The beginning of the boiler loop is inside the boiler at the water jackets where the water is heated. The pump kicks on when the thermostat calls for heat and the heated water is pumped up the boiler loop supply line where it flows through the baseboards, radiators, or coils. As it flows through these heat exchange devices the water loses heat and flows back to the boiler in the return line. As it gets near the boiler the water goes through the impeller which is part of the circulator pump. *I have seen a few boilers where the circulator pump was on the loop supply side but this is uncommon. Most boilers in a residential boiler hot water system have the circulator pump on the return side of the loop.
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The picture on the right is of an oil fired boiler my co-worker and I installed in March 2002. After making several minor adjustments it started up on the first shot. The boiler piping design is ideal for servicing the boiler. The boiler isolation valves can be turned off so that the circulator pump can be changed without bleeding the entire loop. Maintenance can be performed on the boiler without introducing air into the systems loop. Additionally, the air purge drains eliminate the need to bleed air from all the baseboards or radiators on start-up. As a service technician, I often wish all boilers I work on were installed with these features. Problems with hydronic loops develop because air gets trapped in the pipes. It's called hydronic lock by some and air block by others but it prevents the water from circulating in the pipes. The solution is to drain the air out of the pipes and with some systems it is easier said than done. With the proper piping, gate valve, and boiler drain arrangement in the near boiler piping this is a cinch. You simply hook up a water hose to the drain in the piping, close a gate valve, and allow the fast fill (or bypass) to flush the air from the loop. Set up properly, you can even back flush the air from the loop. In both residential and commercial boiler loops there air purges installed in the loops to prevent build up of air. Sometimes these air purges malfunction and the air builds up and causes hydronic block. The customer then goes without heat until a technician is called in to find the block and purge the air. Some boiler loops are more complex than others but it is nice to know that the proper valves and drains were installed strategically in the boiler loop system to accomplish the purge quickly and efficiently.

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