Hello.
I am simulating the simplest of structures, a solid (Fraunhofer) brick with interior plaster, and I am looking at the transient U values as calculated by the thermal transmission module. For 2 months out of 12, specifically July and August, the U values are shown as equal to zero  see attachment. How can this be possible? And how can it be that the corresponding R values (also attached) are not the reciprocal of the U values?
Many thanks,
Apostolos
Thermal Transmission calculation of U values
Thermal Transmission calculation of U values
 Attachments

 Screenshot 20220927 at 09.45.26.png (75.89 KiB) Viewed 259 times

 Screenshot 20220927 at 09.45.18.png (84.21 KiB) Viewed 259 times
Re: Thermal Transmission calculation of U values
Hi aease,
these data are results of an evaluation of the transient heat flows through the component, and under these conditions the relationship between R value and Uvalue is more complex than for the steadystate conditions which are usually considered.
The transient R and Uvalues may even become zero or negative. The help file for the postprocessor says:
Kind regards,
Thomas
these data are results of an evaluation of the transient heat flows through the component, and under these conditions the relationship between R value and Uvalue is more complex than for the steadystate conditions which are usually considered.
The transient R and Uvalues may even become zero or negative. The help file for the postprocessor says:
The red arrow in your second screenshot shows that this has happended here.The postprocessor is intended to assess heat losses. Under warm outdoor conditions, heat may flow into the building, resulting in negative R and Uvalues. These negative values are set to zero, a red arrow at the side of the diagram alerts the user to this.
Kind regards,
Thomas
Re: Thermal Transmission calculation of U values
Thanks, Thomas, your explanation makes sense, although I can't imagine why WUFI would artificially Uvalues to zero if negative and not keep the absolute value, which would carry physical meaning.
How can I find out more about how transient Rvalues are calculated, here?
Many thanks, again,
Apostolos
How can I find out more about how transient Rvalues are calculated, here?
Many thanks, again,
Apostolos
Re: Thermal Transmission calculation of U values
Hi Apostolos,
that's because the Uvalues are usually used as a measure for the transmission heat loss of a building. Negative heat losses should be distinguished from positive heat losses, but they are of no real use to you because they do not compensate the losses that have already occurred. Beyond that it is somewhat arbitrary, yes.
Please refer to Topic 3 of the "Thermal Transmission" postprocessor's help file which explains the calculation in detail.How can I find out more about how transient Rvalues are calculated, here?
Kind regards,
Thomas
Re: Thermal Transmission calculation of U values
Thanks for those pointers, Thomas.
I've read through topic 3 in the documentation for the Thermal Transmission postprocessor but would still like to clarify the following:
1) Are the moisturedependent steadystate Rvalues (straight lines for dry, u80 and typical builtin moisture levels) in the postprocessor graph the resultants of the wall component Rvalues only or do they include the heattransfer resistances of the interfaces? I'm trying to addup all the possible components but can't seem to arrive at the Rvalues calculated by the postprocessor.
2) The documentation seems to state that, for the transient U and Rvalues, reciprocity should hold, since they are both defined in terms of the ratio (or its inverse, respectively) of the average heat flow density divided by the average temperature difference over the calculation period. However, the monthly Rvalues are not the reciprocal of the corresponding Uvalues.
Any clarifications would be very useful and appreciated.
Thanks,
Apostolos
I've read through topic 3 in the documentation for the Thermal Transmission postprocessor but would still like to clarify the following:
1) Are the moisturedependent steadystate Rvalues (straight lines for dry, u80 and typical builtin moisture levels) in the postprocessor graph the resultants of the wall component Rvalues only or do they include the heattransfer resistances of the interfaces? I'm trying to addup all the possible components but can't seem to arrive at the Rvalues calculated by the postprocessor.
2) The documentation seems to state that, for the transient U and Rvalues, reciprocity should hold, since they are both defined in terms of the ratio (or its inverse, respectively) of the average heat flow density divided by the average temperature difference over the calculation period. However, the monthly Rvalues are not the reciprocal of the corresponding Uvalues.
Any clarifications would be very useful and appreciated.
Thanks,
Apostolos
Re: Thermal Transmission calculation of U values
Hi Apostolos,aease wrote: ↑Wed Oct 12, 2022 5:23 am 11001) Are the moisturedependent steadystate Rvalues (straight lines for dry, u80 and typical builtin moisture levels) in the postprocessor graph the resultants of the wall component Rvalues only or do they include the heattransfer resistances of the interfaces? I'm trying to addup all the possible components but can't seem to arrive at the Rvalues calculated by the postprocessor.
no heat transfer resistances are involved in calculating the Rvalues. For example, if your assembly only consists of a 0.3 m thick layer of Baumberger sandstone, the water content corresponding to 80 % RH is 35.6 kg/m3 (as determined by the moisture storage function), and the moisturedependent thermal conductivity for this water content is 1.9445... W/mK (interpolating in the relevant table). The Rvalue of this simple assembly is therefore 0.3 / 1.9445... = 0.15427... m2K/W, the postprocessor says the Rvalue for u80 is 0.1543 m2K/W.
Please note that due to a bug in the data exchange between WUFI and the postprocessor, the postprocessor may show wrong Rvalues if there are monitor positions within the assembly. If so, please remove all monitor positions from the assembly. See the list of known bugs, tab postprocessors.
There is no reciprocity between the transient R and Uvalues because the transient Rvalues use the mean temperature difference between the component surfaces, while the transient Uvalues use the mean temperature difference between indoor and outdoor air.2) The documentation seems to state that, for the transient U and Rvalues, reciprocity should hold, since they are both defined in terms of the ratio (or its inverse, respectively) of the average heat flow density divided by the average temperature difference over the calculation period. However, the monthly Rvalues are not the reciprocal of the corresponding Uvalues.
Kind regards,
Thomas