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I have a one pipe steam heating system, i recently changed out all the vents on the rads to Gortons. Everything was running great , the PSI on the gauge does not go above 4, but I have yet to replace the vent on the main. The system was working great, no hissing or banging, but every once in a while it will hiss from most rads during use and this lasts a day or maybe two and then nothing for a while. Any ideas on what maybe causing this?