The original boiler piping design was going to be a single loop system. Return water would be pumped through the system to the boiler and hot would flow out to replace it. One Taco 009 pump was chosen for the job due to its high head pump curve. Seen below, the 009 pumps fewer gallons per minute but at higher head in feet than the other larger pumps and higher than the smaller pumps that pump similar gallons per minute.
This graphs also compares the 009 with the smaller pump sizes.
This was a beautiful simple system. However, when tested, I could not get the water to pump fast enough through the piping to keep pressure in the boiler. Actually, there were several things wrong with this first installation that might have been changed one at a time and this simple loop system would have worked. I did not know that at the time, and only found the issues one by one. My first step was to abandon the simple system for a more complex and more usually recommended primary/secondary boiler piping design.