I've searched the boards, but haven't been able to find anything addressing the specific situation I believe I have. If I've missed it, please feel free to point me in the right direction.
Background: I have a single pipe steam heating system with an approximately 4-5 year old gas fired boiler. I purchased the house this past January and have done quite a bit to the old steam system since including insulating all the pipes in the basement and changing out all the radiator and main vents to various Gorton sized vents. The system works much much better than when I moved in, with the entire house heating quietly and evenly, but the solution to one problem still escapes me.
The Problem: A cast iron radiator on the 2nd floor rapidly blows air in and out while it is heating up. It sounds like it's panting, or hyperventilating, with several in/out air exchanges occurring per second. (definitely not water hammer) The radiator still gets hot and heats the room, but the noise is usually enough to wake us up when the boiler kicks on at night. (its in a bedroom near the bed) This is the only radiator in the house with this issue, so I believe it is something with this specific radiator or riser and not the system or boiler. I have exchanged the vent with another from a radiator not having this issue, with the same result on this radiator.
- The boiler is oversized, and could be overfired, but wouldn't that affect all radiators, or at least the ones closer to the boiler? This is not the closest radiator to the boiler.
- The pipes are insulated everywhere they are visible, but this riser does run up along the outside wall. So if it is uninsulated in there, that could cause the panting from what I have read on this wall. However the radiator in the next room over is configured in the exact same manner, on the same main loop running up the same side of the house, and it does not have this problem. It would be weird for one riser to cause such severe panting from being on the outside wall and the next one over cause absolutely none right?
- Shortly after we moved in I made the rookie mistake (absentmindedly unpacking boxes) of leaving the boiler feed water on too long (way too long) and had water running out of the radiators before I discovered what I had done. I don't remember the radiator panting before this, but that was for a short time, and before I got the system working well enough to actually get heat up to the second floor. I have checked all the pipes to make sure they are pitched correctly after all the extra weight, and none on that main were affected, and all that seemed to be off and been corrected (or rehung). So I'm running out of ideas...
- The remaining thing I can think of is a partial blockage at the radiator valve or somewhere along the riser. This radiator is on the second floor, so the valve wouldn't have been flooded (and it seems to be all the way open, best I can tell, right now) Where the rise goes off the main it goes up about a foot to above the basement wall, then runs about level for about a foot or two, then turns up into the outer wall towards the radiator. Could the flooding, or something else, have caused a partial blockage here that would cause this panting? If so, can I fix this?
Absent input from here that convinces me otherwise my next step is to detach the radiator from the riser, double-check the valve open-ness, and then try to flush the riser. I was thinking some TSP, followed by a enough water to flush anything all the way back to the boiler. Thoughts from the experts? Am I missing something? Am I going to make things worse? Am I in over my head? I had a local contractor come out and look it over and he didn't seem to have any idea what could be causing it other than a bad vent. (which I have since ruled out)
Thanks for your time. (And thanks for all the other stuff on there, this is an awesome website)