[Equest-users] Slab on Grade Floor Heat Loss

Sun Feb 4 13:44:23 PST 2018

Paul,

Following 90.1 Table G3.1 #5: “…..All components of the building envelope
in the proposed design shall be modeled as shown on architectural drawings
or as built for existing building envelopes…..Equivalent dimensions shall
be assumed for each exterior envelope component type <in the baseline> as
in the proposed design; i.e., the total gross area of exterior walls shall
be the same in the proposed and baseline building designs. The same shall
be true for the areas of roofs, floors, and doors, and the exposed
perimeters of concrete slabs on grade shall also be the same in the
proposed and baseline building designs.”

We can debate how precisely the actual areas should be captured in the
model, but I think most would agree that it is a mistake if the modeled
area is off by 50%, or doubled, or tripled, which is unfortunately not
uncommon in the compliance models. If proposed design has better roof
insulation than the baseline, doubling the area of the roof in the model
would double the associated savings. Reviewers typically don’t have budget
to do sensitivity analysis to evaluate the impact of the grossly
misrepresented areas on the performance rating or incentive, so it is a
standard practice to flag significant deviations in the review comments.

You wrote below: “….In the model I referenced, about 1/2 of the first floor
is over a conditioned space and one half is slab on grade.  So I defined
this floor as over an unconditioned space in the DD wizard as this should
result in a reasonable average. “  This conflicts with the requirements of
90.1 quoted above, but irrespective of that, I think many on this forum
would disagree that it’s a good approximation. Aside from that, when you
specify floor as adjacent to unconditioned space in Wizard, it’s actually
modeled as an exterior surface adjacent to ambient conditions (see below).
Bill described implication of that in his last response. 90.1 2010 Table
5.5-5 does not require insulation for non-residential slab-on-grades, but
mass floors adjacent to exterior must be U-0.064 max. Modeling
slab-on-grade as an exterior surface exposed to ambient conditions will
grossly exaggerate heating use in the baseline and proposed designs,
skewing contribution of heating-related improvements (e.g. more efficient
heating system in the proposed design) toward performance rating and
incentive.

It’s not too hard to create surfaces of the correct area and type (adjacent
to exterior, ground, etc.) in eQUEST Detailed Data Edit interface if you
don’t try to get a pretty 3D view. It’s OK if the building looks like a
cubist painting as long as you reasonably capture the areas of exterior
walls, roofs, etc. for each thermal block. (But watch out for the impact of
geometry on the exterior shading and daylighting.)

Changing exterior surfaces may affect infiltration load because in Wizard
the infiltration is entered as flow per square foot of the *exterior wall
area*. So when you change the area of exterior walls in the model (e.g.
enter slab on grade as an exterior surface), you are also changing the
infiltration flow CFM.

I would also argue that modeling is science, and not an art, but this post
is already way too long J.

*From:* Paul Diglio [mailto:paul.diglio at sbcglobal.net]
*Sent:* Sunday, February 04, 2018 1:07 PM
*To:* Maria Karpman <maria.karpman at karpmanconsulting.net>; Bishop, Bill <
bbishop at pathfinder-ea.com>; Nathan Miller <nathanm at rushingco.com>;
equest-users at onebuilding.org; Glen P. Eigo <glen.eigo at uinet.com>
*Subject:* Re: [Equest-users] Slab on Grade Floor Heat Loss

Maria:

Yes, sometimes I model an entire floor as having roof construction if some
of the floor is exposed to outdoor conditions.  If part of this floor is
under a conditioned space, it is possible to delete the roof constructions
under the conditioned area.  If it is possible, I model several shells per
floor in order to more accurately define which areas are exposed to outdoor
conditions and which areas are adjacent to a condition space.  Often this
is not possible since multiple zones span the condition-unconditioned area
of the shell above and we run into the problem of these zones having
external walls down the middle of them.

The same follows for underground floors.  In the model I referenced, about
1/2 of the first floor is over a conditioned space and one half is slab on
grade.  So I defined this floor as over an unconditioned space in the DD
wizard as this should result in a reasonable average.

If I remove the first floor external slab on grade floors as I have stated,
it is interesting that before and after the removal of the floors, the slab
on grade floor conductance is 0 in both cases.  By removal of the floor,
the conductance remains 0 but the infiltration changes based on the LS-B
reports.

