[bldg-sim] plenum temperature and roof heat transfer using eQuest

[Guo Zhou]

Dear All,

I'm using eQuest 3.54 to model a simple 5 zone (perimeter + core) big box 
retail building with 4' plenum in Fresno, CA. Roof insulation = R-11, Wall 
insulation = R-19. Ceiling is not insulated. not much glazing.

The following are the findings:
On hot summer days(outside dry bulb above 100 degree F), when the rooms are 
controlled to maintain 75 degree F:

1. with R-11 roof insulation and no ceiling insulation, the plenum 
temperature is very close to room temperature( about 2 degree F higher) 
during the day.

2. with R-11 roof and R-19 ceiling, the plenum temperature is 5~8 degree F 
higher than room temperature

3. with no roof insulation and R-19 ceiling, the plenum temperature can 
reach 93 degree with outside is 103 degree.

4. in all above 1,2 and 3 cases, the plenum temperature results from 
return-air-path = plenum-zones and duct are identical.

I think the actual plenum temperature would be much higher than the 
simulation results. Especially in case #3.

Can anybody please tell me what you think about it? What's causing it?

As a result, the cooling load from the roof being largely underestimated...

Thanks.

Guo


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