VFD pumps control in hydronic system 1

[Sahkah]

Hello,
for a hydronic system consisting of two condensing boilers , of dual return, utilizing variable speed pumping primary and secondary circuits , what would control the pumps speed whether on the primary or the secondary circuits? also would it be more efficient for the systems to have the boilers work in a parallel operation such that each work at the same time but with partial load or just have one boiler work all the time and alternating with the other every week? The building has its BAS system
Thanks for your help

 

[GT-EGR]

Condensing boilers that I know are more efficient at part load, so operating the two simultaneously will be the most efficient way to operate. Confirm this with your manufacturer which should have all the boiler efficiency curves both for variable return temperature and variable flows/output.

Your system/secondary pumps should control to maintain the piping system differential pressure needed to satisfy the control valves.

The primary/boiler pumps ideally operate at the exact flowrate as the secondary system, but is hard to do in practice. Flow less than the secondary side and your boilers have to put out a higher temp to keep up, flow more than the secondary side and you will end up raising your return water temperature and lose condensing efficiency.

I believe the simplest way is to modulate the primary/boiler pumps to maintain a constant delta T across the boilers. The boiler manufacturer may have their own control system include that directly controls the pump based on its current entering and leaving temperatures and fire rates.