Are these the Silver Bullet valves?
By the way, it is possible for a compressor air to build up ice in a valve, if the conditions are just right, like when the weather is cold and humid. I have once or twice been forced to open the water supply and run the zones, to warm up the valves enough to defeat this phenomenon.
This post has been edited 1 times, last edit by "Wet_Boots" (Nov 25th 2012, 6:01pm)