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Primary/Secondary plumbing has been used for years in commercial and residential heating and cooling applications. In a radiant heating application, a properly sized pump on the Primary Loop triggers the on-demand heater whenever any zone calls for heat. This creates a circulating loop of hot water between the heater and a pair of closely spaced tees that intersect with a Secondary Loop.

In effect, the closely spaced tees become the “heat source” for the radiant Zone Manifold via the Secondary Loop. When a radiant zone calls for heat, hot water is drawn from the closely spaced tees and sent to the radiant tubing.

Primary/Secondary plumbing solves a couple of problems. One, it prevents overheating of certain types of boilers by guaranteeing that the proper amount of fluid is always circulating through the system’s internal heat exchanger — even if only a small zone (i.e. low flow) is calling for heat.

Also, for very large zones, the Primary/Secondary plumbing prevents the heater from becoming a “choke point” in the radiant system. In other words, the heater may have plenty of heating power (i.e. high BTU output), but the inlet and outlet ports are too small to pass the large volume of water required by a large radiant system.

Important note: For relatively small radiant systems, those using less than 3,000 linear feet of 7/8″ XL PEX, Primary/Secondary plumbing isn’t necessary. Unless, as stated above, a specific boiler manufacturer requires Primary/Secondary plumbing to protect the unit’s internal heat exchanger.

Takagi on-demand water heaters, for example, do not require Primary/Secondary plumbing. Nevertheless, it would be a huge mistake to run even a modest radiant floor system directly off the water heater.

Again, it comes back to flow. The inlet and outlet ports of on-demand units are tiny and that restriction can “choke” the flow rate needed by the radiant floor. For this reason, a properly sized three-way mixing valve is installed between the on-demand heater and the radiant floor Zone Manifold. With this configuration, the radiant circulator pump(s) draw from the “mix” port, a combination of the “hot” port of the mixing valve (the line from the heater) and the “cold” port of the mixing valve (a tee off the line returning from the floor to the heater) solving the volume/flow problem. Tiny ports no longer matter because only a small amount of heated water is required from the on-demand heater — for the simple reason that the floor water returns to the heater only ten degrees cooler than when it left.

In other words: The water going to the radiant pumps is mostly the very same water that just returned from the floor — with a small injection of hot water from the on-demand heater (remember the ten degrees we used to heat the zone?).

So, if a radiant system has lots of tubing (more than 3000 linear feet of 7/8″), we don’t draw the system water from a three-way mixing valve, and we certainly wouldn’t draw it directly from the on-demand unit. Instead, the primary loop becomes the mechanism by which a large, high volume, secondary loop is heated.