Should Millivolt Gas Boiler's Aquastat Relay Be Set to Auto or Manual?

Have an old gas burnham boiler with millivolt system. Had the original aquastat relay replaced yesterday with an L8148J. I just don't know whether or not the setting should be on the auto or manual. When I have it on manual I rarely see the burners come on. What's the difference between these 2 settings and which is the better to use? The thermostat is one floor above basement. Reading the manual to the part is like Japanese to me.


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