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

[NHEngineer037]

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. 

 

[justin-hawaii]

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.  

 

[NHEngineer037]

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

 

[justin-hawaii]

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.    

 

[NHEngineer037]

thanks for doing that!

 

[mselim]

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

 

[Abraham5G PE]

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

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

 

[MikeGlass1969]

There is a new eratta posted for the corrections.

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

 

[mselim]

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

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!