Stainless Steel 304 Coil Tube for Floor Radiant Heating Systems
Product Overview
The Stainless Steel 304 Coil Tube (Product No. 157) is a purpose-engineered coiled piping solution designed for floor radiant heating systems within the HVAC (Heating, Ventilation, and Air Conditioning) sector. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this coil tube is specifically formulated to operate within a hot water temperature range of 35–55°C and is intended for permanent embedding in concrete slabs. The product addresses the unique demands of hydronic underfloor heating circuits, where long-term durability, thermal stability, and resistance to environmental degradation are paramount.
Unlike conventional plastic or copper alternatives, the 304 stainless steel coil tube offers superior mechanical strength, negligible thermal expansion, and exceptional resistance to chemical attack — making it an ideal medium for low-to-medium temperature hydronic heating loops that must endure decades of service within an inert concrete matrix.
Material & Properties
The coil tube is fabricated from AISI 304 / ASTM A213 TP304 austenitic stainless steel, which is the minimum material grade required for this application. The selection of 304 grade is driven by the following property profile:
- Chemical Composition: Chromium (Cr) 18.0–20.0%, Nickel (Ni) 8.0–10.5%, Carbon (C) ≤0.08%, Manganese (Mn) ≤2.0%, Silicon (Si) ≤0.75%. The high Cr and Ni content ensures a stable passive oxide layer and general corrosion resistance.
- Mechanical Properties: Tensile strength ≥515 MPa, yield strength ≥205 MPa, elongation ≥40%. These values ensure the tube withstands concrete pouring pressure and differential settlement without deformation or fracture.
- Thermal Conductivity: Approximately 16.3 W/(m·K) at 20°C, providing efficient heat transfer from the circulating water to the surrounding concrete and, subsequently, to the floor surface.
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C). While stainless steel exhibits measurable thermal expansion, the low operating temperature range (35–55°C) limits dimensional change to a negligible level, minimizing stress on pipe-to-concrete interfaces.
- Operating Temperature Range: 35–55°C (hot water circulation). The material retains full mechanical integrity and corrosion resistance throughout this range.
- Maximum Allowable Pressure: Typically rated for PN16 (1.6 MPa) or PN25 (2.5 MPa) depending on wall thickness, accommodating standard hydronic heating system pressures.
Chloride Ion Penetration and CUI Protection
A critical engineering consideration for this product is the risk of Chloride-Induced Stress Corrosion Cracking (Cl-SCC) and Corrosion Under Insulation (CUI). In floor radiant heating applications, the stainless steel tube is embedded in concrete, which is inherently alkaline (pH 12–13) and may contain chloride ions from deicing salts, seawater-based aggregates, or environmental exposure. Key protective considerations include:
- Chloride Threshold: AISI 304 has a chloride resistance threshold of approximately 200–500 ppm in neutral pH environments. In the alkaline concrete pore solution, this threshold is elevated, but prolonged exposure to elevated chloride concentrations (e.g., >1000 ppm) can initiate localized pitting.
- CUI Mitigation: When the concrete acts as a thermal insulation layer, moisture trapped between the pipe surface and the concrete interface can create a corrosive micro-environment. The passive film on 304 stainless steel provides inherent protection, but proper concrete cover thickness (minimum 30 mm recommended) and low permeability concrete mix design are essential.
- Recommendation: For applications with known high chloride ingress risk (e.g., coastal regions, areas with road deicing salt exposure), upgrading to 316L stainless steel (with 2–3% Molybdenum) is advisable for enhanced pitting and crevice corrosion resistance.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Coil Tube for Floor Radiant Heating | — |
| Material Grade | AISI 304 / ASTM A213 TP304 | ASTM A213 / ASME SA-213 |
| Manufacturing Method | Welded (ERW / TIG welded) | ASTM A213 |
| Outer Diameter (OD) | 15.8 mm / 16 mm (typical); customizable | ASME B36.19M |
| Wall Thickness | 1.0 – 1.5 mm (typical range) | — |
| Coil Length | 100 m / 200 m / 500 m (standard); custom lengths available | — |
| Coil Inner Diameter (ID) | ≥ 150 mm (minimum bending radius compliance) | — |
| Operating Temperature | 35 – 55°C | — |
| Maximum Operating Pressure | PN16 (1.6 MPa) standard; PN25 (2.5 MPa) optional | EN 10217 |
| Surface Finish | Pickled and passivated (2B finish) | ASTM A480 |
| Installation Method | Embedded in concrete slab (post-pour or pre-pour) | — |
| Heat Transfer Fluid | Hot water (glycol-water mixture compatible) | — |
| Design Service Life | ≥ 50 years (when properly installed) | — |
| Chloride Resistance | Threshold: ~200–500 ppm (neutral pH); enhanced by alkaline concrete environment | ASTM G48 |
| Applicable Standards | ASTM A213, ASME B36.19M, EN 10217, ISO 6508 | — |
Manufacturing Process
The production of the 304 stainless steel coil tube follows a rigorous multi-stage manufacturing sequence to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Coil Unwinding and Leveling: Cold-rolled 304 stainless steel strip (slit coil) is fed through a high-precision leveling machine to eliminate coil set and ensure flatness within ±0.05 mm tolerance.
