[Equest-users] Slab on Grade Floor Heat Loss

Fri Feb 2 13:54:53 PST 2018

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"

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>
> *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 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>; 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] *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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