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PEX tubing is the distribution backbone of every hydronic radiant floor heating system. It carries heated water from the manifold through the floor and back, transferring that heat into the concrete or subfloor assembly above it. Choosing the right type, size, and spacing is not a detail to leave to guesswork: these decisions directly affect system output, pressure balance, installation ease, and how the system performs decade after decade.
WBI specifies PEX tubing for every radiant floor system it supplies. Here is what that specification involves and why the choices matter.
The Three Types of PEX and Why Type Matters for Radiant
PEX stands for cross-linked polyethylene. All PEX starts as polyethylene pipe, and all PEX is made more durable by crosslinking the polymer chains. The difference between PEX-A, PEX-B, and PEX-C is the method used to create those crosslinks, and that method significantly affects the tubing’s physical properties.
PEX-A is manufactured using the Engel peroxide method, which crosslinks the material while it is in a molten state. This produces the most uniform crosslink density and gives PEX-A two properties that matter for radiant installation: superior flexibility, and shape memory. A section of PEX-A that has been kinked can be repaired by applying a heat gun to the damaged area, allowing the tubing to return to its original round profile. This does not work with PEX-B or PEX-C, where a kink means a cut and a fitting.
PEX-B is made using the silane moisture-cure method. It is stiffer than PEX-A, has a lower crosslink density, and does not have true shape memory. It is widely available, costs less than PEX-A, and works acceptably in radiant applications where the layout does not require tight bends. PEX-C, made by electron beam irradiation, is the stiffest of the three and is not commonly specified for radiant floor applications.
Oxygen Barrier PEX: Required When It Matters
Standard PEX tubing is slightly permeable to oxygen. In an open loop system or one filled with fresh water regularly, this causes no problem. In a closed hydronic heating system that includes iron or steel components, oxygen permeation over time dissolves into the circulating water and causes oxidation of those ferrous components. The result is rust contamination that clogs manifold ports, damages pump impellers, and shortens the life of every iron component in the system.
Oxygen barrier PEX solves this by bonding a thin EVOH (ethylene vinyl alcohol) or aluminum layer to the outer wall of the tubing. This layer blocks oxygen diffusion into the water side of the system. WBI specifies oxygen barrier PEX-A for all hydronic radiant installations that include a cast iron or steel boiler, ferrous manifold components, or iron pump housings.
Systems built entirely with non-ferrous components, such as stainless steel manifolds, bronze pumps, and polymer heat exchangers, can use standard PEX without an oxygen barrier. WBI designs system component selections and tubing specifications together so the two are always compatible.
Tubing Size: 1/2 Inch for Most Residential Work
The tubing diameter determines how much water flows through each circuit at a given pressure, which in turn affects how much heat each circuit can deliver and how evenly heat is distributed from the manifold supply to the end of the loop. For residential radiant floor heating, 1/2 inch PEX is the standard choice.
A 1/2 inch circuit carries enough flow to deliver the heating output of a typical residential zone, keeps pressure drop within the range that residential circulators handle efficiently, and remains flexible enough to work through the bends of a serpentine floor layout. The practical maximum circuit length for 1/2 inch PEX is 300 feet, which covers a floor area of 225 to 300 square feet at 9 inch spacing.
For longer circuits or higher-capacity commercial applications, 5/8 inch or 3/4 inch tubing allows longer runs and higher flow rates without excessive pressure drop. WBI sizes the tubing diameter for each circuit based on the heat load, circuit length, and available pump head.
Tubing Spacing and Layout Patterns
Tubing spacing, the distance between adjacent passes of tubing across the floor, determines how much heat output the floor can deliver at a given supply water temperature. Closer spacing means more tubing per square foot, higher output capacity, and more even floor surface temperature. Wider spacing means less tubing, lower output, and a slightly more striped temperature distribution if the supply water is very hot relative to the design condition.
For well-insulated residential spaces with moderate heating loads, 9 to 12 inch spacing is standard. Rooms with high heat loss from large glazed areas or poorly insulated walls require 6 inch spacing to deliver adequate output without driving supply water temperatures above the optimal range for the heat source. WBI calculates the required spacing for each zone as part of the heat loss and system design process.
The two common tubing layout patterns are serpentine and spiral (also called snail or double serpentine). Serpentine runs the tubing back and forth across the floor in parallel passes. Spiral winds the tubing inward from the perimeter to the center, alternating supply and return passes so that the hottest and coolest sections of the circuit are adjacent throughout the floor. Spiral layouts produce more even floor surface temperatures and are preferred for spaces where uniform comfort is the priority.
Installation: Embedded Slab, Staple-Up, and Pre-Routed Panels
PEX tubing in a radiant floor system is installed by one of three methods, each suited to a different floor assembly. In a new concrete slab, the tubing is tied to a wire mesh or rebar grid and the slab is poured over it. This is the simplest and most common approach for garage floors, basement slabs, and slab-on-grade construction.
In wood-framed floors, staple-up installation attaches the tubing to the underside of the subfloor from below, between joists. Pre-routed insulation panels install from above, with the tubing pressing into pre-cut channels in an EPS foam panel before the finish flooring goes down. WBI covers the selection between these methods as part of each system design, based on floor assembly, access, and project budget.
Frequently Asked Questions About PEX Tubing for Radiant Floor Heating
Get the Right PEX Spec for Your Radiant Project
WBI supplies PEX-A tubing, manifolds, and complete hydronic radiant systems sized and designed for your specific project. Tell us your floor area and heating load and we will handle the rest.
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