Re: U-value window

[Michaël Kummert <kummert@xxxxxxxxxxxxx>]

Hilde,

>  
> I'm working on a sensitivity analysis of natural night ventilation. 
> Therefore I have to vary all the input parameters in my Prebid-Trnsys 
> simulations.
> For a window, I started to vary the U-value (from 1.07 to 1.13) only by 
> changing the thickness of the cavity filled with Krypton from 10mm to 
> 15mm in the WINDOW 5.0 program and exported the results to the window 
> library in Prebid. When I use these two windows in my simulations, I got 
> exactly the same indoor temperatures in every time step, although the 
> U-values of the 2 windows differ a little bit in the w4-lib.dat file.
> Is there an explanation? And how can I solve this problem.

I think the explanation to your problem might be that you are trying to 
see a too small difference. I don't have the details of your windows and 
glazing systems, but I tried to reproduce your problem as follows:
- I created the following glazing systems:
  1. CEN center U-Value of 1.13 (Double low-E, Krypton 7.3mm)
  2. CEN center U-Value of 1.03 (Double low-E, Krypton 9mm)
- First, the NFRC U values are much closer (1.17 and 1.19), so depending
  on the conditions your U-values will be much closer (Difference = 2%
  instead of 10%)
- I included those glazing systems in "reasonably large" windows so the
  edge effect is small (2000x1500mm, Alu with break)
- The global U value for the window (glazing + frame) are 1.925 and
  1.944 (less than 1% difference)

So if you have stronger edge effects, or particular ambient conditions 
that minimize the difference between the thermal performance of your 
windows, you will not notice any difference between the two of them.

I would recommend that you test both windows with a highly insulated 
frame and steady-state ambient conditions (e.g. 20°C inside, -10°C 
outside) to check that Type 56 is reading their properties correctly.

Kind regards,

Michaël




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_________________________________________________________

Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive
Madison, WI 53706

Tel: +1 (608) 263-1589
Fax: +1 (608) 262-8464
E-mail: kummert@xxxxxxxxxxxxx

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