On Wednesday, December 19, 2018 at 1:36:13 AM UTC-5, dpb wrote:
On 12/18/2018 5:20 PM, wrote:
...

I am thinking one of two possibilities. Either the pump on the furnace
does not have enough pressure to lift the water to that second level, or
else that radistor needs some sort of restriction on it's OUTPUT side to
force the water to the top of the radiator. Currently he just has both
pipes going to the radiator, one to the bottom left, the other to the
bottom right.

(This is a HOT WATER, -NOT- a Steam heat setup).

Yeah, if he's running a single-pipe system, the inlet should be the high
entry point and the exit the low for each radiator...

https://www.diydata.com/planning/central_heating/pipework.php#single

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It's more common that they are both at the bottom. There may be some
advantage to one at top, one at bottom, but either way works. The
water is hot and it rises to fill the whole thing.