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Basement radiators don't work
I just installed two old cast iron radiators in my basement, piped with 3/4" copper. The 3/4" feed comes off the furnace main, which is an old 2" line, then branches to each of the radiators, feeding into the bottoms. A 3/4" return comes out of the top of each, joins together, and then goes to the main return line. The equivalent length of pipe, including all fittings, is about 150' for each radiator. EDRs of the radiators are 50 sq ft and 60 sq ft.
For what it's worth, the system used to be a gravity feed system (house built in 1920). Over the years, some of the radiators were changed to cast iron baseboard with copper feeds. The system now has circulating pumps in a primary/secondary loop arrangement. The pump that feeds the house radiators is a Grundfos 15-58FC 3-speed, set on medium speed.
When the furnace kicks on, the feed line to these radiators get hot, but the radiators are barely warm to the touch. I am certain I've purged all the air out of both the rads and out of the return line back to the main return. I also ran water from a hose through both radiators and it seemed to be running well, so I don't think the radiators themselves are plugged, at least not totally. It's also interesting that if I turn off one of the radiators, the other gets a little warmer than if both are open, but still it's barely warm. Finally it's worth noting these are old rads that were hanging on the basement ceiling that I had moved - don't think they ever got hot while hanging on the ceiling either.
Any ideas of what could be wrong? Is it OK to feed the bottom of the radiators and take the return out the top? I can't seem to find anything that would indicate this is a problem, but I the typical arrangement seems to be bottom in and bottom out. Could it be a flowrate problem and maybe I need a circulating pump on this circuit? I've read some stuff on laminar flow, but if this were the problem, wouldn't the rad be cold but the return line be hot? Or is it likely that both radiators are just blocked so much they won't work?
Any help would be greatly appreciated. I'm just a lowly homeowner trying to educated on all this stuff.