thanx princo.. the help is highly appriciated
thanx nashwan. and, yes, I know that the BT 137 can run the heater directly, but the problem is I have done a wrong when I design the PCB. I realizd later that the power rail I've designed was insufficient to drive the continuous current (6A) of the hater plus, the currents of the other elements (4 ACIM motors and some AC solenoids, driven by separate triacs). thats why Have decided to run a relay from the triac, (without doing a modification to the PCB) and drive the heater from the relay.
ANYWAY, I have figured out what causing the problem, it was the snubber network (0.1uF + 120 ohms) I have placed between the MT1 and MT2 terminals of the triac. the leakage current of the snubber network was sufficient to energize the relay coil and therefore the relay was remain on even though the triac has commutated.
as the solution, I have replaced the 0.1uF with a 0.01uF. now that leakage current of the snubber is insufficient to energize the relay coil and everything is fine.
thanx all for replying me and helping me out
regards,
Nimesh