Aqua Hot 450D troubleshooting cool air zone 2 (front)

Using your manual pump, attach the return pipe for that zone to the intake of pump. Attach output of pump to return fitting on rear of tank. Turn on manual pump and Aqua Hot zone pump. That should purge air in heating loop.

I have done this when installing a new heater in a Alegro Bus whose owner allowed the fresh water loop to freeze and split necessitating the new heater.