Help understanding a heating iron circuit component
I'm not going to trace it out, but it doesn't really look like a temperature controller at all. Looks more like a phase control that's only slightly more sophisticated than a light dimmer (reduced snap-on).
The diode may be a 32V diac similar to DB3, in which case the TO-92 part U10 would be something like a MAC97.
Edit:
Based on your schematic- this is a simple phase control. The pot adjusts the trigger angle. The trimpot is used to trim the minimum power setting. The bridge and R25/26 are used to help reset the timing capacitor C1 to reduce &quot;snap on&quot;
effect. The time constant is between the pot/trimpot,R and C1.
When the voltage across C1 exceeds about 32V it triggers on, triggering the triac. Image from here: