This is a legitimate concern because more and more of us are involved with radiant heating where the water returning from the system can be as low as 90 degrees. It's also a concern when you add a modern boiler to an old gravity hot water system. There's nothing worse than hitting a hot boiler with cold return water.

Primary/secondary pumping lets you use small, inline circulators - even on large commercial jobs - and that's a real plus. And in a multiple-boiler system, this simple method of piping gives you a way to lower boiler standby losses and save fuel. When piped properly, no water will flow through an "off" boiler when its secondary circulator stops. And when the water stops flowing, the standby losses practically disappear.

There's nothing complicated about primary/secondary pumping. It all comes down to what happens when water flows through a tee. If water goes in, it has to come out. That's common sense. But how it comes out makes all the difference in the world. With primary/secondary, you have to set the tees that lead to the secondary circuit no more than 12 inches apart. As the primary flow enters the first of the two tees, it "looks" ahead, and then makes a choice. It can either go straight for 12 inches and be past these two tees leading to the branch circuit, or it can divert through the first tee's branch and flow through the entire secondary circuit. It becomes a question of which is the easier way.

Now, imagine yourself as the water in that primary main. What would you do? If the secondary circuit's circulator were off, wouldn't you choose to flow straight across those 12 inches of straight pipe? I know I would. That's the path of least resistance. This is why that 12-inch maximum spacing is so important. If you place the tees too far apart, primary water will begin to see the secondary circuit as a path of lesser resistance and begin to flow that way.

It's pretty simple when you get right down to it. When the secondary circulator is off, no water will flow through the secondary circuit because the 12-inch "gap" between the tees in the primary main is the path of least resistance. And it doesn't matter how large or small the primary and secondary circulators are either. They operate independently because they're hydraulically disconnected. You size each circulator for the flow rate and pressure drop needs of only the circuit it serves. That's why you usually wind up with lots of small inline circulators instead of one or two large, base-mounted pumps.