[Equest-users] Slab on Grade Floor Heat Loss
Joe Huang
yjhuang at whiteboxtechnologies.com
Mon Feb 5 22:01:44 PST 2018
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"
On 2/5/2018 11:54 AM, Jones, Christopher wrote:
>
> 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
>
> T +1 416-644-0252
>
> 2300 Yonge Street, Suite 2300
>
> Toronto, ON M4P 1E4 Canada
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> *From:*Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] *On Behalf Of
> *Joe Huang via Equest-users
> *Sent:* Friday, February 02, 2018 4:55 PM
> *To:* equest-users at lists.onebuilding.org
> *Subject:* Re: [Equest-users] Slab on Grade Floor Heat Loss
>
> 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 <mailto: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"
>
> On 2/2/2018 12:26 PM, Paul Diglio via Equest-users wrote:
>
> 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> <mailto:bbishop at pathfinder-ea.com>
> *To:* Nathan Miller <nathanm at rushingco.com> <mailto:nathanm at rushingco.com>; Paul
> Diglio <paul.diglio at sbcglobal.net> <mailto:paul.diglio at sbcglobal.net>;
> "equest-users at onebuilding.org" <mailto:equest-users at onebuilding.org>
> <equest-users at onebuilding.org> <mailto: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 StreetRochester, NY 14608
>
> */T: (585) 698-1956/**//*F: (585) 325-6005
>
> bbishop at pathfinder-ea.com <mailto:wbishop at pathfinder-ea.com>www.pathfinder-ea.com
> <http://www.pathfinder-ea.com/>
>
> http://png-5.findicons.com/files/icons/977/rrze/720/globe.pngCarbon 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 <nathanm at rushingco.com> <mailto:nathanm at rushingco.com>; Paul
> Diglio <paul.diglio at sbcglobal.net> <mailto:paul.diglio at sbcglobal.net>;
> equest-users at onebuilding.org <mailto: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] *On Behalf
> Of *Nathan Miller via Equest-users
> *Sent:* Friday, February 2, 2018 2:33 PM
> *To:* Paul Diglio <paul.diglio at sbcglobal.net <mailto:paul.diglio at sbcglobal.net>>;
> equest-users at onebuilding.org <mailto: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%7E>*
>
> *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 <mailto: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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