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# Help Understanding 'Surface Average Face Conduction Heat Transfer Energy' Results?

Hello, I was hoping someone could help me better understand the 'Surface Average Face Conduction Heat Transfer Energy [J]' output variable results? I'm confused by the results I'm getting and wonder if someone can shed some light on what they might mean in this case?

I'm working on a small residential project in a mixed climate (Northeast USA) and while the annual results are all 'normal' (meaning: they look as I'd expect) the Summer Design Day results are very confusing to me? When I look in depth at the output results, all of my opaque surfaces are showing a very peculiar effect where they appear to 'gain' heat (positive conductive heat transfer) for the morning, and then around mid-day they strongly shift to heat 'losing' (if I'm understanding the sign convention correctly here, positive values indicate 'gain' into the space and negative values indicate 'loss' for this output variable?)

You can see the effect here for one example surface (one of the roof surfaces):

The surface temps seem to show that, as I'd expect, the outer surface temp is higher than the interior until sundown, and so I'd expect the surface face conduction to be consistently positive (heat gaining to the space)? But I must be either doing something wrong, or I'm misunderstanding the meaning of what this output variable is reporting?

The surface is using just a simplified assembly with a 1" mass-layer inside and out, and then a single thermal control layer in between:

And I should note, I'm seeing basically the same effect for all my surfaces here. If I total them up by zone, that looks like:

Can anyone help me understand what I'm seeing here, what these results mean and why it would be the case that I'd be seeing such strong heat 'loss' (negative conductive heat transfer) in the afternoons for these surfaces? Thanks so much for any thoughts, input or clarifications on this.

best, -Ed

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I have several thoughts on that.

1. If you extract surface inside face conduction , the surface outside face conduction and the surface heat storage energy, these three variables, it meets the heat balance. Because regard the wall as the system, the input and output are just surface inside face conduction and the surface outside face conduction. The net energy would be stored by wall itself because of thermal mass which can change the its own temperature.
2. surface average face conduction does not have any physical meaning. Just sum them up and divided by 2. If you think that you just use q''=k(To-Ti) to calculate which is right only under static situation. Now we know the temperature of wall itself will also change, it is a dynamic problem.
3. Moreover, surface inside face conduction , the surface outside face conduction, these two variables should also agree to the surface film energy balance which is associated about inside face convection, solar radiation, lights radiation, system radiation, other surface radiation.
4. It is dynamic problem. Think everything dynamically. The heat balance is the root.

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I would recommend reading similar posts here about surface heat storage & inside/outside face conduction and here about the transient nature of surface conduction calculations.

( 2020-08-30 08:39:55 -0500 )edit

Oh, I did not see those. Thanks for sharing.

( 2020-08-30 18:15:26 -0500 )edit

Ah, thank you so much Aaron and Lantonshen. I appreciate your input here. Reading through the documentation and links, it does help to clarify the issue a little bit for me regarding this 'Surface Average Face Conduction Heat Transfer Energy' output variable.

In particular, reading (and re-reading a few times....) the detailed explanation for the:

was helpful, and does clarify a bit the results shown in that 'Average Face Conduction' output.

thank you, -Ed

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Thanks for sharing. I think I understand the sign convention 1. Surface inside Face Conduction Heat rate (W) is positive when the heat flows into zone; but it is negative when it flows out of zone. This is the same as the sign convention of surface inside face conduction heat energy(J). 2. But the Surface outside Face Conduction Heat rate (W) is positive when the heat flows into the outside air; it is negative when it flows into zone. It is opposite to the sign convention which we define conduction heat energy(J).

( 2020-08-31 20:38:55 -0500 )edit

Continue.. That is reason why we should switch to the same sign convention as Surface inside face conduction heat rate (W) and then sum them up and divided by 2 to get the Surface Average Face Conduction Heat Transfer Energy. The sign of Surface Average Face Conduction Heat Transfer Energy will show the energy flowing into zone or not which is also equal to the Surface inside Face Conduction Heat rate (W)

( 2020-08-31 20:42:39 -0500 )edit

I add one figure above. Forget my drawing.

( 2020-08-31 20:48:15 -0500 )edit