hi,
R11 is half of a potential divider with R8, it sets the Vref on the +vinp on R7/R14, pin #3 of U2A.
The current flowing thru R3 develops a voltage across R3 proportional to that current.
The R4 resistor is the Rin of the U2A OPA amplifier, the output of U2A is fed back to the comparator U2B, which controls the 555 trigger rate
Is this what you are asking,?
E
EDIT:
If W2 was a thermistor located in the irons heated body, it would provide feedback to U2A to correct for heat loss, ie: increase the element drive current.