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best circulation pattern for hydronic cast iron radiator (with antifreeze circ. in mind)

Got a sweet little 3 and 1/4" thick cast iron not water radiator that was previously in service connected for feed and return at the bottom tappings. I plan to put this in a back hall on a secondary loop off the main loop to one of the apartments that we pay the rent for in order to get some comfort in the hall. Because it is a cold hall on the north side and would only have hot water when the apartment was calling I was going to keep it circulating in case setbacks would leave the water sitting long enough to freeze.

I probably could leave it to run with the main circuit but throw in an outdoor temp. sensor so if it's below 20 it just stays on.

But given that I'm trying to keep water moving that itself could become fairly cool between cycles I would like to encourage as much of the radiator to be involved in that pumped exchange (versus thermal filling of the sections from the bottom of the radiator when pumping reliably hot water.

I'm a little hazy from a fluid dynamic perspective whether this could best be acheived by opening a top tapping and circulating from a low corner to a high corner (or vice versa).

either that or I could use a control that would just trigger the main say once an hour to cycle til hot returns against an outdoor temp threshold – or super sophisticated have it do so more often the colder it got outside. (if there is such a control or way to implement that strategy) as the apartment would need the heat anyway and having it spread out when their wasn't an occupied demand would help insure this back hall radiator didn't freeze.

thanks for thoughts,