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That typically means either there was a high transient voltage, or your valve solenoid has shorted, causing the entire 24VAC to be applied on that resistor. I agree that the resistor isn’t a good solution for solenoid shorting, hence in later expanders we completely removed it as it’s not very useful.
PTC fuse would not work well either, it’s too slow to trigger. By the time it has any effect, the triac would have already been burned.