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Installing condensing gas combi boiler or boiler only

bob eck
bob eck Member Posts: 930
When you are installing a condensing gas combi boiler or condensing gas heating boiler only I know the boiler manufacturer has certain GPM needed for the primary pump my question is the secondary pump(s) do these pumps need to have a higher GPM than the primary pump or does it not matter?


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,541
    The secondary pump or pumps should be sized to the head loss of the zone they're serving
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  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    To get the lowest return temps the secondary pump should flow a higher GPM than the primary, but that us sometimes not the way a system is.

    This is why I like fire tube boilers which can sometimes direct piped.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    EBEBRATT-Ed Member Posts: 15,622
    Secondary pumps should move the same as the primary as a minimum, slightly more is better. If the secondary pumps less that the primary water backs up "across the bridge" and go in the boiler return
  • Zman
    Zman Member Posts: 7,573
    The flow of the secondary is not particularly relevant to efficiency it should be designed to give the zones the appropriate flow rates to achieve the design delta T. The flow will vary as the zones open and close.
    On the primary side , in a perfect world, the flow would modulate as the boiler firing rate changes. This way the boiler always maintains the maximum delta t and lowest return water temp. Lochinvar and others are including this feature in their firetube models.
    Pages 39-42 do a great job of explaining the math.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • bob eck
    bob eck Member Posts: 930
    Thanks Zman
    I know Lochinvar now includes a ECM circulator pump with their Knight wall hung or floor standing condensing gas boilers.
    I will check out the Caleffi info.