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SHGC is calculated as Tsol + inward flowing fraction, which takes into
account inside temperature and absorbed heat in the glass, so
convective and FIR radiative gain from solar radiation is incorporated
into the SHGC. U-factor that contributes to heat transfer without
solar radiation is always positive and so delta T where indoor
temperature is lower than outdoor temperature would result in a net
heat gain to the space, not the heat loss that you are seeing.
However, note several other factors. Temperature changes over the 24
hours each day, radiant temperature of the sky is normally much lower
than the air temperature, so when you have very small average indoor to
outdoor delta T, these things can switch direction of heat flow. I
guess, you would need to look at each hour and look at the specifics.<br>
<br>
Charlie Curcija<br>
DesignBuilder Software USA <br>
<br>
<br>
<br>
-------- Original Message --------<br>
Subject: Re: [Bldg-sim] Glazing heat transfer<br>
From: Raoni Venâncio dos Santos Lima <a class="moz-txt-link-rfc2396E" href="mailto:raoniv@ig.com.br"><raoniv@ig.com.br></a><br>
To: <a class="moz-txt-link-abbreviated" href="mailto:rgaray@labein.es">rgaray@labein.es</a>, <a class="moz-txt-link-abbreviated" href="mailto:bldg-sim@lists.onebuilding.org">bldg-sim@lists.onebuilding.org</a><br>
Date: 10/13/2009 9:47 AM<br>
<blockquote
cite="mid:927a3ea80910130647h135bab6bs9857a3c798bd29cb@mail.gmail.com"
type="cite">This was my hypothesis, but none of my DesignBuilder
students bought that. Indeed, the glass surfaces temperatures (internal
and external) were higher than the outside temperature. On the other
hand, the internal temperature was lower than outside temperature.
Apparently the glass tends to lose heat from its external surface to
the outside air (probably maximized by convection) and to gain some
heat due to conduction (from outside air to internal air). In this
case, my glass is losing more heat.<br>
<br>
I appreciate your reply,<br>
Best regards,<br>
<br>
Raoni.<br>
<br>
<br>
<br>
<br>
<div class="gmail_quote">2009/10/13 <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:rgaray@labein.es">rgaray@labein.es</a>></span><br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><br>
<font face="sans-serif" size="2">Dear Raoni,</font>
<br>
<br>
<font face="sans-serif" size="2">I´m not completely of the
calculation
method used by Energy Plus, There are several reasons an outward heat
flux
in the outer surface of a glazing system:</font>
<br>
<br>
<font face="sans-serif" size="2">The usual one is when the outside
temperature
is colder than the one inside. As you tell in your post, this is not
the
case in your simulations.</font>
<br>
<br>
<font face="sans-serif" size="2">There is a second reason that only
happens
when both internal and external temperatures are nearly the same (even
beein the internal one a little bit lower than the external one): The
glazing
is heated by the incoming solar radiation and its temperature rises
over
the external temperature. This creates an outward heat flux. For a
clear
glass, the usual absorptance is very low (1-4%) so this phenomenom is
usually
very rare.</font>
<br>
<br>
<font face="sans-serif" size="2">I don´t know is this info will
solve
your problem.</font>
<br>
<br>
<font face="sans-serif" size="2">Kind regards,</font>
<br>
<br>
<font face="sans-serif" size="2">--------------------------------------------------------------------------<br>
Roberto Garay Martinez<br>
Labein-Tecnalia<br>
Unidad de Construcción y Desarrollo del Territorio<br>
c/Geldo. Parque Tecnológico de Bizkaia<br>
Edificio 700<br>
48160 Derio (Bizkaia)<br>
Tel.: 34 94 607 33 00<br>
Fax: 34 94 607 33 49<br>
<a moz-do-not-send="true" href="http://www.labein.es"
target="_blank">www.labein.es</a><br>
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Bilbao, 17-19 de Mayo, 2010<br>
Open and Sustainable Building<br>
Edificación Adaptable y Sostenible<br>
<a moz-do-not-send="true" href="http://www.OpenBuilding-2010.org"
target="_blank">www.OpenBuilding-2010.org</a><br>
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<br>
<br>
<br>
<font color="#800080" face="sans-serif" size="2"><br>
----- Mensaje de Raoni Venâncio dos Santos Lima <<a
moz-do-not-send="true" href="mailto:raoniv@ig.com.br" target="_blank">raoniv@ig.com.br</a>>
en Fri, 9 Oct 2009 16:46:37 -0300 -----</font>
<table>
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<td>
<div align="right"><font size="3"><b>Para:</b></font></div>
</td>
<td><font size="3"><a moz-do-not-send="true"
href="mailto:bldg-sim@lists.onebuilding.org" target="_blank">bldg-sim@lists.onebuilding.org</a></font>
</td>
</tr>
<tr>
<td>
<div align="right"><font size="3"><b>Asunto:</b></font></div>
</td>
<td><font size="3">[Bldg-sim] Glazing heat transfer</font></td>
</tr>
</tbody>
</table>
<br>
<tt><font size="2">Dear colleagues,<br>
<br>
I've noticed a slight heat loss through glazed surfaces during the whole<br>
year. I'm usind DesignBuilder and the situation is the following:<br>
1) climate: Brasilia (Brazil). Average temperatures around 22°C, but
during<br>
occupied periods (daytimes) the temperatures are higher (around 26°C).
Solar<br>
radiation is considerable.<br>
2) Conditioned zone with 24°C set point temperature<br>
3) Using a single pane glass 3mm (SHGF around 89%) form the EnergyPlus<br>
dataset.<br>
<br>
I wonder why the zone is losing heat through glazing even when internal<br>
temperatures are lower than external (conditioned zone). Heat gains due
to<br>
solar radiation are ok, but I'm not sure the glazing results are fine.
The<br>
interesting thing is that the zone never gains heat from the glazing
(except<br>
due to solar radiation).<br>
<br>
Is this a common thing? Any thoughts?<br>
<br>
Thanks in advance,<br>
Raoni</font></tt>
<br>
<font face="sans-serif" size="2"><br>
</font>
<br>
<br>
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