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Test Your Knowledge: Recirculation Noise

Noise created during steam recirculation noise can not only be annoying, but cause damage to steam systems. So what causes recirculation noise in a steam system?

  1. 1.High differential pressure across orifice
  2. 2.Insufficient back pressure downstream
  3. 3.Improper sizing of baffle 
  4. 4.Orifices discharging into tank below water line
  5. 5.Too much back pressure cause by globe value

And the answer is…

Both 1 and 2!

Recirculation noise is a result of high differential pressure across orifice which causes cavitation downstream (formation and consequent collapse of steam bubbles). It can also be caused by insufficient back pressure downstream.

If a circulator pump or hydronic heat piping contains unwanted air, cavitation in the pump due to the presence of the air can make a pump abnormally noisy as well as make a bubbling or gurgling sound in the building hot water heating pipes.

The noise happens most commonly with pump duty point 150 psig and higher at shut-off. Recirculation noise typically isn’t heard at lower pressure. A manifold sized to accommodate multiple recirculation flows can also cause it as it reduces back pressure when running less pumps.

The effect…

  • Recirculation cavitation creates loud reverberating popping noise similar to pump cavitation
  • May pit pipe wall and nearby fittings over time
  • Unhappy employees in boiler room
  • Recirculation flashing does NOT damage pump or affect pump performance


When re-working a noisy installation there are three approaches you can take:

Add a Globe Valve

Add a glove valve on header to tank to introduce back pressure and inhibit flashing.

Add Second Orifice

If the tank includes a single recirculation connection, requiring a recirculation manifold, consider adding a second orifice nipple downstream as space allows. Size each orifice to take half the drop.

Install Individual Recirculation Lines

On the front end, install individual recirculation lines with orifices discharging into tank below the water line, letting the water be the baffle. This is the most mechanically “sound” solution.
Eventually air leaking into a heating system (or low water volume in the hot water heating system) leads to loss of heat so it is something that should be corrected. For more information on which solution may work best to resolve your recirculation noise issue contact the team at Campbell-Sevey.

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