Flow Issue? One Cast Iron Radiator Won't get Hot

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Yonkers, NY
In the Spring, I added a new heating zone and circulator pump for my basement (zone B) to my hydronic heating system. While I was at it, I relocated the circulator pump for the pre-existing zone (zone A) from the return side of the boiler to the supply side, so that it is located upstream to the expansion tank. When I fired up the heating system, every radiator in the house is hot and toasty except one, located on the 3rd floor.

I have purged the system of air and I have water coming out of the bleeder. If I bleed this cold radiator long enough, the radiator will get hot. The supply pipe of the radiator will get hot but the return side will not.

Today I disconnected the radiator and flushed it with a garden hose. Black particulates were flushed out and the radiator had steady flow from the inlet to the outlet. The radiator shutoff valve looked fine, but I replaced it with a new one anyways (one with a larger opening). I then reconnected the radiator and purged the system of air. When I fired up the heating system, this radiator is still the only cold one. If I leave the bleeder valve open, the supply side will get hot but the return side stays cold.

I have isolated the supply side from the system, and with the circulator pump off and the radiator valve (on the supply side of radiator) in the shut position, opened the bleeder valve of the cold radiator. When I do this, I still get a steady steam of water, which tells me there isn't a blockage in the return line.

It's a three-story house built in 1909. Zone A is the pre-existing zone and is circulated by a Bell & Gossett Pump (runs very quiet). Last winter, all radiators were functioning perfectly. I have isolated zone B from the rest of the system via shut off valves, so the system is essentially what is was before.

The cold radiator worked before the circulator pump was moved. It seems that the flow on the radiator's return pipe isn't being pulled adequate enough to pull the hot water on the supply side, but what would cause this? What can I do or try next? Did I screw something up? All other 17 cast iron radiator get piping hot. What I missing something?



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