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If you are using the ASHRAE F-factors and want to capture the season
average heat flux differences for each zone your approach seems
good. You might consider whether treating the zones separately is
worth the effort. Note the ASHRAE F-factors are designed to be used
with air temperature (for UA trade offs with other air temperature
components) so using these factors for a DOE2.2 /Equest UNDERGROUND-FLOOR
or UNDERGROUND-WALL, which has a ground temperature backside, will be
underestimate heat fluxes.<br><br>
In E+, the <b>Construction:FfactorGroundFloor</b> handles the
calculations for you. This object uses the shallow ground
temperature which is close to air temperature.<br><br>
If your purpose is to determine actual performance of the insulation
configuration, and the zones are conditioned without a lot of zone
temperature variation, the E+ slab model (auxiliary programs) can be used
to calculate ground temperatures. Outputs are an average, perimeter
and core back side ground temperature and the zone floors can be
specified with the appropriate value as the backside boundary
condition. The slab model can be run as a stand alone module
and the outputs used in other programs. Equest doesn't seem to
allow two ground temperatures so if you really wanted to capture the zone
differences you might be out of luck.<br><br>
Mike Kennedy<br><br>
<br>
At 07:49 AM 8/24/2011, Talbert, Joshua wrote:<br>
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Hello Bldg Sim Folks,<br>
<br>
Could anyone comment on how they “translate” the F-factor requirement for
on grade slabs into a U-value / material for a floor construction in
Energy+ or eQuest<br>
<br>
My method has always been to create a fictional material with negligible
thickness that approximates the equivalent R-value for the whole floor
area of the zone based on the edge requirement. This results in me
creating a separate one of these fictional materials for each zone.
This is because even though a zone might have the same F-factor
requirement the difference in perimeter to floor area ratio effects the
actual heat flux through the floor, or so I think.<br>
<br>
Here is an example of how I have modeled this, see attached picture for a
layout:<br>
<img src="cid:7.1.0.9.0.20110824151846.04893310@madronagrove.com.1" width=411 height=483 alt="[]">
<br>
<br>
F-Factor = 0.64 (equivalent to R-10 insulation for 48” and for an
“Unheated Slab” per ASHRAE 90.1 Table A6.3)<br>
<br>
Zone 1<br>
Area
=
25 sqft<br>
Edge Perimeter
=
10 ft<br>
<br>
Zone 2<br>
Area
=
375 sqft<br>
Edge Perimeter
=
70 ft<br>
<br>
Perimeter * F factor = Area * U value equivalent<br>
<br>
Zone 1 equivalent U = 0.256<br>
Zone 2 equivalent U = 0.119<br>
<br>
This process has made sense to me, but in reviewing this technique with a
colleague we wondered what does everyone else do. Am I under/over
predicting slab loss for certain zones with this method? Your
comments and insight are always appreciated.<br>
<br>
Thanks,<br>
Josh<br>
<br>
<br>
Joshua W. Talbert P.E., LEED-AP<br>
Mechanical Engineer<br>
Hallam<b>///</b>ICS<br>
38 Eastwood Dr. Suite 200<br>
S. Burlington, VT 05403<br>
P: (802) 658-4891<br>
F: (802) 658-1457<br>
<br>
<br><br>
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