Basic U-factor Tag components questions

[Jeremiah Burhite]

I have read section 6.2.4 of the THERM manual about Defining U-factor Surface Tags and many of the threads in this forum I could find on the subject. I need clarification on a few fundamental points.

In the manual it says that it is common practice to define the U-factor on the interior boundary surface. If U-factors are desired for the exterior boundary surfaces, these surfaces should be tagged separately:

-Why would you typically define the U-factor on interior side and not typically on the exterior side?

-In what circumstances would you tag the exterior side and why?

The standard U-factor tags:

-I understand "None" is used for adiabatic boundary segments.

-"Frame" is used for interior frame boundary segments and for glazing segments that are between the frame and the sight line. Could someone show an example of a "glazing boundary segment that are between the frame and the sight line". Also, if a part of the structure, like structural steel, is an integral part of the framing, would that also use a "Frame" U-factor tag?

-"Edge" is used for all the glazing system boundary segments that are above the sight line. Could someone show an example that clarifies what is meant by "above" the sight line? If I'm modeling a curtain wall spandrel area, and instead of glass it has a metal back-pan, would I use "Edge" for this area?

New U-factor tags:

-I've read how to add new U-factor tags and have been able to do so, but I'm not sure why and when you would do so, and I that stems from not being sure what the tags do in THERM.

     - In another thread called "Interpreting a model for a full building analysis w Comcheck", it was recommended to divide the exterior surface into 3 U-factor tags and make the transitions where the isotherm lines are parallel. If you give it three tags, THERM will produce a U value for each of the tags? So, is the purpose of the U-factor tags just to define where U values are calculated?

     - What are the differences between one U-factor tag from another? When you add a U-factor tag to the library, basically all you are doing is adding a name. Are there any variables for a U-factor tag?

Boundary Condition Type dialog box:

-When I double click a boundary condition segment and it brings up the Boundary Condition Type dialog box, I see the Boundary Condition name, the U-Factor Surface name, Emissivity value, Shading System Modifier, Blocking Surface check-box.

     - The Boundary Condition drop-down menu is obviously the BC I've chosen and the U-Factor Surface drop-down menu is where I assign the U-Factor tag.

     - I'm not sure about the Emissivity box. I see that it is editable, but am I correct that this value is assigned when the calculation is ran? I've noticed if I change the U-Factor Surface name it doesn't effect the Emissivity value after I run another calculation.

     - Shading System Modifier drop-down menu only shows "None". I take it that this is to be used if say you have some sort of sun shade device and your surface isn't getting direct sun light. But why does it show only "None" in the drop down menu?

     - What is a blocking surface and when should this check-box be checked or not?

     - The 3 other items in the Boundary Condition Type dialog box, Temperature, Hc, and Radiation Model:

         - Am I correct that those are all uneditable because they are results of the Boundary Condition type defined in the Boundary Condition Library?

         - What does "Hc" stand for?

Thank you for any clarifications anyone can give.

Jeremiah Burhite

[Jeremiah Burhite]

In partial response to my original question: An example of "glazing below the sightline (which is something I can't recall encountering before) would look like the attached image.