My boiler has 4 zones. Zone 1 is the bottom floor.

There are two radiators in zone 1. I am replacing one of them, an old baseboard radiator, with a cast iron with similar BTU.

We hooked up the new radiator and got nothing. But the other radiator on the same zone is working fine.

In the process of troubleshooting, we confirmed that whoever installed the boiler was reading the diagram backwards and put valves on the supply lines, not the return lines, so there is no easy way to bleed the air out of each zone other than the air vent taps on the radiators themselves.

Naturally, all 3 air vent taps on the new radiator are frozen, so we had to drill a new tap. After that, we confirmed there is no air in the system.

We thought, blockage. We disconnected it, flushed the lines, got a little gunk but nothing significant, and are able to easily blow air through the whole circuit.

The two radiators in zone 1 are connected in parallel, that is, the supply line is split right near the boiler and they have separate return lines.

The radiator that isn't working shares the return line with radiators on the 2 floors above. The way the return line is configured is it has to push water up 6 feet, and it joins the return line with a standard T joint, meaning there is also pressure coming from the floors above.

Is the problem the pressure in the system?

I figure either the two radiators connected in parallel are not balanced, so the pump is not circulating enough water in the line to the new radiator, which is the longer of the two.

Or, the pump can't generate enough pressure to push the water up 6 feet and against the water coming down from the floors above.

Does that make sense? Do I need to reconfigure it? Constrict the supply line on the one so the two are more balanced? Hook up the return so it's not fighting the pressure of the water from the floors above? Just get a stronger pump?

  • What happens if you turn off the values on all the readators apart from the one that does not work. – Walker Jan 31 '14 at 17:26
  • There are no valves installed, so I can't turn off the other radiators. We just put in a new circulating pump and that didn't fix it. Installing a valve just so I can do that is probably the next step. – pduey Jan 31 '14 at 18:30
  • Was the old baseboard working properly? Is the other radiator baseboard type? The cycle time for cast iron radiators is much longer than baseboards. The whole system may be cycling on then off before the cast iron has a chance to start working. The resistive return is not helping either. – bcworkz Jan 31 '14 at 19:44
  • The return line is warm (not hot), I guess because warm water is seeping in from the shared return. The old baseboard did get warm, but maybe that seepage was enough for the baseboard, so it is possible it was not working correctly. Too late to check now! The other radiator on that zone is also cast iron, but it is much smaller. It heats up very quickly. – pduey Jan 31 '14 at 19:51
  • @bcworkz I don't know about cycle time. My understanding is, if the water/aquastat is at top temp, the boiler stops firing. But if the room/thermostat is not at temp, the circulating pump keeps circulating. That is, the boiler firing and the hot water circulating are independent of each other. Is that wrong? Is there some way I can keep the water circulating to see if it finally heats up the new radiator? – pduey Jan 31 '14 at 20:08

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