is also a function of airflow,
inlet supply temperature, and how the supply air gets injected to the
plenum. More inlets and inlets closer to the exterior can minimize
plenum heat gain.

Also, be very conscious on the type of eQuest system used versus the
type of UFAD system designed. In general, a PIU system for a UFAD
system that has swirl diffusers on the interior and fan powered boxes on
the perimeter is acceptable (you just have to understand how the
perimeter box operates). All UFAD systems are unique, so be sure to
understand the design carefully.

Here is what I would recommend:

-Do not just model the system as the EDR link describes. If you do, you
will probably over estimate UFAD savings.

-Read through the ASHRAE article from October 2007 titled "Cooling
Airflow Design Calculations for UFAD"

-Read through all of CBE's free online resources. They have information
on many UFAD topics, including airflow calcs, energy modeling, comfort,
and more. http://www.cbe.berkeley.edu/research/research_hvac.htm

-Also remember that in the latest eQuest update, you can input user
defined Radiant/Convective splits. This is a huge development for UFAD
systems.

Andy

PS: If you're just looking for a quick simulation of UFAD, without all
the headache of trying to manipulate eQuest, I'd try another simulation
engine. There are some out there that have routines written for UFAD.
EnergyPlus is probably the best, since they base things on CBE research.

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