Installing condensing gas combi boiler or boiler only

bob eck · July 2019

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 · July 2019

The secondary pump or pumps should be sized to the head loss of the zone they're serving

Solid_Fuel_Man · July 2019

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.

EBEBRATT-Ed · July 2019

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 · July 2019

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.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_15.pdf

bob eck · July 2019

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.

Installing condensing gas combi boiler or boiler only

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