NCEES HVAC+R Practice Exam #506 and #508 Solution Errors

The solution for this problem says that the air gap has an R-Value of 1.83.

ASHRAE fundamentals lists an air gap R-value of R-3.41 for a 3.5" air space with a 0.2 emittance that has heat flowing down (the problem says 90 deg F summer conditions, so assuming A/C inside, flow is down)

Did anyone else get hung up on this?

Also..

There is an obvious mistake in #506 where the solution uses the wrong unconditioned space temp. 

I concur that the solution for problem #506 uses the wrong conditioned space temp.  76 degrees is used instead of 74 degrees in the solution.  

For problem #508, I found that the ASHRAE Fundamentals 2017, Chapter 26, Table 3 Effective Thermal Resistance of Plane Air Spaces, 90 deg F mean temp air space and temp diff of 10 deg F, Horiz/Direction of Heat Flow Up, 3.5 in air space, Effective emittance of 0.2 results in a value of 1.83 h-ft^2-F/Btu.  I got hung up on this as well.  I spent a good amount of time trying to figure out how they got the 1.83 value as opposed to the 3.41 value.  

Hi Justin,

I see that.  We agree that is a mistake, right?

If the problem was for a winter condition with a cold exterior and warm interior, id select the 1.83, but since it says summer, wouldn't heat flow be down?

-Ryan

I concur and also NCEES agrees for #506.  https://ncees.org/wp-content/uploads/2016-Mec_HVAC_errata_ALL.pdf

I am going to email NCEES to let them know about #508.  I will let you know what they say.    

thanks for doing that!

Any updates on this?

Also, could someone please tell me where they came up with the Acoustic Tile resistance value of 1.89 for problem# 508?

Thanks

mselim said:

Any updates on this?

Also, could someone please tell me where they came up with the Acoustic Tile resistance value of 1.89 for problem# 508?

Thanks

Click to expand...

Acoustical Tile Thermal Resistance = 1.89 from Table 4 in Ashrae Fundamentals 2005 Chapter 25, Thermal and Water Vapor Transmission Data

There is a new eratta posted for the corrections.

https://ncees.org/wp-content/uploads/2016-Mec_HVAC_errata_ALL-2.pdf 

Abraham5G said:

Acoustical Tile Thermal Resistance = 1.89 from Table 4 in Ashrae Fundamentals 2005 Chapter 25, Thermal and Water Vapor Transmission Data

Click to expand...

Thanks for the quick response! Not sure why I didn’t get an email notification that someone responded.

Thats interesting... I have the 2017 ASHRAE Fundamentals handbook and can’t find that chapter or the value. The closest thing I found is Table 1 (Building and Insulating Materials: Design Values) in chapter 26, but only the conductivity value (k) is listed for acoustical tile. If I divide the conductivity value by the thickness (3/4”), I get an R value of 2.0, which is close enough, but I’m still a little confused because acoustical tile is only listed under the Insulating Materials category or the Building Board and Siding category, but problem# 508 specifies it as the ceiling material (which is neither insulation or building board/siding).

I guess I gotta just go with it.

Thanks again!