You need to isolate the triacs from the logic as they produce switching spikes from the back EMF of the solenoid valve so using an optocoupler like the MOC3081 triac driver for each triac and having a separate supply for your 24v AC is the way to go. Your fuse blowing could be spikes firing all triacs at the same time. It also makes the circuit simple as all you are driving is a LED well within the abilities of an AVR.
Regards Paul
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