U values W/M2/K

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u values ....W/M2/K

I am getting confused by the definition of the u-value .
I want to do some calculations for heat loss but I do not understand the timescale for the U- value calcs
I understand that a U value describes the energy in watts that is lost per Sq Mtr for each degree (K) diferential , but is that per hour , day ,year ?
I may have missed something obvious but any advice would be gratefully accepted
Kev
 

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  • #2
russ_watters
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A watt is a joule per second.
 
  • #3
Andy Resnick
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If the 'U-value' is the concept as the 'R-value' for insulation, then there is no time dependence.

http://en.wikipedia.org/wiki/R-value_(insulation [Broken])

Heat transfer (as a thermodynamic application) has no time dependence. The amount of heat flow depends only on the temperature differential.
 
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  • #4
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Thanks for the help
I have sussed now that the figure relates to the rate of energy transfer and that the amount of energy transfered is calculated by the the timescale .
I probably was being thick, but at the time I just couldn't see it .
Kev
 
  • #5
russ_watters
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If the 'U-value' is the concept as the 'R-value' for insulation...
They are inverses of each other.
...then there is no time dependence.

http://en.wikipedia.org/wiki/R-value_(insulation [Broken])

Heat transfer (as a thermodynamic application) has no time dependence. The amount of heat flow depends only on the temperature differential.
What? That's like saying velocity has no time dependence!? Quite obviously, for a given piece of insulation, "the amount of heat flow" in an hour is 60 times larger than "the amount of heat flow" in a minute.
 
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