No, my domestic hot water is heated by a "sidearm heater" (heat
exchanger that is installed in the boiler).

The boiler system (and as a result, the domestic hot water heat
exchanger) was off while I was away.

The only expansion tank is the one on the boiler. That tank is
charged to 12 psig. I set the pressure of that tank (through the
shraeder valve on the bottom) while the boiler was pressurized.
Should I have set it when the boiler is depressurized (by closing the
make up water valve and draining some heating water from the boiler?)

Thanks,

Linc

Do you have a separate storage water heater? Was it firing
during this last test while you were away? If so, is it fitted
with an expansion tank? Is the tank plumbed in such a way that
overpressure (due to expansion) can be telegraphed to the boiler
feed circuit?

Jim