Mike is right on. That works, most of the time. On our '06 Dynasty, we had a different problem with precisely the same symptoms you are experiencing. It turned out that the pump for zone 2 had "failed".
Turn your thermostat off for a few seconds (to let the pump motor stop spinning), and back on and see if it starts working again. Ours did. Every time! Almost! It happened rarely, then more often, then more often, and finally it got to where most nights it would fail to work during the night. I tried what Mike suggested, and it didn't work, but I kept thinking I wasn't hitting it with the right "touch".
Anyway, each time I would shut off the thermostat for several seconds and then turn it back on, it would work. It got to where it maybe took a couple of tries. If you can, go out to the AquaHot and get as close to it as you can. Removing the top half of the cover is best, but not required. Listen to the zone 2 pump run when it's not working (blowing cold air). Have someone turn the thermostat off for four or five seconds, and then back on. See if it sounds different.
Those pumps are sealed. The motor is in one half and the pump is in the other. The coupling is magnetic, only. There's no shaft through the wall of the pump. As the pump ages, it can suffer from the same contamination causes as the flapper (check) valves. When the pump does "catch", it runs at a somewhat lower RPM than it does when the magnetic coupling fails to catch. If you do that, and you hear the difference in the RPM of the pump, it is highly likely that is the problem.
It's probably what Mike is suggesting, but if it isn't that, this is your next place to look. I was able to get a replacement pump, although it looks a little different and mounts a little different, with a Dremel to grind off the attachments for the old and a drill to make a way to mount the new, I was able to install the new one where the old one was, and it works awesomely!!! Just get a couple of cheap needle nose Vise Grips to clamp off the hoses when you remove it. I didn't have to drain the unit at all. I might have lost ¼ cup of coolant, if that. Yay! Sometimes we win!
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Jim & Angie, Boise, ID
2021 Entegra Esteem 27U
2017 GMC Acadia Denali
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