ISO 11855-2:2021

Overview

ISO 11855-2:2021 specifies methods to determine the steady-state design heating and cooling capacity of water‑based embedded radiant heating and cooling systems. The standard defines procedures and boundary conditions to calculate the heat flux between the building space and an embedded surface (floor, wall, ceiling) as a function of the medium differential temperature. Results include average and extreme surface temperatures, temperature uniformity, nominal heat flux, the associated nominal medium differential temperature, and characteristic curves linking heat flux to controlling variables.

Key technical topics and requirements

• Scope and outputs
  • Applies to water‑based surface heating and cooling systems integrated with the building structure (not panel systems with open air gaps).
  • Calculation outputs: average/max/min surface temperatures, uniformity, nominal and design heat fluxes, and characteristic curves.
• Calculation methods
  • General numerical method using finite element method (FEM) or finite difference method (FDM) for two- or three-dimensional modelling.
  • Simplified calculation methods tailored to system construction (pipe position, pipe spacing, screed, insulation, heat-conducting devices).
• Heat exchange and characteristic relationships
  • Definition and use of the heat exchange coefficient between surface and space (convection + radiation) and basic characteristic curves for floor, wall, and ceiling surfaces.
• Documentation and verification
  • Requirements for calculation documentation and verification (including normative annexes for verification of FEM/FDM programs).
• Supporting annexes
  • Annexes cover heat flux calculation, general resistance methods, wooden constructions, verification procedures, material thermal properties, and maximum surface temperature guidance.

Practical applications and who uses it

ISO 11855-2:2021 is used to ensure uniform, reproducible assessment of embedded radiant systems in design and compliance work:

• Architects and building services designers for sizing and specifying embedded radiant systems.
• HVAC and mechanical engineers performing thermal performance and heat load calculations.
• Simulation/modeling specialists using FEM/FDM or simplified calculation tools.
• Manufacturers and system integrators validating product performance and characteristic curves.
• Certification bodies, consultants, and project QA teams for design verification and compliance with comfort criteria.
• Energy modellers using outputs as inputs for dynamic or whole‑building energy analyses (see other parts of ISO 11855 series).

Related standards

• ISO 11855-1: Definitions, symbols and comfort criteria (normative reference)
• Other parts of the ISO 11855 series for design/dimensioning (Part 3), dynamic TABS calculation (Part 4), installation (Part 5), control (Part 6), and energy input parameters (Part 7)

Keywords: ISO 11855-2:2021, embedded radiant heating and cooling systems, water-based surface heating, design heating and cooling capacity, finite element method, finite difference method, simplified calculation methods, heat flux, surface temperature, nominal medium differential temperature.