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Hissing sound after boiler shuts off...

Hey All!

Been a while, but I think I may have an issue with my boiler and wanted to get some feedback.

I drained my converted gravity system a week ago to have a radiator removed due to a bathroom remodel. While it was emptied I also had a Caleffi air separator installed. Bathroom was completed earlier this week, the radiator was reattached, and the system was filled. I bled all the radiators from the top down and fired up the system to get the fresh water circulated through it. The system ran for about 15 minutes and then I shut it off. Once off, I heard something hissing which I thought was coming from the air separator, but it was coming from inside the boiler case.

So..My question to you experts and Pros: How\What can I check to see what the cause is?




  • GordyGordy Member Posts: 7,122
    Entrained air

    Once the system is filled, and purged/bleed. There will still be entrained air in the new fill water that will come out of solution once the water is heated. The air separator will take care of this if installed correctly.
  • Sweet_LewSweet_Lew Member Posts: 104
    I forgot one thing...

    Hey Gordy,

    Thanks for the reply. I actually didn't finish my initial post. The hissing noise was coming from inside the boiler, NOT the air separator.

     Also, I think I need a more accurate gauge. Before draining the system cold pressure was at 12. Now, it's at 8. The feed\reducing valve has shut as well.
  • GordyGordy Member Posts: 7,122
    Type of expansion tank

    You have? I'm assuming this is a non converted gravity system with out a circulator. The expansion tank in the attic, and is not a bladder type.

    What leads you to believe the pressure gauge needs replaced?
  • Sweet_LewSweet_Lew Member Posts: 104
    edited September 2013

    It's a converted Gravity system. I have a Taco circulator and a WATTS bladder style tank.

    I'm ASSuming that my built in gaugue might be sticking or possibly off due to the initial reading before I drained the system. When the system was empty, it still registered 12psi until the next day. When I refiled the system, it still read 0 until I started bleeding the radiators and started the system. Now it reads 8.

    Do you think it's safe to adjust the reducing valve to allow more water in or just open the valve manually? I was thinking to get a more accurate read from my local Pro before I start adding more water. Luckily, it's still warm here in Chicago and won't need the heat anytime soon.
  • GordyGordy Member Posts: 7,122
    Fix it

    Replace the tridictator gauge. You would not put air in a tire with out a tire pressure gauge. How do you know?

    Check the bladder tank isolated from the system for proper psi 12- 15 if a two story home with boiler in basement.p
  • Sweet_LewSweet_Lew Member Posts: 104

    I will. My local Pro is coming Saturday to clean the boiler for the season and will have him confirm that the gauge is indeed faulty.

    Appreciate the assistance.

  • Sweet_LewSweet_Lew Member Posts: 104
    edited September 2013
    Saturday cleaning\check up

    So my local Pro showed up for a routine cleaning and checkup for the season. The hissing was most likely due to condensation since there was fresh\cold water in the system hitting the heat exchanger. If there was a crack, there would be water on the floor. WHEW! Dodged a pricey bullet.

    The current cold reading on my gauge is 16PSI. He hooked up his gauge to the drain valve and his read 16. So the gauge is accurate but now a new question arises:

    Last year I was at 12PSI cold and now I'm at 16PSI cold. I have not played with the reducing valve and the other two variables would be that I had the system drained to install an air separator and then refilled. Is the fresh water causing the pressure to rise due to the oxygen in the water or something else?
  • GordyGordy Member Posts: 7,122
    Leave it

    At 16 yes the fresh water will keep more oxygen in solution. once the system starts operating. Cold water holds more oxygen tin solution than warm water. In the same token a higher pressure will keep oxygen in solution more than cold water...... If that makes sense.

    So as your air separator starts removing oxygen in solution your fill pressure will drop a bit.

    Lesson learned let system equalize in temperature before firing the boiler. Or do not add fill water while boiler is at temp. Situations to avoid.
This discussion has been closed.


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