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NIBCO Partner
AHR 2022
Plumbing Museum

Addressing Trapped Air in Primary/Secondary Piping:
The Closely Spaced Tee that Works Smarter.

Primary/secondary piping systems are commonly used in condensing boiler installations as a simple and effective way to maintain proper flow through the boiler and the various heating loops. Although this method has grown increasingly popular, it is not without its challenges.

Primary/secondary piping success is contingent upon hydraulic separation, which is most commonly achieved via closely-spaced tees. Once hydraulic separation is established, each secondary heating loop can operate on its own decoupled circuit, independent of the primary boiler loop. The primary loop flows straight through the tee while the secondary loops branch off. This is useful when the secondary loops need to be controlled with different sized circulator pumps at different flow rates, since the secondary loops do not interfere with the primary loop or any of the other secondary loops.

In order for hydraulic separation to be possible, the center-line to center-line distance between the closely spaced tees must fall within a specific range of no more than 4 times the diameter of the primary loop piping. Spacing beyond that distance would cause a pressure drop across the tees resulting in ghost-flow in the secondary piping loop.

Fabricating the closely spaced tees is the first challenge for the installer. The work, which is often done in the field, is tedious and restrictive due to the small size and multiple angles for solder joints. Precise spacing between the tees is a must for proper boiler performance.

Once a system is in place, the next step is to fill and purge the air that’s accumulated during the installation. Failure to do so creates a noisy nuisance and can result in reduced efficiency, excessive wear on system components, and poorly controlled zone temperatures.

Heating systems can be designed in a number of ways to eliminate the small amounts of air that occur naturally during the course of operation. However, most solutions, like centrally located air eliminators and high point venting devices, use low-capacity vents incapable of efficiently handling the large amounts of air present after installation. To compensate for this and facilitate future service, systems are often also outfitted with strategically-placed supply and drain valves. Through a process called “power purging”, fluid is rapidly forced through the system via the supply valve and out the drain until the flow is free of any accumulated air.

Herein lies the second challenge unique to primary/secondary piping systems - the desirable hydraulic separation makes it impossible to force all of the air in the loop through one path where it can be consistently purged. Instead, the air would become trapped in isolated pockets throughout the heating system. These pockets would eventually exit the drain depending on the pressure across the different secondary loops, but because the pockets would all be moving independently, you could never really be sure that the system was completely purged of air. In the case of future heating system service, a portion of old fluid would continue to circulate in the secondary loop.

Circulator Pumps Installed with Isolators

If the user could close a valve between the tees, they could easily suspend the hydraulic separation by recoupling the loops and divert all of the flow into the secondary circuit for purging through the aforementioned drain valve.

However, there is no way to install a valve between two closely spaced tees as the distance between them would be too great to achieve hydraulic separation. The problem simply cannot be resolved by assembling off-the-shelf components.

The Webstone Pro‑Pal® Purge Tee™ is the solution to these challenges. It combines the precision crafted closely spaced tees with an integrated ball valve at just the right spot to recouple the loops.

Circulator Pumps Installed with Isolators

This heavy-duty forged brass valve not only resolves these problems, but does so with added benefits as well. Its patented space-saving design eliminates four leak paths and over 30 minutes of labor that would otherwise been spent on field fabrication. The Purge Tee is available with sweat connections in primary loop pipe sizes of 1" & 1 ¼" with secondary loop pipe sizes of ¾" to 1 ¼". The Webstone Pro‑Pal® Purge Tee™ also features a reversible handle for flexibility during installation, blowout-proof stem, and adjustable packaging gland for long term product life.

Simply put, the Purge Tee™ is the only way to effectively eliminate all of the air from a Primary/Secondary Loop System and like all other Webstone products, it's guaranteed for life.

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