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

[Jeff Haberl]

FYI. 

You must also enable CWFs, and have properly layered roofs and walls or the hourly temperature profiles can be meaningless. Even with this it remains difficult to match meas temps from an attic without manipulating a proxy variable such as infiltration. 

Jeff
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Jeff S. Haberl, Ph.D., P.E.............................jhaberl at esl.tamu.edu

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-----Original Message-----
From: bldg-sim at gard.com <bldg-sim at gard.com>
To: bldg-sim at gard.com <bldg-sim at gard.com>
Sent: Mon Sep 19 11:31:24 2005
Subject: [bldg-sim] plenum temperature and roof heat transfer using eQuest

Glenn,

Thank you for the trick. I will try it out. the problem is I don't have complete data on measured peak attic temperature for all the cases I want to study, i.e. with different insulation level, different climate zone, etc.
 
Guo.



On 9/19/05, Glenn Haynes <glenn.haynes at rlw.com> wrote:

	Guo,

	 

	Accurate treatment of radiant heat transfer in attics, etc., is too cumbersome for an overall building simulation code like DOE2 (maybe future generations with faster computers…).  A little trick I discovered to help compensate for the underestimation of attic temperatures is to put a little glass in the roof deck.  If you get the right amount, it tends to correct for both summer and winter attic temperatures.  How much glass depends on a number of variables, but if you have some idea what that peak temperature is (measured data indicated about 118-122 degrees F. in St Petersburg, Florida), you can find the right amount by trial and error.  I found that about 5% was sufficient to obtain the result I desired.  If you have enough attic temperature data under differing conditions, you can experiment with emissivity, U-value, etc.  I just never had enough time to explore those variables, but it seemed that the right glass area was the most important variable.

	 

	Glenn Haynes

	 

	________________________________

		From: bldg-sim at gard.com [mailto: bldg-sim at gard.com <mailto:bldg-sim at gard.com> ] On Behalf Of Guo Zhou
	Sent: Friday, September 16, 2005 2:47 PM
	To: bldg-sim at gard.com
	Subject: [bldg-sim] plenum temperature and roof heat transfer using eQuest

	 

	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