Spreadsheet Tricks for Thermal Engineers
: Solving Thermal Networks Eliminate the ambient node, and treat its value as zero when you write up the equations for the
matrix. Then, add the ambient each temperature value. For fan on/off options, use an indicator with allowable values of zero and one. For example, choose a heat transfer coefficient based on
the fan indicator, "htc = IF(fanID=0, htc_NC, htc_FC)". For an optional component (for example, heat sink fins on a surface), use a very small length dimension instead of zero. That way,
when you calculate a resistance that involves dividing by an area, you'll eliminate a divide by zero (#DIV/0!) error. For complex networks, Excel's labels capability is phenomenally useful. Set
up a table of resistances for each component to every other component. For example 3, where many nodes are linked to the internal air and to the ambient, the table looked partially like this
table:

node1 
node2 
node3 
case 
Rinternal 
R1i 
R2i 
R3i 
Rc1 
Rambient 
R1a 
9999 
9999 
Rca 
Rsource3 
9999 
R23 
R23 
9999 
The large values (9999) are placeholders in case you ever want to change the linkages in the matrix – then you
don't have to rewrite the matrix coefficients later. Of course, you may know that there is no way that link will ever exist, so you can leave it out.
Using this table setup, the matrix coefficients can be written – and debugged – very easily, for example: =1/node1 Rinternal + 1/node2 Rinternal + 1/node3 Rinternal + 1/case Rinternal
… (etc.) and =1/case Rinternal + 1/case Rambient + 1/case Rsource3 … (etc.)
Yes, just like that, with spaces  Excel figures out that you want the value at the intersection of the row and column having the labels you've typed in (sortof) plain English. 