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Very high CIBSE Solar Gains

Gary Jackson
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I modelled a simple 5mx5m glass box with no heat transfer across the floor or ceiling and I created a custom glass to replicate a Pilkington glass (U-Value of 0.6 and g-factor of 0.55).  When I checked the solar gains using CIBSE Gains they were 10.8kW.  However when I calculate the gains manually using the CIBSE guide I calculate the solar gains to be 6.2kW.

Furthermore the CIBSE Gain results on Heavacomp gives the average solar gain as 410W/m2 where as in the CIBSE guide, 410W/m2 is the solar gain for a southern aspect only (E&W are 189W/m2 and north is 117W/m2).  Therefore this is leading to conclude that Heavacomp has not adjusted the solar gains for each wall aspect but rather used the max solar gains for all four walls.  If I do the same in my hand calcs, I get a total solar gain of 11.3kW so pretty close.

Also I have noticed that the CIBSE Solar Gains are double the Carrier and ASHRAE Solar Gains.

Can you think of an explanation for the large discrepancy for the two values?

Gary

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    Gary - It would appear that you have defined your glazing incorrectly. The steady state calculations (CIBSE Gains, Carrier Gains etc) use the information from the Glazing Properties box in the top right corner of the glazing program.

    Please consult this document for more information. Note that all the information in the Glazing Properties section should be populated. Simply changing the U value is not sufficient. SHGC (Solar Heating Gain Coefficient) is analogous with the Carrier F Factor.

    regards,
    Duncan

    Duncan Brown



  • 0 Offline in reply to Duncan Brown

    Duncan, Thank you for the swift response.  For reading the linked document, I believe I entered the correct data.  I have attached the screenshot of the Glazing Data Page and the Manufacturer's datasheet I am using (I am using the top glazing product in the list).  As you can see the calculated data is very close to the manufacturers data and I also enter the figures into the Glazing Properties Fields.

  • 0 Offline in reply to Gary Jackson

    Gary - A couple of things stand out - the SHGC is unlikely to be the same as the Shading Coefficient - this (3rd party) document provides information on this. And my other concern is where did you get the Environmental figures from? These are paramount to the CIBSE Gain calculation.

    Regards,
    Duncan

    Duncan Brown



  • 0 Offline in reply to Duncan Brown
    Duncan,

    I will look into the shading coefficient. Reference the environmental figures, I used the default figures from Heavacomp as I do not have any manufacturers data covering these parameters. I had assumed that the CIBSER method used the Solar Gains data set published in the guides and simply multiplied them by the g-factor and area but it looks like this is not the case. is there anything in the wiki about calculating the environmental figures?

    Gary
  • 0 Offline in reply to Gary Jackson

    Gary - the first document I linked to you contained a link to the calculation methods. You can go directly to it here. Information on the method used for calculating solar gains can be found in the software manual, which is usually available by pushing F1 or clicking on the ? icons that are scattered through the software.

    Regards,
    Duncan

    Duncan Brown



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