FAQ
TL;DR: On old cast‑iron radiators, install the vent in the upper plug of the last rib; one expert notes, "The vent should be installed on the radiator cap in the last section" and reports success on heaters up to 10 ribs. [Elektroda, Zbigniew Rusek, post #16672907]
Why it matters: This FAQ helps homeowners retrofit or service cast‑iron radiators without leaks, stuck plugs, or poor bleeding performance.
Quick Facts
- Best location: upper end plug on the last rib (highest point when pitched). [Elektroda, Zbigniew Rusek, post #16672907]
- Many old "blind" end plugs use left‑hand threads—check direction before removal. [Elektroda, Zbigniew Rusek, post #16672907]
- Radiator tilt is of minor importance; a level cast‑iron unit vents fine when equipped. [Elektroda, kgs245, post #16674512]
- Loosening valve unions to bleed can gush boiling water if over‑turned. [Elektroda, kgs245, post #16674512]
- Outlet shutoff valves aid mid‑season isolation and help with venting via open/close. [Elektroda, jaga134, post #16672910]
Where should I install the air vent on a cast‑iron radiator?
Place it in the upper plug of the last rib, at the radiator’s high point. Clean the seat, fit a gasket, and seal threads as specified. Quote: "The vent should be installed on the radiator cap in the last section." This approach worked even on 10‑rib heaters. [Elektroda, Zbigniew Rusek, post #16672907]
Can I vent from the supply valve instead of the end plug?
Old systems often bled through the valve, but those radiators were pitched toward the supply to move air there. If you add a proper vent, the end plug remains the most reliable high‑point location. [Elektroda, FRANKOX, post #16673181]
Do end plugs really have left‑hand threads? How do I tell?
Yes, many old “blind” end plugs use left‑hand threads. Try turning clockwise to loosen; if it tightens, switch direction. Use a punch wrench for stuck plugs and avoid forcing the wrong way. [Elektroda, Zbigniew Rusek, post #16672907]
I can’t remove a seized plug. What’s a safe workaround?
Out of heating season, drill the existing plug, tap the hole, and screw in a vent using flax or PTFE. This avoids disturbing the plug threads. Failure case: one user over‑loosened a union and flooded the floor with hot water. [Elektroda, kgs245, post #16674512]
Is radiator tilt required for proper bleeding?
Tilt helps move air to the chosen vent point, but a cast‑iron radiator with a dedicated upper‑plug vent will bleed effectively even when level. [Elektroda, kgs245, post #16674512]
Can I combine a flow‑control outlet valve with an air vent?
Use standard inlet/outlet shutoff valves for isolation and balancing, and place the vent in the upper end plug. Outlet valves aid service and venting via open/close, but they don’t replace a high‑point vent. [Elektroda, jaga134, post #16672910]
How do I bleed without a dedicated vent?
Start the pump, close the radiator valve, then slightly loosen the union to release air. Retighten once water appears. This works but is messy and may need repeats; add a vent when possible for safety. [Elektroda, arturdip, post #16671672]
Should the vent be on the supply or return side?
Place the vent at the supply side’s high point when using supply‑end venting practices. With end‑plug vents, prioritize the physically highest plug regardless of pipe orientation. [Elektroda, Zbigniew Rusek, post #16675442]
What sealants and gaskets should I use when refitting plugs and vents?
Clean mating faces to bare metal. Fit a suitable gasket, then apply liquid sealant or thread paste. If the vent has an O‑ring, you can install it without extra sealing; otherwise, use paste or PTFE. [Elektroda, Zbigniew Rusek, post #16672907]
Quick how‑to: add a vent to an old plug without removal
- Drain the radiator and confirm no pressure.
- Drill and tap the plug to the vent’s thread size.
- Wind flax or PTFE on the vent threads and screw it in firmly.
This takes about 20 minutes per radiator with basic tools. [Elektroda, kgs245, post #16674512]
Common mistakes when servicing cast‑iron radiators?
Over‑packing tow prevents proper tightening, and corroded threads leak after reassembly. Another mistake is misreading what’s loosened: caps versus valve inserts. Inspect threads and components before force. [Elektroda, Zbigniew Rusek, post #16673075]
I’m on the top floor—anything special to check?
Ensure radiators have upper vents and confirm riser vents on the last floor. Air accumulates at system highs after startups and neighbor adjustments, so plan to vent at season start. [Elektroda, Zbigniew Rusek, post #16673075]
