Primary/Secondary Piping Complexities

This is the new primary/secondary piping. It follows several boiler piping rules of thumb, although the 009 pump is now a bit over-sized for the primary loop. There is a secondary 011 pump that also is capable of pumping at high head because a radiant loop system consists of several turns of pipe that create a high head condition.

New Challenger Boiler Piping

New Boiler Piping Illustrated

Primary Secondary System

Primary Secondary System Piping

In place of the simple in/out system piping we now have a primary loop that circulates from hot to cold directly as well as a secondary piping loop that draws hot water from the primary loop and returns it to the primary loop. This is similar in function to the simple system except that the primary loop allows the boiler to circulate at a different and constant speed, while the secondary loop that feeds the heat emitters (radiators and radiant) can vary in speed and pressure without reducing the pressure to the boiler.

The main feature of the primary/secondary design is the “closely spaced tees” that cannot be more than 12 inches apart but should be only 4 pipe diameters (4″ for 1″ pipe) apart. The hot water line length to the tees is 8 times the pipe diameter while the cold water line needs only be 4 pipe diameters long. The Taco 009 pump runs only the primary loop while another Taco 011 pump runs the secondary loop. Both pumps are stainless steel even though cast iron is acceptable in a closed loop system, I was concerned about water quality for boiler longevity so made the whole system brass, copper, and stainless steel as well as using oxygen barrier pex.

I added several gauges to the return line: temperature, pressure, and flow. That way I can compare the pressure differential between the hot water supply as it leaves the boiler and the cold water return. This difference is called a delta T and has a role in the system design. I originally was planning for a delta T of about 20 but it appears we are getting closer to 10. Sometimes when only the radiators are calling for heat in this combination system, the boiler is not firing because the return temperature is higher than the outdoor reset curve. That is how it should work–unless the radiators are not hot enough to warm the space. So I have to keep an eye on these temperatures.

Some of the other components would have been added to the simple system as well. The boiler feed valve is a Caleffi and maintains a pressure of about 10-12 psi in the system. The valve can be closed to keep new water from entering the heating system. There is a Spirovent air separator. When the system was first purged of extra air (by opening the drains at the Honeywell valves and running the loop water into a bucket until no more air bubbles appear) the system was still airy enough that water was burping out of this vent so I attached a pipe to drain it to the sump, later I removed the pipe.

The system also has a Spirotrap dirt catcher on the cold water return side. There is an acid neutralizer for the condensate overflow so that the water from the boiler condensation process is treated with limestone before entering the sump. There is a boiler pressure relief valve that is piped to the sump too. There are shutoffs before and after each pump and a boiler drain valve as well as a hose (green in the photos) that is hooked up to the Honeywell control valves in the manifold that drain each loop as it is opened.

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