250 Out & 250 Back
A common design approach in such situations is to locate several commercial size manifold stations along a common wall, and pipe them together in a parallel direct-return or reverse return arrangement. This approach creates very closely spaced tubing near the manifold stations where the tubing “fans out” from its tight spacing at the manifold, to its nominal spacing within the slab. An example of this undesirable tube placement is illustrated in Figure 1.
Closely spaced tubing near the manifold stations creates unnecessarily warm floor surfaces and uses more linear feet of tubing per square foot of floor area than necessary.
This approach often creates circuits having significantly different lengths. The longer the circuit, the greater its hydraulic resistance, and the lower its flow rate. Different length circuits also make it difficult to use stock tubing coils without creating “remnants” that are too short for use as another circuit. Often, the option for using these remnants as floor circuits is to join them with couplings, and then bury those couplings in the floor slab. Although some manufacturers allow this, conservatively minded designers may not feel comfortable with it.
A straightforward approachA recently completed Big-R Farm, Ranch, and Home Supply Store in Winnemucca, NV, took a different approach. With the exception of a small vestibule, this slab-on-grade building is a simple rectangle measuring 200 feet by 250 feet. The vast majority of the floor space is for retail display and check-out counters. The small remaining space is for offices, employee break room and restrooms.
Given the corner location of the building’s mechanical room and the dimensions of the building, the initial thought was to locate several manifold stations along the 200-foot side wall and route all floor circuits from there. However, it quickly became apparent this would lead to significant differences in circuit length, generate many tubing remnants and crowd the tubing together near the manifolds as described earlier.
Extended manifoldSome rethinking lead to the idea of a site-built “extended manifold” that would run almost the entire length of the 200-foot wall. Each floor circuit would tee into this extended manifold at the exact location where it emerges out of the floor slab. This would eliminate the crowding of tubing. It would also allow stock 1,000-foot coils of PEX tubing to be cut in half and used to create two 500-foot circuits (250 feet out, one return bend, and 250 feet back).
The single return bend in each circuit needed to stay back about 3 feet from the opposite wall due to planned cabinetry. This allowed sufficient length within the 500-foot circuit to form the return bend and short risers for connecting each end to the extended manifold. Wasted tubing and remnants were reduced to essentially zero.
The 53 gpm flow rate required 2.5-inch size manifold tubing to keep head losses relatively low. Fabricating an extended manifold from copper tubing of this size, and using 106, 2.5-inch copper tees along with reducer fittings was a possible, but costly option. Likewise, fabricating the extended manifold using T-drilled connectors would require tooling and installation techniques that the installing contractor was not equipped for.
After more research and discussion between the owner and design engineer, a decision was made to fabricate the extended manifold from 2.5-inch size Climatherm glass fiber-reinforced polypropylene tubing (PP-R) from Aquatherm Inc. This piping system and the necessary tooling to assemble it comes from Germany. Although used in many countries for 35 years, it is relatively new in North America.
About that PP-R tubingPP-R tubing is now available in sizes from 3/8-inch to 10-inch, and in compliance with ASTM F2389 (standard specification for pressure-rated polypro-pylene piping systems). The glass fiber-reinforced composite core layer limits thermal expansion and allows the piping to remain rigid at sustained operating temperatures up to 160°F with a corresponding pressure of 70 psi, and 200°F at 20 psi.
PP-R tubing is joined to polypropylene fittings by socket fusion. The mating surfaces of the tube and fittings are simultaneously heated using a special fixture. After the outer surface of the tube and inner surface of the fitting are heated to 500°F, they are separated, the heating fixture is removed, and the semi-molten surfaces are immediately pushed together and held in position as they cool.
A distinct advantage of using this piping system for the extended manifold was the ability to create tappings using saddle fusion. The process begins by using a special rotary cutting tool to create a precisely sized hole in the side of the tube. This tool removes all cut material to prevent it from falling into the pipe as seen in Figure 2.
The extended manifold systemFigure 5 shows one of the 56 pairs of circuit supply and return tappings used on this project. A mini ball valve equipped with a special adapter for a direct compression connection to 5/8-inch PEX tubing was used at both ends of every circuit. This allows each circuit to be completely isolated in the event the tubing is ever damaged by future remodeling of the space. These valves also make it easier to purge air from the floor circuits when the system is commissioned. The middle horizontal pipe without tappings seen in Figure 5 is the full flow return of a reverse return layout. More on this later.
Desirable effectsThe reverse return layout in combination with constant header size results in slightly higher flow rates through the circuits near the end of the headers compared to those near the center of the headers. Slightly higher flow rates mean slightly lower circuit temperature drops. The latter implies somewhat higher average water temperatures and thus somewhat higher heat outputs. This was a desirable effect in this application because the higher output circuits are near exterior walls where heat loss effects are more dominant.
Heat is supplied by three modulating/condensing boilers that are fired and modulated by an outdoor reset control. This multiple boiler system was interfaced to the distribution system using a pair of closely spaced tees for hydraulic separation as seen in Figure 7. This detail was chosen in lieu of a hydraulic separator because the air separator was already owned and available to be included.
The building’s very small domestic hot water load did not justify an indirect water heater. However, the boiler piping and controller used would allow an indirect water heater to be easily added and operated as a priority load if future use of the building required more domestic hot water.
This system has almost two full winters behind it and is working well, according to the owner.
In many respects this design is “routine” in its use of modern hydronics hardware. The use of a site-built extended manifold rather than multiple conventional manifolds was simply a more appropriate choice given the specifics of the building and availability of materials. This adaptation demonstrates the versatility of modern hydronics in meeting specific requirements, while at the same time creating synergy between hardware and intended operation.
John Siegenthaler, a licensed professional engineer, is principal of Appropriate Designs, a consulting engineering firm in Holland Patent, NY, and author of the text “Modern Hydronic Heating” now in its third edition. He also serves as hydronics editor for Supply House Times and its sister publications, Plumbing & Mechanical and PM Engineer. He has more than 30 years of experience in designing modern hydronic heating systems and is an Associate Professor Emeritus of engineering technology at Mohawk Valley Community College, Utica, NY. Contact him at firstname.lastname@example.org, or visit his Web site, www.hydronicpros.com, for publications and software on hydronic system design.