- Forming (Roll Forming / Welded Tube Making): The leveled strip is passed through a series of progressively shaped rolls that gradually form the flat strip into a circular cross-section. This cold-forming process introduces beneficial work hardening, increasing the yield strength of the final tube.
- Welding: The formed strip edges are joined using Electro-Resistance Welding (ERW) or Tungsten Inert Gas (TIG) welding. TIG welding is preferred for higher-quality applications as it produces a cleaner weld seam with minimal heat-affected zone (HAZ) distortion. The weld seam is oriented at the 12 o'clock position (top) to minimize stress concentration during bending.
- Weld Seam Inspection: Each weld seam undergoes Eddy Current Testing (ECT) or Ultrasonic Testing (UT) in accordance with ASTM E164 or ASTM E1149 to detect any discontinuities, porosity, or lack of fusion.
- Pickling and Passivation: The welded coil tube is treated with a nitric acid/hydrofluoric acid (HNO₃/HF) pickling solution to remove weld scale and oxide discoloration. Subsequently, a passivation treatment (typically 20–30% HNO₃) restores the chromium oxide passive film, ensuring optimal corrosion resistance.
- Dimensional Inspection: Final products are measured for OD, wall thickness, straightness, and coil diameter compliance using laser gauges and calipers.
- Coiling and Packaging: The finished tube is coiled to the specified length and diameter, wrapped in protective film or paper, and secured for transportation to prevent mechanical damage and moisture exposure during transit.
Applications
The primary application of this product is floor radiant heating systems (hydronic underfloor heating), where the coil tube is embedded within a concrete slab or screed layer. Additional application contexts include:
- Residential Underfloor Heating: Low-temperature hot water (35–45°C) circulates through the coil loops to provide comfortable, even radiant floor heating in homes, apartments, and villas.
- Commercial and Industrial Facilities: Large-floor-area buildings such as shopping centers, hospitals, schools, and manufacturing plants where uniform floor temperature distribution is required.
- Agricultural Greenhouses: Floor heating for crop cultivation, where precise temperature control (35–45°C) ensures optimal growing conditions without direct air heating.
- Swimming Pool Deck Heating: Embedded in concrete pool deck areas to provide anti-slip, comfortable walking surfaces and prevent condensation.
- Snow Melting Systems: Integration into concrete pavements, sidewalks, and parking lots for controlled snow and ice melt (operating at the lower end of the temperature range).
Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and product certification portfolio to ensure compliance with international standards:
- ISO 9001:2015 — Quality Management System certification, demonstrating systematic control of manufacturing processes, supplier management, and continuous improvement.
- ISO 14001:2015 — Environmental Management System certification, covering waste management, emissions control, and sustainable resource utilization.
- CE Marking (EU) — Conformity with European directives including the Pressure Equipment Directive (PED 2014/68/EU) where applicable, and the Construction Products Regulation (CPR 305/2011).
- EN 10204 3.1 Material Test Certificate — Each batch is accompanied by a third-party verified material test certificate confirming chemical composition, mechanical properties, and heat treatment status.
- ASTM A213 Compliance — Product manufactured and tested per ASTM A213 standard for seamless and welded austenitic stainless steel tubes for high-temperature service.
- NACE/AMPP No. 0286 — If applicable for coating or passivation processes, demonstrating compliance with corrosion prevention best practices.
Unique Selling Points
- Exceptional Longevity: With a design service life exceeding 50 years, the 304 stainless steel coil tube eliminates the need for replacement that is common with plastic (PEX/PERT) or copper alternatives, reducing lifecycle costs significantly.
- Superior Mechanical Strength: The high tensile strength (≥515 MPa) of 304 stainless steel ensures the tube withstands concrete pouring loads, differential settlement, and structural stresses without deformation — a critical advantage over plastic tubing.
- Chemical Inertness: The stable austenitic microstructure and passive oxide layer provide inherent resistance to chloride-induced pitting, general corrosion, and thermal degradation within the 35–55°C operating range.
- Dimensional Stability: Low thermal expansion at operating temperatures minimizes stress at pipe-to-concrete interfaces, reducing the risk of cracking or delamination over thermal cycling.
- Hygienic Surface: The pickled and passivated 2B finish provides a smooth, non-porous surface that prevents biofilm formation and maintains water quality within the heating circuit.
- Customizability: Available in multiple OD, wall thickness, and coil length configurations to accommodate diverse system designs and installation methodologies.
- Weldable and Repairable: Unlike plastic tubing, stainless steel coil tube can be field-welded for repairs or system modifications, extending serviceability and reducing waste.
Engineering Note: For applications involving prolonged exposure to high chloride environments (e.g., coastal concrete structures, deicing salt-heavy regions), it is strongly recommended to evaluate the upgrade to AISI 316L stainless steel to ensure long-term immunity against chloride-induced stress corrosion cracking (Cl-SCC) and Corrosion Under Insulation (CUI). Wenzhou Jixing Metal Materials Co., Ltd. offers 316L variants upon request.