Rob R. wrote:Why do all the zones call at once?
I've seen mine do it randomly on very rare occasions. Not all zones calling at once as in 100% simultaneous, but close enough for all practical purposes. The independent zones thermostats dictate this.
My cure is that an aquastat shuts down my single circulator (in a 4 zone valve system) whenever the boilers internal temp falls to 145 degrees. This permits the boiler to recover and be strongly on the temperature upswing when it permits the circulator to run again at 155 degrees and once again supply heat to the calling zones.
Since my boiler fires at 170 degrees it is quite rare indeed when it is brought down to 145 degrees, but all 4 zones opening at once after each loop has been off long enough to cool down appreciably can do it.