Got up this morning to find an over temperature condition (temperature
way above thermostat setting) on the first floor at home. At first I
thought it was a thermostat or a flow control valve (FCV) issue but it
turns out that the heat motor piston in the power head (PH) for that
zone is stuck in the "expanded wax" position. Due to the FCV spring
pressure, it was one hell to remove the PH from the FCV housing even
after disconnecting control voltage and waiting five minutes.

It's a Taco 555-102 PH. With it removed from the system, the piston
remains in the expanded position. Am I missing something or can the
wax in the heat motor change properties like this?