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Noisy Hot Water Loop Off Steam Boiler

I have a hot water loop that runs off my steam boiler to the 3rd floor.  Seems to heat fine but is very noisy especially when the 3rd floor gets up to temperature and the circulator shuts off while the boiler is still running.  My guess is it is steam flashing but not sure why it is happening.  Based on reading through Dan's books, everything appears to be installed correctly.  Thought maybe it was dirty water, but regular skimming only reduces the problem slightly.

Any ideas on what could be wrong?  Any help or ideas would be greatly appreciated.


  • Mark N
    Mark N Member Posts: 1,094
    Hot water loop

    Are you using a heat exchanger to supply the loop? If you aren't, are you using a 3/4" bypass line to blend the return water with the water going to the loop? What is the temp of the water feeding the loop when the pump shuts off?
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Boiler bypass?

    Do you have a boiler bypass installed in the hot water loop? If you are sending boiler water at 212F up three stories, the lower pressure at the top of the loop may be causing the water to flash to steam at the reduced pressure found there. Remember that atmospheric pressure can hold the water up to an altitude of 32 feet, but the pressure decreases linearly to zero at that height. So with the reduced pressure found at the third floor height, water boils at much lower temperatures. The circulator is adding to the pressure there when it runs, but when it stops the pressure decreases suddenly, causing the water to flash to steam.

    If you have a bypass at the boiler to blend cooler return water with the supply to reduce its temperature, the water temperature at the top of the loop will be lower and may then be low enough to avoid flashing.

    The best solution would be to isolate the third floor loop with a heat exchanger so that the heating loop could be pressurized, eliminating the flashing problem entirely.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    As a non-professional, I have a question.

    If you have steam heat on the lower two floors, it seems to me that adding heat to the third floor would have been done with steam also. I wonder why it was decided to use hot water instead. Much more complicated.
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,111
    Hot Water Loop vs. Steam

    If the boiler and steam system is sized properly for two floors of steam radiation, it would probably be too small to supply the additional steam load.  It would, however have the capacity for hot water radiators in its pick up factor.  

    Even if the boiler was oversized and the new third floor radiators could be placed right above the existing ones on the second floor by extending the risers, the rizer and runout size is probably insufficient to carry the added load. 

    A hot water loop can be more easily added using soft copper or Pex tubing and a circulator. 
  • Steamhead
    Steamhead Member Posts: 15,773
    One way to add to a one-pipe steam runout's capacity

    is to drip the end where it rises to the radiator. This eliminates counter-flowing condensate in the runout, so it can handle more steam without banging. It helps to also re-pitch the runout toward the drip, if possible.

    In most cases, the riser has much more capacity than the runout, because the vertical riser can handle much more counter-flowing condensate than a horizontal runout, even if the runout has the proper pitch.

    The drip must go to a wet return, or to a water seal if the system has a dry return.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Scott1313
    Scott1313 Member Posts: 4
    Heat Exchanger/Bypass

    Thanks for the response.  There is no heat exchanger, but there is a 3/4" bypass.  Runs at about 180F and will jump up to nearly 200F for a bit after the circulator shuts off.  Even at 180F though, it is fairly noisy after it runs for awhile (but worse at 200F), so I guess it is the pressure drop that Mike K is referring to in his post?

    I should probably try and adjust it down, but don't know how low I can safely go with the return water?  Seem to remember reading something about that being bad for the boiler?
  • Scott1313
    Scott1313 Member Posts: 4
    Heat Exchanger/Bypass

    Thanks very much for the response.  Based on your description,  I'm guessing it is the pressure drop at the 3rd floor that is causing the flashing.  There is a 3/4" bypass installed, but no heat exchanger.

    Any idea how low I can safely set the water temperature in the loop until I can look into an exchanger.  I seem to remember reading setting the temp too low could be bad for the boiler?  Currently running the loop at about 180F though it does jump up a bit when the circulator cuts off.
  • ttekushan_3
    ttekushan_3 Member Posts: 944
    Like you,

    I have fun finding the best way to add onto a steam heating system when rooms are added or extra heat is required somewhere.   If the boiler has the capacity, you can usually find a main that has extra capacity, or an alternative piping method, etc. to do it.

    The trick is knowing how to do it ;-)

    Sometimes I feel like a Steam Zombie, like the "living Dead Men" are speaking through us - "you can do it."   Or maybe I should speak for myself on the zombie part :-)

    Happy New Year
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Maximum loop temperature

    I don't know the details of your system, but if the 3rd floor radiation is 22 feet above the boiler, then the absolute pressure at that point is 5 PSIA, or about 1/3 of normal atmospheric pressure of 14.7 PSIA where water boils at 212F.

    At 5 PSIA water boils at approximately 162 degrees F, so that would explain your problem of flashing to steam with 180F water. Try to adjust the bypass to get a water temperature of 160F into the loop and see what happens. I have a feeling that this may resolve the problem.

  • Scott1313
    Scott1313 Member Posts: 4
    Weekend experiment

    I experimented with adjusting the bypass over the weekend which has lowered the temperature a bit and improved how the hot water loop is running, but even with the bypass fully open the loop is still running at 175F. 

    I took a look at a another diagram for piping the hot water loop and realized that while I have a ball valve on the bypass, but there isn't one on the pipe coming from the boiler.  Could this be why I can't get the temp lower or could there be another problem?
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Valve at boiler supply

    Yes, an additional valve at the water outlet of the boiler will reduce the loop temperature further. Right now, the relative flow through the bypass is based on the difference in flow resistance of the bypass and the boiler.

    By restricting the flow through the boiler with the valve, more loop water will chose the path through the bypass, thereby reducing the water temperature.
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