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Hi everyone,
I'm currently working on a thermal simulation using TRNSYS 18 and I’ve run into a conceptual issue regarding the TSGRD input parameter in Type 56.
In the official TRNSYS example file "restaurant", TSGRD is connected to the dry bulb temperature from the weather file.
However, there’s no explanation provided as to why this connection is appropriate or what assumptions are behind it.
Despite searching through the documentation and forums, I haven’t found a clear answer to the following:
- What is the physical meaning of TSGRD?
The manual says:
“This temperature represents all surface temperatures except the sky temperatures which are in the view to the outside of an external surface.”
→ How can this be represented simply by the dry bulb temperature?
- Is TSGRD required to be connected?
If not connected, is there a default value assumed internally by TRNSYS?
- What kind of values should be assigned to TSGRD?
For example, in summer conditions, the ground or surrounding surfaces can be significantly warmer than the air due to solar absorption and thermal mass. Should this be reflected in TSGRD?
- What is the impact of TSGRD on simulation results?
Especially in summer overheating or passive cooling simulations, how sensitive are results to this input?
I noticed a clear difference in overheating degree hours depending on whether TSGRD is connected to the dry bulb temperature or left unconnected.
However, I couldn’t find any documentation explaining the mathematical basis for this impact.
- Is there any official guidance or best practice on how to define or calculate TSGRD?
Any insights, references, or experiences would be greatly appreciated. I’m particularly interested in ensuring that my summer thermal protection simulations are not skewed due to incorrect assumptions about this parameter.
Thanks in advance!
Andrea
Hello @andreas,
Welcome to the forum! Answers to your questions below.
1) For continuity and in lieu of more site-specific modeling or data, it's common to assume the ground surface temperature is approximately equal to the adjacent air temperature. It is a simplifying assumption, and users are welcome to use other analytical models or measured data as they see fit (see below).
2) If TSGRD is not connected, TRNSYS will use its default value (0) as the ground surface temperature for the entirety of the simulation. That is probably less accurate than using the air temperature as a proxy for the surface ground temperature. So yes, it needs to be connected to something that is reasonably realistic to the model.
3) You are welcome to use any model or measured data you like to simulate the ground surface temperature. For example, TESS has a ground coupling model, Type 1267, available as a standalone component (not included with any TESS libraries) that uses 3D finite element analysis to solve soil temperatures given convection and radiation at the soil surface, as well as other elective influences (buried pipes, buried thermal zones, and the like). The horizontal ground heat exchanger model in the TESS GHP library, Type 997, offers fewer elective features, but similar functionality, if all you want is an undisturbed ground temperature based on energy balances (you could leave the pipes disconnected). I am not aware of any models in the standard library that model ground temperature based on solar absorption and thermal mass. You could also try using the Kusuda undisturbed ground temperature model, Type 77 (in the Physical Phenomena folder of the standard TRNSYS library), at a very thin depth. Type 77 approximates ground temperature as a sinusoidal wave with an annual period influenced by annual average and min/max air temperatures, but it does not explicitly adjust for solar radiation. With Type 77, you would still get warm surface temperatures in the summer and cold temperatures in the winter, but without the daily diurnal swings of using the dry bulb air temperature. You could also use an equation block to take the average of the air temperature and the Kusuda model temperature, and connect that to the building model as the ground surface temperature. There are many options!
4) Looks like you already know how to determine how sensitive the simulation results are to this input :-). It's impossible to say, as a blanket statement, whether or how much a simulation will be affected by the ground surface temperature assumption. The best way to determine the sensitivity to the ground surface temperature assumption is to run the simulation multiple times with different (reasonable) assumptions and see how they compare. If the difference in results is significant, it's probably worth putting the time and effort into either obtaining measured ground surface temperature data at the site, or building out a model of the surrounding ground surface that accounts for as many influences as possible (solar radiation, thermal mass, buried pipes, buried thermal zones, shading, etc.).
5) I would consult the standards and guidelines of a body like ASHRAE (www.ashrae.org), or similar organization in your region, for recommendations and best practices regarding modeling ground surface temperatures in the built environment. TRNSYS allows users to connect the ground surface temperature from the model or input of their choice, but does not provide official guidance or recommendations on best practices for inputs to its models.
Good luck with your modeling!
Best regards,
~Angela Weiss
AndreaS Topic starter
17/10/2025 6:46 am
hi @a_weiss,
Thank you so much for your incredibly helpful and detailed response! I really appreciate the time and care you put into explaining the different options and considerations. 🙂 This gives me a much clearer path forward in my modeling work.
Thanks again and all the best,
Andrea
