Cold Second-Floor Radiator on a Converted Gravity System
A two-story house has a converted gravity system with large cast-iron radiators, 1-1/4 in. mains, one circulator on the supply, and zone valves. The second-floor rear bedroom radiator was replaced with a smaller modern unit. The supply pipe to the new radiator gets hot, but the radiator stays cold and its return pipe is cold. Other zones heat. The circulator is noisy, and the system pressure gauge reads 12 psi cold and 18 psi at 180°F. The bladder-type expansion tank is on the boiler return, and the air separator is on the supply. A manual bleed valve on the new radiator releases a short hiss of air, then water. The installer purged the zone by opening the boiler drain for about 30 seconds. The new radiator is connected to the existing supply and return with the original pipe sizes, and there is no automatic air vent at the high point.
1. What is the most likely reason the radiator stays cold, and what evidence supports it?
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The supply pipe is hot while the radiator and return are cold, so hot water is reaching the branch but not circulating through the radiator. A short hiss followed by water at the bleed valve indicates an air pocket, not a completely dry radiator. On a converted gravity system, the large mains and radiators can trap air at high points, and a 30-second boiler-drain purge does not move air through a second-floor branch with low head. The new smaller radiator may also have a different internal volume and piping connection that makes the air pocket harder to clear. The noisy circulator supports air in the system. The pressure rise from 12 to 18 psi is not automatically a failure, but the expansion tank precharge and acceptance should still be checked as part of the repair.
2. Describe a proper purge procedure for this zone and explain why the installer’s method failed.
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Isolate the other zones, connect a purge hose to the zone drain or boiler drain, and make sure the fill valve can maintain pressure. Open the purge point and feed water through the zone at full flow, working from the boiler outward, while opening the high-point bleed at the radiator. Close the bleed when water runs steadily without air, then continue purging until no air comes from the purge hose. Restore pressure, open all isolating valves, and run the circulator. The installer’s 30-second boiler-drain purge probably pulled water through the boiler and near piping, not through the second-floor loop. Air remained at the high point, so the circulator churned it and the radiator stayed air-bound. Adding an automatic air vent or a high-point bleed at the radiator will help prevent a repeat.
3. What else should be checked on this converted gravity system before declaring the repair complete?
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Check the expansion tank precharge and acceptance; a waterlogged or incorrectly charged bladder tank can cause pressure swings and pump noise. Verify the circulator is sized for the remaining radiation and not producing excessive velocity through the large gravity mains. Check that the new radiator’s supply and return are not reversed or connected in a way that creates a trap, and confirm the zone valve opens fully. Inspect the air separator and make sure it is at the correct location with proper flow direction. Verify fill pressure matches the system height, usually about 12 psi for a two-story house plus a few psi margin. After purging, recheck the pressure cold and hot, confirm the radiator heats evenly, and listen for air at the circulator.