Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Here's one theory about what you are seeing (until I can confirm with a weather file):
The deep ground boundary condition is going to be fairly shallow in your case and will likely be the strongest driver of heat transfer from the slab. You input file specifies "Autoselect" for this boundary condition, which will estimate the depth of the water table based on your location and give it a constant temperature equal to the annual average dry-bulb temperature.
This deep ground boundary temperature is likely:
In the winter, the driver of heat loss through the slab is much lower than through the above-grade envelope, but in the summer it could be a much stronger driver.
(I'll update with more details when/if I can get a weather file to test this hypothesis.)
| | 2 | No.2 Revision |
I worked with a colleague to dig into this a bit, and the explanation is actually a bit different from what I originally theorized:
What is happening here is related to the heat balance at the surface, where convection gain + radiation gain = conduction loss.
The slab surface temperature is actually very close to the air temperature for most of the year and convection is relatively low in magnitude. The largest driver of heat flow through the surface is radiation exchange with the other surfaces in the zone. Because the other surfaces are significantly warmer than the slab in the summer there is considerably more radiative heat transfer to the slab surface. This radiative heat needs to go somewhere, and with such a low convective temperature difference, the path of least resistance is to conduct into the slab.
The conclusion here is that conduction is not a good indicator of the thermal load on a space. If you look at the convective heat gain rate instead, you will see the behavior you were probably expecting to see with conduction.
Here's one theory about what you are seeing (until I can confirm with a weather file):
The deep ground boundary condition is going to be fairly shallow in your case and will likely be the strongest driver of heat transfer from the slab. You input file specifies "Autoselect" for this boundary condition, which will estimate the depth of the water table based on your location and give it a constant temperature equal to the annual average dry-bulb temperature.
This deep ground boundary temperature is likely:
In the winter, the driver of heat loss through the slab is much lower than through the above-grade envelope, but in the summer it could be a much stronger driver.
(I'll update with more details when/if I can get a weather file to test this hypothesis.)
