Understanding this TRIAC circuit
If there is no load on the output, the voltage will approach the peak line voltage of \$\sqrt{2}V_{RMS}\$, minus maybe a bit from the triac drop and two diode drops. If there is a load, the voltage will droop between peaks as the capacitor discharges and you will get a lower average voltage.  



You have a zero-voltage switching opto, which "should" prevent the triac from turning on mid-cycle, however there may be odd conditions under which that won't be true. Such a turn-on could damage the triac, capacitor or diodes.