This is a subset of a larger sheet. Delta T should be 0, 15, 15 and I get 0, 23, 15. I zeroed out the first heat input and got a large imbedded matrix for unknown reason, that is secondary SMath.smz (5 KiB) downloaded 32 time(s).

Wrote

... and got a large imbedded matrix for unknown reason ...

It carries 'i' from previous use and it has to be cleared in some way.
That is one of many cases of shadow in context, very annoying has it
does not clear the previous matrix.

ElsidDoctored.sm (10 KiB) downloaded 25 time(s).

Appears to carry everything though and just evaluates the last value
Forgot to post. I changed q.w to check values on the top SMath.smz (5 KiB) downloaded 29 time(s).

It's in the loop. If I write an equation dt[1:f.dtv(1) above the program, I get one value array. As I scroll it down, it becomes a 3 value array

Jean, it should not clear the matrix, but build it. Maybe look into stacking it

Looks like it was a structure in the program. I had a "double" program line to start the loop. Cleaned it up and appears to work correctly. Input is always welcome
SMath.smz (5 KiB) downloaded 35 time(s).

Wrote

Maybe look into stacking it

SHOULD BE 15, 15 and .0017. What's wrong?
Just collect as you figure it should be.

SMath [2].smz (5 KiB) downloaded 28 time(s).