[Equest-users] Slab on Grade Floor Heat Loss
Jones, Christopher
Christopher.r.Jones at wsp.com
Mon Feb 5 11:54:13 PST 2018
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
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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.
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
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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
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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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