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Hissing rads

This is my first winter in my house. I have an older (mid 80's) oil fired boiler in my 100+ year old house. The boiler has been rebuilt and seems to be operating well. Every single radiator in the house hisses. Not just a quiet hiss, but a chorus of them.

The pressure gauge barely moves above 0 as the system cycles. I know I have main vents, but not sure the brand. They are located at the ends of the two mains. The radiator vents are all Dole. While listening to the "chorus" last night, I listened as one of the rads in the master would hiss, then click, finally becoming silent which lasted a few seconds, then it would click back open. All of the others never click or stop hissing. In one of the other bedrooms there is a sound in the radiator that sounds just like a water drip (enough so that an overnight guest thought there was a water leak somewhere).

So, no hammering, no pinging, rads all heat evenly and at about the same time, just TONS of hissing.

I plan on replacing main vents from reading the books (lost art) and reading this forum. But I want to start troubleshooting from the beginning rather than just taking shots in the dark. Thanks!!


  • EricinVT
    EricinVT Member Posts: 5
    additional info...

    I forgot to mention when the hissing starts. It is after the rads begin to get hot, and continues through the end of the cycle.
  • Abracadabra
    Abracadabra Member Posts: 1,948
    hissing vents

    #1 - What range is the gauge on your boiler.  If it's a 0-30psi, please add a 0-3psi and tell us what range the boiler is running and what max pressure it hits. Check the pigtail of the existing gauge and make sure it's not clogged with gunk.

    #2 - Radiator valves should eventually stop "hissing".  If they continue to hiss, then either the steam has not reached the vent to shut it, or the vent is not shutting and needs to be cleaned or replaced. The vent that clicks, stops, hisses again and repeats sounds like it's working like it's supposed to.  If the "clicking" bothers you, you may  need to go to a different brand of vent.

    #3 - I would track down the "drip drip" sound.  May be a leaking valve packing, or valve union.

    #4 - please take pics of main vents and a couple of radiator vents and post.

    #5 - radiator vents should be installed vertically.  Not diagonally, or sideways.
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