THERM custom length feature question

[Linda Whaley]

I am trying to understand the custom length feature in THERM.  I built a wall model with homogeneous layers and set a u factor tag called entire exterior along the outside.  I then ran THERM and got an expected U value of .0241.  I doubled the length of the model by extending all components and still got the expected U value of .0241.  However, if I halve the length of the long version of the model or double the length of the short version using the Custom Length feature (without actually changing the model) it changes the U value.  Why?

[Jose Li]

Hello Linda，

the u value equals the heat loss through the entire section divided by the "length".

So If you double the length, the u value will be half.

I remember the custom length is only used when the therm calculated the wrong length.There is a case in the manual.