I have just been looking at the Keaton link you provided Doitall.
On page 4 a hydraulic layout is shown which shows a pump fitted between the the header and the boiler which Keston calls a "shunt pump".
Surely this is the "primary" pump.
If that shunt pump (as Keston call it) was not fitted , then water would never pass through the boiler because the secondary pumps after the header would just circulate water through the header alone and no heat would never reach the system .
On page 4 a hydraulic layout is shown which shows a pump fitted between the the header and the boiler which Keston calls a "shunt pump".
Surely this is the "primary" pump.
If that shunt pump (as Keston call it) was not fitted , then water would never pass through the boiler because the secondary pumps after the header would just circulate water through the header alone and no heat would never reach the system .