Sizing Secondary Systems

Artman

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?