# HVAC NCEES practice problem #508



## xmickmaniax4 (Apr 18, 2017)

This is from the NCEES 2017 practice problem.
See attached the question and solution and ref material.

I believe the solution is wrong. The values of R for the air gap and the inside film are incorrect. 

For the air gap it looks like the direction of heat flow was chosen as upward instead of downward. Because it a summer condition heat is moving from outside in or the roof down. 

Also for the inside film , .68 is incorrect that is only for vertical walls. Just wanted some confirmation here.


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## xmickmaniax4 (Apr 18, 2017)

Seems I've messed up the attachments


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## Audi Driver P.E. (Apr 18, 2017)

I don't do HVAC, so take the following for what it's worth.

I looked at various references and about the only one I can't find with any certainty is the value they use for the air gap.  I believe 0.92 to be the correct value to use for the inside film (based on the tables I looked at and using horizontal, flat ceiling, downward/summer), but at least I see where they got their value of 0.68.  I believe that to be in error.  However,  If I use a typical 3.5" air gap value of 1.01 and .92 in the calculation the answer is still closest to B.

If I am reading air gaps right, it looks like maybe they took the galv. layer to be reflective? I'm not sure how or where they got their value for that. 1.00 to 1.01 for 3.5" air spaces seems typical.  And maybe that galv layer affected their choice for the inside air film as well.  I really don't know.


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## xmickmaniax4 (Apr 18, 2017)

Thanks for the response.

If you look at my first photo attached below, this is how they got to the 1.83 r value.  I think the correct r value is 3.41, if you follow horizontal space with downward heat flow across.

In winter conditions you have the hot air rising from the ceiling to the roof. In the summer the hot air gets to the air gap and stay to the top of the air gap with little to no air convection. So in theory the R value for a air gap in winter should be less than a r value in the summer. This problems been driving me crazy for the past week.


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## Audi Driver P.E. (Apr 18, 2017)

Makes me wonder if they posted a solution for a vertical roof. :wacko:


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## xmickmaniax4 (Apr 18, 2017)

Yeah they really need to review these solutions before printing the books,

Hopefully there isn't a roof air gap question on the exam friday. If there is I'll go with my gut about the direction of heat flow being down in the winter.


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## mp57078 (May 4, 2017)

I noticed this same problem you are showing with the air space R-values.  The only way I could rationalize their number is that they mention it's 90 deg outside, and 90 deg average for the air space.  Would this mean that sometimes the interior space is above 90 deg, therefore there would actually be heat loss from the interior space to the outdoor space.  Therefore the direction of heat transfer would be upwards.


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## Ramnares P.E. (May 4, 2017)

Please don't post photos of copyrighted material.  A problem description or reference to the specific book should suffice.


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## Audi Driver P.E. (May 4, 2017)

Ramnares P.E. said:


> Please don't post photos of copyrighted material.  A problem description or reference to the specific book should suffice.


Ram, there should be no issue with it. Copyright permits reproduction of limited sections, if the original referenced.


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## Ramnares P.E. (May 4, 2017)

We have had complaints before specific to this material.  I'm not trying to be a stickler but I don't want anyone getting in trouble.  The link to the external site is still up.


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## xmickmaniax4 (May 5, 2017)

So how do i edit the post to remove the link. I used a guest to post the link on imgur so I cant remove it from there.


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## Ramnares P.E. (May 5, 2017)

Edited.


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## Audi Driver P.E. (May 5, 2017)

Ram, I don't want to belabor the point more than necessary because I understand the concern.  However, it is pretty easy to make a fair use case for this type of post: 1) it's not reproduced in anything for commercial gain.  This site is free to use and reference. 2) it is for educational purposes 3) it is a small sampling of the work 4) it is properly referenced, and not being plagiarized and would likely encourage sales of the original work (heck we promote the original a lot here actually).

See the following, for reference:

http://www.bitlaw.com/copyright/fair_use.html


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