What Is the Optimal Temperature Rise for Secondary Heating System Circulators?

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The discussion centers on optimizing secondary circulator and piping design for a heating system with two boilers, each handling 75% of the heating load. The boiler manufacturer recommends a specific formula for calculating secondary flow based on temperature rise, which raises questions about the balance between flow rate and temperature. Participants debate whether to prioritize higher temperatures with less water flow or to use more water at lower temperatures for efficiency. A compromise temperature rise, such as 40 or 50 degrees Fahrenheit, is suggested as a potential solution. The conversation highlights the need for careful consideration of system design to achieve optimal performance.
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I would like to get some opinions on secondary circulator and piping design for heating system.

System Description:

Two (2) boilers each one is 75% of building total heating load. Each with a secondary circulator pumping to a manifold into a primary loop. The return piping comes from a second manifold adjacent to to supply manifold.

Primary loop with two (2) parallel circulators moving the heating water 100% of the heating load using a 20 deg F TD to the building and through the equipment.

Okay, the boiler manufacturer suggests the formula:

Gross Output BTUH/(Temp Rise x 500) = Secondary gpm

Temp Rise = 230 deg F - Return Water Temp

I know that the boiler can handle higher flows than will be obtained through this formula (with a 70 deg Temp Rise and a 515,000 btuh boiler I only get 14.7 gpm).

So which is better, use less water at higher temperature, or use more water at lower, closer to system, temperature? Or is it best to figure a compromise and go with that? Say a 40 deg F or 50 deg F temperature rise in the boiler?
 
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