Heating Help

weird gravity piping

I did a load survey at a 2-story school from the 40's I think, and it is gravity hydronic. Every radiator on both floors was piped the same way, with the radiator as a bypass off a downcomer that came out of the ceiling and went into the floor. This system has evidently been running OK for 75 years or so, and I can't figure out why the radiators have flow. Oh, duh - I think I just got it. The cold water in the rad is heavier than the hot water in the pipe so it drains out and is replaced by hot water. I guess I asked the right guy. Anybody agree or disagree? Thanks for thinking about it.

Yup

it's what we call an overhead or Mills system. The supply from the boiler goes up to the top of the building where it feeds horizontal mains, and the risers feed down from there.

John Mills was a heating genius.

thanks

Thanks for confirming. I was clear on the downfeed but the bypasses seemed to not make sense until they suddenly did

Dan...

Describes it in this article:

http://www.heatinghelp.com/article/332/Gravity-Hot-Water-Heating/72/Gravity-Hot-Water-Heating-FAQ

overhead downflow forever

There's even older buildings with those heating systems working just fine.
I figure that the system you describe was less expensive to install than the more common ladder (double) piping for gravity circulation. Sometimes less expensive is superior.