Six Minute Solution - HVAC - Ventilation Problem

saloms · Oct 7, 2013

There is a problem in the PPI Six Minute Solutions for HVAC/Refrigeration that requires you to calculate whether or not a reheat coil should be added to classroom HVAC unit. The unit supplies 1000 CFM of supply air to the room. The classroom contains kids, age 7.

My problem is not understanding how this problem is completed it is how PPI calculates the minimum ventilation air. I opened my ASHRAE 62.1 book and found the ventilation rate for classroom (age 5-8), 10 CFM/person and 0.12 CFM. The occupant density is 25 people/ 1000 ft2. Based on this information the space will need about 463 CFM of outdoor air.

When PPI calculates the outdoor air required they only take the portion of the ventilation calc (people x CFM/person) and they do not include the second portion of the calc (area x cfm/ft2). This scares me a bit because now I do not know if I should be including this second part of the outdoor air calc when I go to take the exam.

Anyone run into this? I do not have my 6 minute solutions book on me right now, so I can't give you the problem number. It is almost at the end of the book problem 75 to 85, somewhere in there.

sycamore PE · Oct 7, 2013

I remember that problem. I'm pretty sure that's an error in the 6MS book, because I read ASHRAE 62.1 forwards and backwards and came up with the same thing you did.

JapeEscape · Aug 21, 2014

I just got to the problem referenced in the original post.

Can anyone confirm that this is an error? This is in regards to problem 76. (The same approach is taken in problem 67)

Mike M PE · Sep 2, 2014

Maybe I took the formula the wrong way but (going by memory) I understood the formula as one or the other. In other words if I knew the number of children then I use the first part but if I only had space type and purpose then I used the second part if the equation.

I can say that I didn't see anything that ambiguous on the exam so I wouldn't really sweat it too much.

Jonhnny123 · Sep 2, 2014

Mike M PE said:
Maybe I took the formula the wrong way but (going by memory) I understood the formula as one or the other. In other words if I knew the number of children then I use the first part but if I only had space type and purpose then I used the second part if the equation.

I can say that I didn't see anything that ambiguous on the exam so I wouldn't really sweat it too much.

There are some cases where you just use a default value (like corridors or storage rooms) based on square footage. But when you're talking about a room with a known occupancy, you should be using: [ People x CFM/People + SF x CFM/SF ] to calculate the outside air requirement. ASHRAE 62.1 should have a fairly thorough list of occupant densities for various types of rooms.

In practice, we'd use the exact occupancy if we had it. Otherwise, we'd use the densities.