I have spent time experimenting and have found that deleting the roof
constructions in zones adjacent to conditioned areas in both the baseline
and proposed models results in approximately a 1/10 of one percent change
in the annual MBTU consumption.

For example, when removing a roof construction in a zone with a plenum
under a conditioned zone, the wall conductance increases and the
infiltration increases.  Looking at the unconditioned plenum for this zone,
when removing the roof construction, the wall conduction increases, the
roof conduction decreases and the infiltration decreases.

It is my expectation that the utility reviewer is at least as competent as
I am in eQuest.  And even though I have been submitting models over 10
years, I have a lot to learn.

Going in circles to correct a 1/10 of one percent difference is
unreasonable.  I have gone back on unpaid time to calibrate models based on
the annual energy consumption post model and post project completion.  The
annual utility cost of the model is often 30-50% greater than the actual
cost since the building is not operated per the requirements of Appendix G,
90.1.  For example, in a residential high-rise, the apartment fans do not
run whenever the zone is occupied, and this makes a huge difference in the
annual energy consumption.  Per 90.1, the model is not intended to predict
actual energy consumption, but is just a comparison between a code
compliant model vs. the proposed model operated to the requirements of the
standard.

Sure, anyone can nitpick any one of the several hundred assumptions used in
models, but a competent reviewer should know which will affect the model
significantly and would need to be revised.  Modeling is not a science, it
is an art.

Thank you,

Paul Diglio
------------------------------

*From:* Maria Karpman <maria.karpman at karpmanconsulting.net>
*To:* Paul Diglio <paul.diglio at sbcglobal.net>; "Bishop, Bill" <
bbishop at pathfinder-ea.com>; Nathan Miller <nathanm at rushingco.com>;
equest-users at onebuilding.org
*Sent:* Friday, February 2, 2018 4:34 PM
*Subject:* RE: [Equest-users] Slab on Grade Floor Heat Loss

Some eQUEST models do have redundant roofs, exterior or underground floors
sandwiched between the Shells created in DD Wizard. These can be easily
identified by hiding some exterior and interior surfaces from 3D view,  to
look “inside” the building, like in the screenshots below. There is also a
nice summary of surface areas at the end of LV-D report, which can be used
to check that the modeled “UNDERGROUND” and “ROOF” areas are as expected
for the projects. That said, reviewers too make mistakes. We all know that
sometimes they are hands full J.

*Maria Karpman *LEED AP, BEMP, CEM

________________

Karpman Consulting

www.karpmanconsulting.net

Phone 860.430.1909

41C New London Turnpike

Glastonbury, CT 06033

*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] *On
Behalf Of *Paul Diglio via Equest-users
*Sent:* Friday, February 02, 2018 3:27 PM
*To:* Bishop, Bill <bbishop at pathfinder-ea.com>; Nathan Miller <
nathanm at rushingco.com>; equest-users at onebuilding.org
*Subject:* Re: [Equest-users] Slab on Grade Floor Heat Loss

Thanks Bill, you are always such a big help.  That is what I thought, but
just wanted to run it by the forum.

Thanks to you too Nathan.

Paul Diglio

------------------------------

*From:* "Bishop, Bill" <bbishop at pathfinder-ea.com>
*To:* Nathan Miller <nathanm at rushingco.com>; Paul Diglio <
paul.diglio at sbcglobal.net>; "equest-users at onebuilding.org" <
equest-users at onebuilding.org>
*Sent:* Friday, February 2, 2018 3:13 PM
*Subject:* RE: [Equest-users] Slab on Grade Floor Heat Loss

Paul,

Your reviewer may have thought that if the slab was showing up as an
“exterior surface” (such as on the LV-D report) than the model was
simulating heat transfer between the zones and the outside air through the
slab. But eQUEST/DOE-2 simulates heat transfer across underground surfaces
differently, as described in the UNDERGROUND-WALL and UNDERGROUND-FLOOR
section of the DOE-2 help documentation. The main difference is that ground
temperature is used instead of the OA temperature as the outside
temperature. The monthly ground temperature is taken from the weather file,
or can be entered as SITE-PARAMETERS:GROUND-T.

