"hobbes" wrote in message

After a power outtage, when the system came back on. All the hot water
went to Zone one the garage circuit. After the garage was warmed up,
then the rest of the house was supplied.

Is it usual for such a HVAC system to only supply one Zone at a time
with hot water because of the amount of hot water required?

No. It should be able to run each independent of others.
The pwer failure may have tripped something or a su rge whent he power came
back on may have caused damage. I'd do some experimenting to see if each
will work on its own by turning the garage off and tu rnign each of hte oth
ers way up to see if they come one. If not, you need repairs.