# Slab on Grade Floor Heat Loss

12 posts / 0 new

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

Offline
Joined: 2011-09-30
Reputation: 400

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) * 5 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

Offline
Joined: 2011-09-30
Reputation: 200

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

Offline
Joined: 2011-09-30
Reputation: 200

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

[cid:image007.jpg at 01D39C38.6333FF10] [cid:image008.jpg at 01D39C38.6333FF10]

134 South Fitzhugh Street Rochester, NY 14608

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

bbishop at pathfinder-ea.com www.pathfinder-ea.com

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

Offline
Joined: 2012-02-25
Reputation: 7

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

Offline
Joined: 2011-09-30
Reputation: 400

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 ; 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"
*To:* Nathan Miller ; 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
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
] *On Behalf Of *Nathan Miller
via Equest-users
*Sent:* Friday, February 02, 2018 2:35 PM
*To:* Nathan Miller ; 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 *

*From:* Equest-users [mailto: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
; 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
*

*From:* Equest-users [mailto: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

Offline
Joined: 2011-09-30
Reputation: 0

The use of F2 factors for foundation heat flows has always been flawed.?? The simplest
explanation for the F2 factor is that it's the effective U-value per perimeter length.??
This formulation assumes that there are no heat loses through the foundation away from the
perimeter, which is simply not true (think of the case in Minnesota where the constant
deep ground temperatures is between 55 and 60 F).? The other problem is? the delta-T used
to derive the F2 factor is not clearly defined.? All the studies I've seen (albeit? all
these are from 30 years? ago or more) derived? them against the outdoor air temperature,
or more exactly, Tin - Tair.?? This then opens up the question whether in DOE-2 the slab
should be modeled as an exterior wall or an underground wall? The third problem? is that
by treating the slab as a monolithic surface, it lumps the heat flow characteristics along
the perimeter with those of the slab core , which differ in both magnitude and time? lag.

When I last had a project on ground heat flows 17? years ago,? I proposed a simplified
method where the foundation was divided into three regions? (perimeter? 1' ,? near
perimeter 1-4', and core >4' deep),? each with 3 conductances? to the daily average air
temperature , average air temperature over the past three weeks, and the deep ground
temperature.? This method was implemented in an enhanced version of DOE-2.1E, and from
what I hear, adopted into California Title-24.?? If anyone is interested? to read the
report, just send me an e-mail.
<#yiv9572653599Picture_x0020_4>

The good news for everyone here is that there is now a RTAR/WS (ASHRAE terminology for a
RFP) being circulated? in TC 4.7 (Energy Calculations)? that aims to develop a better
simplified ground model to replace the F2 factors. Stay tuned but be forewarned that it
takes ASHRAE at least a year to approve? a project and another half year to pick a
contractor, so don't expect any products for another 3 years.

Joe

Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

Offline
Joined: 2011-09-30
Reputation: 406

Maria:
Yes, sometimes I model an entire floor as having roofconstruction if some of the floor is exposed to outdoor conditions.? If part of this floor is under a conditionedspace, it is possible to delete the roof constructions under the conditionedarea.? If it is possible, I model severalshells per floor in order to more accurately define which areas are exposed tooutdoor conditions and which areas are adjacent to a condition space.? Often this is not possible since multiplezones span the condition-unconditioned area of the shell above and we run intothe 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 thefirst floor is over a conditioned space and one half is slab on grade.? So I defined this floor as over anunconditioned space in the DD wizard as this should result in a reasonable average.
If I remove the first floor external slab on grade floors asI have stated, it is interesting that before and after the removal of thefloors, the slab on grade floor conductance is 0 in both cases.? By removal of the floor, the conductanceremains 0 but the infiltration changes based on the LS-B reports.
I have spent time experimenting and have found that deletingthe roof constructions in zones adjacent to conditioned areas in both thebaseline and proposed models results in approximately a 1/10 of one percentchange in the annual MBTU consumption.
For example, when removing a roof construction in a zonewith a plenum under a conditioned zone, the wall conductance increases and the infiltrationincreases.? Looking at the unconditioned plenumfor 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 leastas competent as I am in eQuest.? And eventhough 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 differenceis unreasonable.? I have gone back on unpaidtime to calibrate models based on the annual energy consumption post model and postproject completion. ?The annual utilitycost of the model is often 30-50% greater than the actual cost since thebuilding is not operated per the requirements of Appendix G, 90.1.? For example, in a residential high-rise, theapartment fans do not run whenever the zone is occupied, and this makes a hugedifference 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 modeloperated to the requirements of the standard.
Sure, anyone can nitpick any one of the several hundredassumptions used in models, but a competent reviewer should know which will affectthe model significantly and would need to be revised.? Modeling is not a science, it is an art.??Thank you,
?Paul Diglio

Offline
Joined: 2011-09-30
Reputation: 400

Thanks Joe.? I have spent the time to accurately calculate the F-factor of foundation heat flows, but have reverted to using the tables in Appendix A of 90.1.? It does not seem to make any significant difference.

Hopefully at some point, eQuest would allow us to plug in the F-factor instead of the U-factor for underground and slab on grade floors.? It would also be nice to plug in the C-value instead of U-factor for below grade walls.
Regards,?Paul Diglio

Offline
Joined: 2011-09-30
Reputation: 400

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 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 ; Bishop, Bill <
bbishop at pathfinder-ea.com>; Nathan Miller ;
equest-users at onebuilding.org; Glen P. Eigo
*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
*To:* Paul Diglio
; "Bishop, Bill" <
bbishop at pathfinder-ea.com>; Nathan Miller ;
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 ; 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"
*To:* Nathan Miller ; 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
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
] *On Behalf Of *Nathan Miller
via Equest-users
*Sent:* Friday, February 02, 2018 2:35 PM
*To:* Nathan Miller ; 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 *

*From:* Equest-users [mailto: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
; 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
*

*From:* Equest-users [mailto: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

Offline
Joined: 2011-09-30
Reputation: 0

It seems to me that the centre of building underslab temperature gradient would change over time. When the building is first turned on, the underslab may be close to the average deep ground temperature. Depending on soil type, with the room temperature approximately constant, I would expect that underslab soil temperature to warm up?

I haven?t been able to find a paper addressing the subject.

Christopher R. Jones, P.Eng.
Technical Specialist
Sustainability & Energy

[cid:image001.png at 01D39E91.2EECBE30]
T +1 416-644-0252

2300 Yonge Street, Suite 2300
Toronto, ON M4P 1E4 Canada

wsp.com

Please consider the environment before printing...

Offline
Joined: 2017-10-12
Reputation: 0

Christopher,

You're exactly right. It probably takes the building several years to warm up the ground
beneath the foundation to a long-term steady-state.

When I was doing thousands of foundation simulations for the Building Foundation Design
Handbook (Labs, Carmody, Sterling, Shen, Huang, and Parker, Underground Space Center,
University of Minnesota 1988),? Lester Shen and I discussed whether to initialize the
simulations with or without the building. We ended up deciding to initialize without a
building, i.e., for an empty site, on the logic that the results would be more
representative for new construction.? However, if you're trying to calibrate your model
for an existing building, you might want to initialize for ten years to pick up the fact
that the building warms up the ground over time.? Even initializing for bare ground took
about five years.? We solved the runtime issue by doing four years at a monthly time step,
one year at a daily time step, before finally doing the run.? Worked out fine.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

Offline
Joined: 2011-09-30
Reputation: 406