Munchkin 140 Super Stor 45 No Daytime Hot Water

jwork

I have a Munchkin 140M for radiant heating (3 zones) and for hot water (zone 4) using a Super Stor SSU-45 and a Taco ZVC404 control. The Munchkin has the Vision 1 upgrade and a few years ago I had the controller upgraded to the 926. For the past few years since the 926 upgrade the system has worked just fine. Until a few days ago. Toward the end of the day I noticed that the hot water wasn't as hot as normal. When I looked at the Munchkin I saw that the water temp was only about 80 F and the boiler wasn't firing even though the zone 4 yellow and red lights were on. The Taco is set up for zone 4 priority. The Super Stor has a Honeywell aquastat which activates a zone valve (Z-zone Z111000). The outside temps have been no higher than the low 50's and usually colder. We have 2 wood stoves which we use during the day so the heating zones 1-3 aren't calling for heat. After noticing the problem I thought I would look into it the next morning. But the next morning when I looked at the system I saw that the zone 4 lights were out and that zones 1-3 were calling for heat (yellow and red lights on all) and that the Munchkin showed a water temp of about 160 and that the flame indicator was on. The Super Stor lines were hot and the zone 4 lights were out. But during the day the same problem occurred: the Super Stor called for heat, but the Munchkin didn't respond. This pattern has continued for 3 days now. This evening I tried setting the ZVC priority switch to OFF, and I manually adjusted the zone 1 thermostat to a higher setting. After a few minutes the zone 1 red light came on, and the Munchkin fired up and began heating. The zone 4 lights were still on and water in the Super Stor began heating. After a half-hour I reset the zone 1 thermostat, and the zone 1 lights went off. At that point, so did the boiler. The Super Stor water was hotter, but it was still calling for heat and the boiler wasn't responding. I can imagine that perhaps the ZVC is malfunctioning somehow, but it is strange that during the night even though the zone 4 (priority) is still calling for heat that it doesn't get during the day somehow the zones 1-3 manage to come on even though they won't during the day. Before buying a new ZVC that may not solve the problem I wanted to ask if anyone else has had a similar problem or has an idea about what might be causing this. Because of the day/night difference I wonder if the Vision 1 system might be at fault?