Alright, here is a bit more up to date save format, just check the source

Header, three chars, PSv

some simulation options to load with, all in one char
legacy(old heat sim) 1 bit, pause 1 bit, gravity mode 2 bits, air mode 3 bits, newtonian gravity 1 bit.

save version (major) 1 char

CELL size, most likely 4, 1 char

width in CELLs (wall tiles) 1 char

height in CELLs 1 char

last 4 chars is length of data, but not directly as an int.

Below is everything compressed
Walls 1 char
--for each fan wall, fvx 1 char
--for each fan wall, fvy 1 char
All particle types (0 if none) 1 char
All particle velocities, one char x, one char y
All particle life, two chars
All particle tmp, two chars
All particle tmp2 one char
All particle deco alpha, one char
All particle deco red, one char
All particle deco green, one char
All particle deco blue, one char
All particle temp, two chars
certain ctype saving elements, one char
number of signs
For each sign
--x location, two chars
--y location, two chars
--text