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Gravity boiler system

My radiators on the second floor, most will not bleed water. Last year I opened the pressure relief valve to force water into the system in order to force the to bleed water. Not sure if that's the best way to do that.  Does that indicate that the pressure relief valve is bad ? 


  • Ironman
    Ironman Member Posts: 7,376
    Not the pressure RELIEF valve, but the pressure REDUCING valve (aka the fill valve). The relief valve lets water out when the pressure gets to 30 psi.

    What’s the pressure showing on the boiler gauge? It should be a minimum of 12-15 psi cold.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    JUGHNE Member Posts: 11,061
    What pressure is showing on the gauge?
    That should be about 15 PSI for a 2 story building.

    That will get you to a standing water height of 30' which should cover your house.

    Your pressure reducing valve is what sets the system pressure.
    You may have to manually add water to get up to the 15 PSI.

    The pressure relief valve will open at 30 PSI and pass water out.

    What type of expansion tank do you have?
  • Alan (California Radiant) Forbes
    Last year I opened the pressure relief valve..............
    You must mean the pressure reducing valve, also called the fill valve. They often get stuck in the closed position and will not fill. Either replace the fill valve or fill the system with a hose using a drain valve if there's one on your piping.
    If it is, in fact, a gravity system, you will hear water coming down the roof gutters when it's full. The expansion tank in the attic is usually vented through the roof.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    JUGHNE Member Posts: 11,061
    Also the overflow could be piped down thru the walls into the basement and tied into the sewer drain system. So you may not hear any water.
    The gauge is the next best indicator.
  • DIYBruce
    DIYBruce Member Posts: 2
    I'll have to check the gauge tomorrow since I'm not home rn. The expansion tank is in the basement and not the attic.  FMI, could you briefly outline the steps that I should do and the order to do them in, in order to properly bleed the air from all of the radiators in a two story home w a gravity system. There are no circulatory on this system btw. Thank you.
  • Jamie Hall
    Jamie Hall Member Posts: 23,297
    Not hard -- but it helps to have a helper. What you need to do is to get your helper down in the basement and using the fill valve to set the pressure to 15 psi. If there's a valve which you can use to close off the expansion tank, close it. Then tackle one radiator, and open the bleed valve until you get a good solid stream of water. Helps to have a bucket... meanwhile, your helper is adding water a bit at a time to make up for the air or water you are bleeding out. He or she should try to hold 12 to 15 psi. Close the bleeder and proceed to the next radiator. And so on. When you are done with all of them, open the valve to the expansion tank if you closed it, and adjust the pressure to 12 to 15 psi with the system cold.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England