I recently had an expansion module fail with one of the 2.2 ohm resistors burning up. No sure exactly what happened – a zone stopped working and my initial voltage checks showed voltage at the output I think. Replaced the solenoid and still not working. Then didn’t have output voltage and opened up the case to find a burned resistor. The removed solenoid works but maybe has an intermittent short.
I have seen at least one other posting of a burned 2.2 ohm resistor. Apparently it is there for current limiting but if the output shorts the part burns up and overheats the pads enough they come off the board making repair difficult. This doesn’t seem like very effective protection but it could prevent more serious damage. Bigger pads would help get rid of the heat but wouldn’t be enough to handle the large amount of heat of a short circuit. Some foil on the back side of the board would help too and the board mostly empty on that side.
A PTC thermistor might work well. There are versions made to be self resetting “fuses”.
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.