I had the same issue on one zone two years ago. The certified Aqua Hot Tech in Paso Robles, CA had me open the bay and feel the two pipes coming out of the top of the unit. One was warm and one was cold. I lightly tapped the cold one near the base and it instantly started warming up. He told me there is a flapper valve at the base of each tube that sometimes sticks. The light tap freed it up. Has worked fine ever since. Maybe a quick fix.