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Near boiler piping of Condensing Boilers

steamfittersteamfitter Member Posts: 161
Do Condensing boilers pump away or into the boiler?
Does it depend on the manufacturer? I'm guessing the lower temps leaving the boiler do not cause the same issues relating to Henry's Law that non-condensing boilers have. However, why would they be piped with the circulator on the return?
Just trying to understand the theory because someone told me that Lochinvar condensing boilers suggest that.


  • SWEISWEI Member Posts: 7,356
    The first decade or so of mod/con boilers were based on water-tube heat exchangers which had high head losses. They were piped pumping into the HX in in order prevent cavitation. The newer Lochinvar designs use fire-tube heat exchangers which have almost no head loss. You generally have the freedom to pipe and pump them in whatever configuration works best for the job. In tall buildings, we pipe them pumping away from the boiler (on the hot side) in order to prevent adding the pump pressure to the static pressure, which allows us to run the system closer to the relief valve setting.
  • steamfittersteamfitter Member Posts: 161
    Thank you! Very Interesting!
    I understand everything but the last sentence regarding the relief valve setting. Can you please explain running the system closer to the relief valve setting? When you prevent adding pump pressure to static pressure, are you trying to keep the pressure low? Aren't you trying to keep the pressure away from the relief valve setting?
  • SWEISWEI Member Posts: 7,356
    We like to keep static fill pressure such that there is about 5 PSI at the highest point in the system. For a system with 50 feet of height, that comes to about 27 PSI. If we add the pump pressure to that, we run the risk of burping a 30 PSI relief valve. By pumping away from the boiler, the pump pressure gets subtracted from the 27 PSI fill pressure. It works.
    jonny88Rich_49GordyPaul S_3
  • TomSTomS Member Posts: 53
    For Lochinvar you can go to their website and download any of their manuals for condensing boilers. Take a look at their recommended piping. Most that I see pump away.
  • steamfittersteamfitter Member Posts: 161
    Thank you Tom!

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