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All radiators but one are toasty. The cold one is up on the second floor, and is the last one on the line, farthest from the boiler.

It has been bled of air. It shoots out a thin stream of (cold) water in a small arc when the bleed valve is opened.

There is a supply main and a return main (black iron pipe from the 1940s).

Boiler is cutting off as expected at about 188F, with pressure reaching ~21psi.

The expansion tank is empty. It rings hollow. (I know from firsthand experience the dull sound of one whose bladder has failed so that the tank is full.)

This radiator was working fine until we lost electrical power a couple of weeks ago after a storm, and so the boiler was non-functioning for 24 hours. I thought perhaps the line had become frozen, but wouldn't the small arc of water from the valve indicate that there is no blockage?

EDIT: On the cold radiator in the new room, there is a shutoff valve on both the return and the supply. There's an arc of water from the bleed vent when supply is open and return is shut, and when the supply is closed and return is open (and of course when they're both open). But the water is cold even when the boiler is firing and the circulating pump is on and the system temp has reached 185F and about 22psi.

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The water that is being vented or bled out when you open the vent could be coming up either the supply or the return line. You could freeze one line and not the other. Is the circulating pump for the boiler running?

  • Circulator pump is running. I can hear it, and all other radiators are toasty. But we cannot turn them off indidividually; apart from this one, in a renovated room, the others are actually convectors without shutoff valves. I misspoke when I called them radiators. But thanks for that tip about the vented water. – TRomano Mar 23 '18 at 14:32
  • On the cold radiator in the new room, there is a shutoff valve on both the return and the supply. There's an arc of water from the bleed vent when supply is open and return is shut, and when the supply is closed and return is open (and of course when they're both open). But the water is cold even when the boiler is firing and the circulating pump is on and the system temp has reached 185F and about 22psi. – TRomano Mar 24 '18 at 14:20
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    If you can, trace the piping from the cold radiator back to where it hooks into the warm/hot piping. Also see if there is another pump on the cold lines as you trace them. Is there a thermostat in the new room, indicating a pump or zone valve that could the problem. If you can bleed water from the radiator when either valve is closed usually indicates that the lines are open. (bleed at least a quart or even a 1/2 gallon for each bleed test) – d.george Mar 24 '18 at 16:03
  • Thanks. After bleeding about a pint the water began to get tepid -- and then hot. What does that mean? – TRomano Mar 24 '18 at 22:33
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    If you ever have to replace the pump I would buy one with a little more capacity (maybe 20-25% more, and maybe a multi speed ).. If it is heating, I would not buy another pump unless this one gives you more trouble. By the way, the pump only moves the water in the system to overcome the friction loss of the piping and the radiation, it does not pump the water up to the radiator. The pressure of the water in the system holds the water up to the top of the radiators and the pump moves the water. Good job I do not know where you are but I am north of Pittsburgh, pa. – d.george Mar 26 '18 at 13:51

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