The eQUEST wizard creates custom constructions for each underground surface
based on inputs for Exterior/Cavity Insulation and Perimeter Floor
Insulation. Underground floors are given constructions with low U-factor
(high R-value) consistent with the lower heat transfer that would be
expected.

Regards,

~Bill

*William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers &
Architects LLP*

*Senior Energy Engineer*

134 South Fitzhugh Street                 Rochester, NY 14608

*T: (585) 698-1956*                        F: (585) 325-6005

bbishop at pathfinder-ea.com <wbishop at pathfinder-ea.com>
www.pathfinder-ea.com

[image: http://png-5.findicons.com/files/icons/977/rrze/720/globe.png]Carbon
Fee and Dividend - simple, effective, and market-based.

*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org
<equest-users-bounces at lists.onebuilding.org>] *On Behalf Of *Nathan Miller
via Equest-users
*Sent:* Friday, February 02, 2018 2:35 PM
*To:* Nathan Miller <nathanm at rushingco.com>; Paul Diglio <
paul.diglio at sbcglobal.net>; equest-users at onebuilding.org
*Subject:* Re: [Equest-users] Slab on Grade Floor Heat Loss

Fixed my typo in the F-factor heat loss calc (should have read 50 ft of
perimeter)… See below.

*Nathan Miller, PE, LEED AP BD+C – **Mechanical Engineer/Senior Energy
Analyst*

*RUSHING* | *O* 206-285-7100 | *C* 207-650-3942

*www.rushingco.com <http://www.rushingco.com/>*

*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org
<equest-users-bounces at lists.onebuilding.org>] *On Behalf Of *Nathan Miller
via Equest-users
*Sent:* Friday, February 2, 2018 2:33 PM
*To:* Paul Diglio <paul.diglio at sbcglobal.net>; equest-users at onebuilding.org
*Subject:* Re: [Equest-users] Slab on Grade Floor Heat Loss

In the past I’ve done a conversion of F-factor (perimeter based)
slab-on-grade heat loss to U-factor (area) and input that in on a
zone-by-zone basis if I really wanted to nail down SOG heat loss.

Example- Space is 20’ x 30’, but the exposed perimeter is only two of the
faces (20’ + 30’). Lets say it is uninsulated SOG.

F-factor from 90.1: F = 0.73 Btu/(hr*ft*F)

Perimeter based heat loss: 0.73 Btu/(hr*ft*F) * 50 ft = 36.5 BTU/(hr *F)

Then we can back-calculate the U-factor to assume for the whole floor to
equate to that heat loss: 36.5 BTU/(hr *F) = 600 SF * X BTU/(hr*SF*F) -> U
= 0.0608 BTU/(hr*SF*F)

You can create a custom floor construction for each space to provide the
U-factor to produce equivalent heat loss that the F-factor calcs are
telling you lose. For purely core zones (no exposed perimeter), I guess the
F-factor calcs are telling us were not really losing heat through that slab
(doesn’t’ seem right, but it probably is relatively small)

*Nathan Miller, PE, LEED AP BD+C – **Mechanical Engineer/Senior Energy
Analyst*

*RUSHING* | *O* 206-285-7100 | *C* 207-650-3942

*www.rushingco.com
<https://url.emailprotection.link/?a6X-5ry0mOOudUi-eMfQ7K9LfplwNoMV9idJhgEyf85c~>*

*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org
<equest-users-bounces at lists.onebuilding.org>] *On Behalf Of *Paul Diglio
via Equest-users
*Sent:* Friday, February 2, 2018 2:19 PM
*To:* equest-users at onebuilding.org
*Subject:* [Equest-users] Slab on Grade Floor Heat Loss

I recently submitted a model to the local utility company for an
incentive.  Part of the buildings' first floor is slab on grade.

One of the comments I received back is that this erroneously shows up as an
exterior surface and I should delete all the slab on grade floors in the
component tree.  Doesn't seem right to me.

I did a few and it does reduce the overall energy consumption.  In the 3D
model, no floors show, just open to the interior of the zone.

So is eQuest assuming that without this floor, there is no heat loss out
the bottom of the zone?  Is this reasonable?

Thank you,

Paul Diglio
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