Primary-secondary pumping has become pretty popular nowadays, especially with boiler manufacturers. They love it because it offers a simple way to protect their boilers against low-temperature return water and the resulting flue-gas condensation that low-temperature water causes. 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.
The compression tank belongs in the primary main, as does the air separator and the fill valve. Make sure you install your primary circulator so that it pumps away from the compression tank. That way, you can take advantage of the primary circulator's full differential pressure. This makes it much easier for you to get rid of any system air that works its way into the piping. Oh, and if you put full-port ball valves in the common piping between the primary and secondary circuits, you'll be able to get the air out of the system a lot quicker as well.
The secondary circulators use the common piping between the primary and secondary circuits as their "compression tank." Always pipe your secondary circulators so they pump away from the primary loop and toward the radiation.
On most jobs, you'll connect your secondary circuits in a manifold off the two primary-to-secondary tees. Pipe the manifold the same way you would if you were connecting it to a boiler. If one or more of your secondary zones are going to serve a radiant heat zone, use a two-, three-, or four-way valve to mix the water returning from your zone into the hot supply water from the primary circuit. Pipe your secondary circulator on the radiation side of the two-, three-, or four-way valve. Use flow-control valves to stop gravity circulation from the primary to the secondary circuits. I didn't always preach this but I've leaned from experience that it pays to have those flow-control valves in place. And depending on the piping configuration, you might need them on both the supply and return sides of the secondary circuits to stop gravity circulation.
We call the system I've been describing "one-pipe, primary-secondary pumping." The primary circulator moves the boiler water through the primary circuit. When the flow of cooler water returns from the secondary circuit, the hot primary water, which jumped the "gap" between the two tees mixes with the return water. The hotter water instantly raises the returning water's temperature and protects the boiler from flue-gas condensation and thermal shock.
If every zone in a one-pipe, primary-secondary system should call at the same time, however unlikely this might be, the primary circuit's temperature will drop by its full design temperature drop. In large commercial systems, this can lower the supply water temperature to the secondary circuits at the end of your primary loop.
To get around this potential problem, you can use two-pipe, primary-secondary pumping instead. With this system, you pipe the primary circuit as a two-pipe direct- or reverse-return system. This gives you a way of delivering the same water temperature to each secondary circuit. You make your connection from the primary to the secondary circuit from a crossover "tunnel" that drops below the level of the primary supply and return mains. The drop in the pipe makes it easier to get rid of the air.
Balancing is more of a challenge in a two-pipe, primary-secondary system. You have to look carefully at the flow rate needs of each circuit and "tunnel" and make sure you're delivering the right flow to those secondary circuits. This type of system usually calls for some thoughtful engineering, so take you time. If you get stuck, bring your questions to the Wall. There are some very bright people there!
Learn more in Dan Holohan's book Primary-Secondary Pumping Made Easy!