ISO 13675:2013

Overview

ISO 13675:2013 - Heating systems in buildings - Method and design for calculation of the system energy performance - Combustion systems (boilers) - is the ISO system standard for energy calculation of combustion-based heat generation sub‑systems (boilers). It covers oil, gas, coal and biomass boilers and defines the required inputs, calculation method, and resulting outputs for space heating and combined space heating/domestic hot water production, including control strategies and auxiliary energy use.

Key topics and technical requirements

• Scope and objective: Provides a systems‑level method to calculate energy performance of combustion sub‑systems rather than product specifications. Useful for compliance checks, design optimization and retrofit assessment.
• Inputs and outputs: Specifies the data needed (e.g., generator performance characteristics, gross calorific value, operating modes) and the output energy indicators for system performance evaluation.
• Boiler types: Addresses a range of boilers - condensing, non‑condensing, biomass boilers, low‑temperature, modulating, multistage and on/off units - and how their operating characteristics affect energy use.
• Calculation method: Defines a calculation period/step approach (hourly, monthly or seasonal), heat balance of the generation sub‑system, and methods to account for boiler efficiency, combustion power, and part‑load behavior.
• System losses and recovery: Requires accounting for heat losses to flue/chimney, generator and storage tank envelope losses, and distinction between recoverable and non‑recoverable system thermal losses.
• Controls and auxiliary energy: Includes effects of control strategies (on/off, modulation, cascading) and auxiliary electrical energy for pumps, fans and electronics in the system energy balance.
• Documentation and annexes: Includes informative annexes with formulas, default values, calculation examples (e.g., modulating condensing boilers) and guidance on gross calorific values.

Practical applications

• Energy performance compliance assessment against regulatory targets.
• Detailed design and comparison of alternative heating system configurations.
• Energy audits and retrofit analysis to quantify savings from upgrades (e.g., switching to condensing or modulating boilers, adding heat recovery).
• Performance modelling for mixed space heating and domestic hot water systems in residential, commercial and institutional buildings.

Who should use ISO 13675:2013

• HVAC and building services engineers
• Energy assessors and consultants
• Building designers and architects
• Boiler and system manufacturers
• Regulatory bodies and policy makers evaluating system‑level efficiency
• Researchers and educators in building energy performance

Related standards

• ISO 13790 - Energy performance of buildings - Calculation of energy use for space heating and cooling (input for system calculations)
• ISO 7345 - Thermal insulation - Physical quantities and definitions
• ISO 13602-2 - (Referenced for calorific value conventions)
• ISO 15927 - (Referenced for external temperature measurements)

Keywords: ISO 13675, heating systems, boilers, combustion systems, energy performance, boiler efficiency, condensing boiler, biomass boiler, space heating, domestic hot water.