Is there a counterpart to the advice given in this video except for late 1940s-era convector-style radiators which do not have a valve on their return and have only an air-bleed-valve? Basically a pipe with fins arranged around it. The convector farthest from our boiler is staying cold though all others are getting warm. All of the air has been bled from the system and this rad doesn't sputter when its bleed valve is opened but shoots out a thin stream of (cold) water.
These convectors are not daisy-chained; each one has its own supply to the main 1.5" NPT black iron supply and its own return to a 1.5" NPT black iron return.
EDIT1: Last night I installed a 3-speed TACO circulating pump yesterday (Model 0010-MSF1-IFC), refilled the system and carefully purged it of air one radiator at a time. Both sides of the house get warm, all radiators, except for the cold one, which shoots out a stream of cold water from its bleed valve, and this happens even when the pump is on "high" setting. So I don't think it's an issue of not having enough pump.
EDIT2: I'm going to make an edit to this abstract diagram in case it's significant in terms of the diagnosis: each zone has a return stop valve. The 1/2" supply and return pipes for all radiators run verticaly from the basement except that with RAD5 (the cold one) the 1/2 pipes turn 90 degrees when then reach that room and then run horizontally for about 12 feet between the floor joists.
|| | | | |
|| RAD1 RAD2 RAD3 RAD4
|| | | | |
|| ============================= return======================
|| x front zone return stop valve
|| x rear zone return stop valve
|| \ REAR ZONE
|| | | | | \________
|| RAD1 RAD2 RAD3 RAD4 RAD5
|| | | | | ___________|
I'm thinking the only way to solve this, absent a way to restrict the return flow at each radiator, is to pump the water into the supply line with greater force, so the radiators first in line simply cannot consume all the water and some will get past them to supply radiators farther downstream. Or possibly close the return stop-valve at the boiler and then back it open just a little so the return is a "bottleneck" which will force water further along the supply line.